FOCUS

THE BRAIN

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FOCUS

THE BRAIN

At the beginning of the twenty-first century, understanding the mecha-

nisms that govern how the brain works represents a major challenge in

fundamental and applied research, especially in public health.

Neuroscience as a discipline is in full expansion and is going to contin-

ue growing rapidly in the years to come. Extending our knowledge in

this area means consolidating today’s multidisciplinary approach to the

study of cognitive function in both animals, especially primates, and

human beings.

The French National Center for Scientific Research (CNRS) intends to

broaden its activities in the neurosciences over the next few years,

notably by establishing a number of centers of excellence in France

where scientists from different disciplines can be brought together at

sites equipped with all the necessary technical resources. This policy

will result in exchanges of information derived from varied methods and

state-of-the-art experimental approaches, and it is to be hoped that the

knowledge thus obtained will open new avenues towards therapeutic

applications.

This publication, Focus The Brain, surveys the current situation and out-

lines future directions, in the context of the ambition of CNRS to foster

brain research—one of today’s great scientific endeavors.

Bernard Larrouturou

CNRS Director General

September 2005

Introduction 6

Seeing the Brain at Work 8

Better Understanding Means Better Treatment 12

Analyzing Behavior 18

Decoding Nervous Information 32

Understanding Cerebral Development 38

Conclusion 41

FOCUS

THE BRAIN

6

Understanding the Human Brain—a Major Challenge in Biology

The second half of the twentieth century was an exceptionally fruitful peri-

od in biological research. Spectacular progress in genetics, cell biology and

molecular biology has led to an understanding of many of the fundamental

mechanisms which control how cells work, as well as providing enormous

amounts of valuable information which can be used to analyze physiological

and pathological processes at various levels, from living tissue through the

organ to the whole organism. Brain research has profited from this conceptual

flowering in which not only molecules and genes but also environmental regu-

latory factors have been shown to be indispensable for living organisms.

A Center of Excellence Strategy to Understand the NeuronThis edifice of knowledge, built up over decades, remains incomplete and

is still, no doubt, far from the biological reality. Deciphering the neuronal

codes which underlie higher cognitive functions—perception, vigilance, the

planning of movement, learning, memory, language, thought, etc.—is an

essential line of research to be pursued if we are to dissect the complexity of

cerebral mechanisms.

This goal, modest as it might be given the complexity of biological systems,

can only be realized if a genuinely multidisciplinary approach is adopted and

effectively carried through in primates and humans. An imperative in such an

approach is to consolidate and expand those sectors of the integrative, com-

putational and cognitive neurosciences in which CNRS already excels. To

achieve this, it is now indispensable that we create conditions to facilitate the

exchange and pooling of results derived from diverse techniques and method-

ological approaches. This means establishing European centers of excellence

to bring together critical masses of neuroscientists with the necessary tech-

nical resources at their disposal.

This center of excellence strategy could be launched by establishing a

number of imaging units housing the kind of equipment on which will depend

progress in the neurosciences in the future. Important capabilities include:

magnetic resonance imaging (MRI), in small animals, primates and humans

(diffusion MRI, functional MRI [fMRI]); positron emission tomography (PET);

electroencephalography (EEG); magnetoencephalography (MEG); multichan-

nel multisite recording; and all the imaging techniques relevant to brain func-

tion at the scale of the neural network or the individual neuron (calcium imag-

ing, confocal and electron microscopy, atomic force microscopy). Centers,

which bring together these diverse capabilities, will certainly facilitate the

quest for knowledge in the field. In these conditions, we will be able to define

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the general rules that govern how the brain works in primates and in

humans, by studying large populations to define the universal anatomical

features and functional parameters associated with a given form of behavior.

In this way, it will be possible to establish functional links between morpho-

logical substrates, metabolic variations and coding of nervous activity. This

approach will also mean developing computational neuroscience to improve

our ability to simulate and model biological situations.

An Interdisciplinary ApproachInformation about the cerebral mechanisms that underlie higher cogni-

tive functions will shed light on the processes involved in the various forms

of mental disease. Knowledge in this area will open new avenues towards

innovative therapeutic strategies to improve the condition of patients suffer-

ing from psychiatric disorders and neurodegenerative diseases such as

Parkinson’s and Alzheimer's disease.

A multidisciplinary approach is equally indispensable in this endeavor.

Investigations at the molecular and cellular levels will be essential, as well

as at the level of the neural network in rodent and primate models relevant

to mental problems. In this way, it will be possible to elucidate the mecha-

nisms that cause the akinesia and hyperkinesia that are typical symptoms in

a range of human mental problems. In addition, animal models of impaired

cerebral function will be used to evaluate technological improvements in

established treatment modalities such as deep electrical stimulation of the

sub-thalamic nuclei as used in patients with Parkinson’s disease.

In this respect, the spectacular advances in nanotechnology and materi-

als science mean that we may be able, in the near future, to construct minia-

ture stimulators and generate interfaces to compensate for functions lost as

a result of neurodegenerative processes. The implementation of such treat-

ment modalities will obviously necessitate preliminary testing in small ani-

mals and primates.

Finally, in years to come, it will be important to capitalize on any progress

made in the field of stem cell research—both adult and embryonic stem

cells. Again, rigorous testing in animal models will be necessary to establish

that a candidate product is both effective and safe before it can be used to

treat neurodegenerative disease in human subjects. This Focus The Brain is

intended to point out the strengths of CNRS in brain research, and inform the

reader about the resources dedicated to this field of research with a view to

extending our knowledge and, insofar as possible, meeting social needs.

8 Focus The brain

Seeing the Brain at Work

Context and Issues

One of the major issues in the fundamental and clinical neuro-

sciences is finding ways to localize cerebral activities accurately in the

living human brain, and follow them over time. Such dynamic imaging

of how neural networks function means using non-invasive, trauma-

free methods with high resolution in both space and time. Current tech-

nology can localize regions of the brain of a few cubic millimeters in vol-

ume over a time frame of the order of one millisecond.

Broadly speaking, there are two types of method for studying how

the living human brain works: metabolic and biochemical imaging on

the one hand and, on the other, electroencephalographic and electro-

magnetic recordings. Using these different imaging techniques for spe-

cific functional investigations, we can monitor which parts of the brain

are activated in the course of sensory and motor functions or complex

cognitive tasks in both normal subjects and patients. These methods

produce distinct and complementary pictures.

Metabolic and biochemical methods look at local changes in blood

flow in situ. After some delay, a stimulus induces increased blood flow

in those regions of the brain which become activated. Electromagnetic

methods record the electrical and magnetic activities induced by stim-

ulation in real time—with no delay but at some distance from the active

sites.

To investigate brain function, it is necessary to be equipped with a

functional and anatomical imaging system coupled with an electroen-

cephalographic acquisition system and a mixed system including a

magnetoencephalograph (MEG) to make it possible, in the course of

experiments, to record respectively the position of the activated sites

and the duration of activation in various parts of the brain. Only ten such

centers exist in Europe (compared to about fifteen in North America)

with none in France. Acquiring such instruments is essential for devel-

oping an ability to “see the brain at work” which is why CNRS includes

this type of investment on its list of priorities.

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Brain Imaging

Brain research today is reaping the benefits of twenty years ofprogress in brain imaging, with anatomical and functional imag-ing providing insight into brain structure as well as into the bio-chemical reactions which occur in the brain at work. Above andbeyond applications in the cognitive sciences, these methods canbe applied to the diagnosis and treatment of disease.

The experience of several CNRS research units has already been translatedinto medical practice, highlighting the diagnostic and prognostic power ofintegrative, non-invasive investigations of the central nervous system usingmodern imaging techniques, e.g. MRS metabolic imaging can be used todiagnose various different types of brain tumor, to predict the likelihood ofrecovery following stroke, and to identify epileptogenic foci. MRI and PEThave become essential tools in the characterization of a number of neuro-logical conditions, including Alzheimer's disease, epilepsy and braintumors. In pharmacology, evaluating the efficacy of candidate neurophar-macologically active substances is increasingly dependent on the monitor-ing of specific markers using integrated brain imaging techniques.

The New Brain Imaging Techniques

Medical and Pharmacological Applications

Since 2001, CNRS has played a leading role in Europe in the field of imag-ing techniques in small animals. In the context of a multidisciplinary pro-gram including CEA and INSERM, CNRS has been promoting the develop-ment of ten resource centers specializing in imaging in murine models ofimportant human diseases. High-resolution (micrometer) MRI/MRS and,more recently, micro-PET techniques have revealed subtle changes inbrain structure and metabolism in transgenic mice whose genomes havebeen manipulated in order to mimic abnormalities observed in the humanforms of certain neurological disorders.

Animal Models

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Three-dimensional image of sectionedcerebral tissue generated by MRI.

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Angiogram of the murine braingenerated using NMR microimag-ing (11.75 teslas).

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Magnetic resonance imaging machinededicated to research—in physics as wellas medical applications.

Seeing the Brain at Work

The anatomy of the brain is mainly investigated using magnetic resonanceimaging (MRI) and CT-scanning. These techniques, which can yield three-dimensional images, are complemented by the analysis of electrical andmagnetic signals in the brain (electroencephalography [EEG] and magne-toencephalography [MEG]). The biochemistry of the normal and diseasedbrain can be investigated directly by magnetic resonance spectrometry(MRS) and functional MRI (fMRI). Chemical reactions can also be charac-terized by injecting radioactive tracers and following their movements andconversions in the brain using positron emission tomography (PET).Increasingly, these techniques are being exploited in research hospitalswhere CNRS, together with other research and public-sector institutions,is providing support as part of a policy to develop a national resource net-work. At many centers, CNRS resources are being put at the disposal ofmultidisciplinary teams which bring together physicists, biochemists,geneticists, physiology experts, physicians and computer scientists.

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Combining Approaches

Our understanding of the neurological basis of mental function is currently expanding rapidly becauseof new imaging techniques which can generate maps of various parameters of brain activity. Recentdevelopments in equipment and image processing technology should, in the very near future, makemultimodal imaging possible, i.e. multidimensional mapping integrating both structural and function-al parameters at different levels of cerebral organization.

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Cerebral slices acquired using high-resolutionmicro-PET in rats after the injection of an[F18]-labeled tracer molecule.

MRI (gray) and PET (yellow-green) imagessuperimposed on an EEG map (red andyellow circles).

Micro-PET imaging which generatesfunctional images of the brains of smallanimals.

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While MRI generates both anatomical (aMRI) and functional (fMRI) brainmaps at a high resolution (of the order of one millimeter), blood flow limitsits temporal resolution to a few seconds. Similarly, positron emissiontomography (PET) can follow chemical reactions and neurotransmissionphenomena at a centimeter scale but its temporal resolution does notexceed a few minutes because it depends on the use of radiolabeled tracers.In contrast, the temporal resolution of magneto- and electroencephalogra-phy (MEG, EEG) is compatible with synaptic events (which occur at the mil-lisecond level) although their spatial resolution is relatively low. Integratingimages generated using these various different but complementary meth-ods is therefore indispensable although such integration represents a majormethodological challenge.

Complementary Brain Imaging Techniques

Although very complex, combining hemodynamic and electromagneticimages should make it possible on the one hand, to define the positions ofdifferent neuronal activities in cognitive networks, and on the other, to fol-low temporal changes. This empirical approach adopted at a number ofCNRS research units involves making sequential recordings—first fMRI andthen EEG or MEG—in a given subject performing the same cognitive task.These recordings are then superimposed on one another. An even morepromising (but more difficult) approach is being investigated by a CNRSteam in Marseilles: this consists of making simultaneous EEG/fMRI record-ings. Combining images generated in different subjects enhances the sta-tistical power of detecting cerebral events. This type of approach is indis-pensable when it comes to determining the general rules which govern thebrain's anatomical and functional organization. The substantial topologicalvariability observed from subject to subject in the human brain means thata reference neuroanatomical map will have to be defined, and a reliabledeformation method will have to be developed to compare images from dif-ferent individuals with this frame of reference.

Image Fusion

Understanding the mechanisms underlying cognitive processes means inte-grating information acquired at different levels of brain function. This goal isbeing pursued in international programs, including the InternationalConsortium for Human Brain Mapping (ICBM) in which CNRS is a key play-er. ICBM is collecting physiological and psychometric data and correlatingthem with anatomical and functional images to construct databases corre-sponding to the healthy state and various neurological diseases.

Towards a Human Brain Program

Seeing the Brain at Work

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Another important line of research is based on exploiting differences with-in and between different individuals to gain insight into the fine, subtlemodulations of brain function associated with attention, motivation andlearning. A major research focus involves integrating information generat-ed using disparate methods in order to make the most of, on the one hand,the excellent spatial resolution of fMRI and, on the other hand, the tempo-ral precision of EEG or MEG. MRI affords complementary information onthe anatomical details of cerebral structures (anatomical MRI), the whitematter (diffusion MRI), and the neuronal activities involved in performingbehavioral tasks (functional MRI). Electrophysiological methods (elec-troencephalography and magnetoencephalography) yield high-resolutioninformation about the dynamics of cerebral activity. Success in this enter-prise is going to depend on a multidisciplinary approach: these days, closecooperation between experts in different fields is essential, e.g. in experi-mental cognitive neuroscience, theoretical modeling and image analysis.

Assessing Differences Between Individuals

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Surface map on which the cortical sulci of30 subjects are shown after normalizationusing the Talairach frame of reference.

Seeing the Brain at Work

Analyzing Images of the Brain

Interpreting the data generated by cerebral imaging methodsmeans applying ways of processing signals to correct measure-ment artifacts and take into account variation between individu-als. With such methods, the information acquired using imagingtechniques can be assimilated into theoretical models describ-ing cerebral structures, how they work and how they interactwith one another. Great advances have been made in imagingtechnologies in recent years, to the point that these techniquesnow afford unique ways of studying the human brain.

An essential prerequisite in cerebral image analysis is setting up anatomi-cal and functional databases based on the findings of researchers in differ-ent countries. Thus, it is important to define a universal frame of referencewhich takes into account the cerebral anatomy of each individual, while alsoallowing direct comparison between different sets of imaging data. In con-sequence, harmonizing analytical approaches is crucial. The ultimate goalis to help neuroscientists and neurologists perform meta-analyses to eluci-date the neuronal mechanisms that regulate how the human brain works.These analyses concern the neuronal pathways involved in the processes ofperception (olfaction, vision and proprioception) and all types of behavior,notably learning, memory, planning and the origin of thought.

Global surface map based on the individualsulci of one hemisphere generated fromanatomical sulci.

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Computer Software

Better Understanding Means Better Treatment

Context and Issues

Despite undeniable recent advances, the neurologist's therapeutic

arsenal is still limited. Whatever the kind of disease under consider-

ation—mental, acquired, genetic or developmental—understanding is

an essential prerequisite to treatment. Research in this field mainly

depends on experimental approaches aimed at probing pathogenic

processes and ascertaining the mechanisms that underlie the clinical

expression of the specific disease in question.

The investigation of any disease will depend on a multidisciplinary

approach. Full understanding usually means combining diverse

approaches at different levels of analysis, from the simplest to the most

integrated. Comparison must always be made between the pathophys-

iology in patients and the physiology in normal subjects which is why

experimental models of neurological and psychological disorders are so

important.

In the neurosciences, three broad types of model are used. Cellular

and molecular models—usually studied in vitro—yield information

about the various mechanisms involved in pathogenesis, e.g. a great

deal of information has been obtained in this way about neuron death in

degenerative neurological diseases. Other models based on knocking

out or over-expressing specific genes have shed a great deal of light,

although these methods have their own limitations. This type of

approach cannot be dissociated from genetic analysis and proteomics,

the future basis of gene and cell therapies, or even diagnostic tests.

Integrative physiology, experimental neurology (in primates) and neu-

ropharmacology are all other important approaches in both the investi-

gation of disease and the development of novel, innovative therapeutic

strategies. Recent evidence of the success of such approaches comes

from cell therapy in Huntington's chorea, and the efficacy of deep, sus-

tained, high-frequency stimulation in the treatment of Parkinson's dis-

ease, dystonia and obsessive-compulsive disorder.

These research strategies are often combined with modeling and

simulation (computational neuroscience) which extend conceptualiza-

tion—in perfect synergy with functional brain imaging—and help visual-

ize the living brain. Thus, the aims of neuroscientists and clinicians

come together: better understanding means better treatment.

12 Focus The brain

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Aging and the Nervous System

In a society in which life expectancy has been rapidly rising fordecades, the incidence of a range of neurodegenerative diseasesis, in parallel, on the increase, notably Alzheimer's disease. Inthis context, understanding the mechanisms involved in the deathof nerve cells and developing treatments capable of blocking suchprocesses represent major challenges in today's neurosciences.

Research at CNRS and other institutions is showing that nerve cell degen-eration is the result of diverse mechanisms which all lead inexorably to thedeath of cells. Apoptosis, inflammation, cerebral edema, stroke-relatedhypoxia, and neuroglial scarring are all cell-based phenomena which leadto neuron degeneration. These mechanisms are investigated at the cellularand molecular levels in neurons or glial cells, as well as at the higher lev-els of neural networks or behavior—the latter largely in murine geneticmodels based on spontaneous mutations or transgenic animals. These ani-mals can be used for in-depth post-genomic analysis with a view to definingthe functions of genes that have been either knocked-out or which are beingover-expressed.

Formation of a double-membraned,autophagous vacuole or autophagosome(arrow) in the cytoplasm of a Purkinje cellfrom the cerebellum of a mutant mouse(Grid2Lc/Grid2ho).

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The death of neurons may play a centralrole in neurodegenerative diseases likeAlzheimer's disease.

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Although the genetic and environmental factors associated with cell deathhave not all been identified, recent progress has led to the characterizationof the majority of genes involved in normal and pathological aging process-es. These genes have recently been described (together with some of theintracellular mechanisms leading to apoptosis) in a worm Cænorhabditiselegans and homologous genes have been detected in mice and humans.Very recently, a CNRS team discovered that the RORα protein—a neuronaltranscription factor—plays an important role in survival and differentiationmechanisms. Other transcription factors such as CREB and Elk– have beenidentified: intracellular activation of these factors seems to be involved incellular aging processes.

Specific Genes are Associated With Neuron Death

BetterUnderstandingMeans BetterTreatment

Murine models of neurodegenerative diseases, such as Huntington's dis-ease, have been used to show that neuron death is associated with extraglutamine residues in the huntingtin protein. Recently identified intracellu-lar metabolic pathways probably trigger a pro-apoptotic pathway: under-standing such mechanisms is making it possible to contemplate using spe-cific compounds to block these pathways, not only those involved in neurondeath but others which lead to the disappearance of synapses between cellsand the loss of the associated functions.

New Tools Against Neurodegenerative Disease

Mechanisms Underlying Degenerative Processes in Neurons

Micrograph of dissociated rat fetal cellsfrom the brain stem (after 12 days of cul-ture). Cultured neurons can be used to testcandidate compounds for activity affectingcell survival.

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Amyotrophic Lateral Sclerosis and Multiple Sclerosis

Amyotrophic lateral sclerosis (ALS; also referred to as Lou Gehrig’s disease) is a disease in whichmotor neurons degenerate, both in the spinal cord (responsible for muscular contraction) and themotor cortex (pyramidal neurons which control voluntary movement). The disease is fatal becauseeventually the phrenic motor neurons which contract the diaphragm (and are therefore essential inbreathing) die off. Understanding the mechanisms which lead to degeneration is an essential steptowards developing new, effective treatments.

Therapeutic Trials in the Pipeline

Research in this field (which is conducted by several CNRS research unitsand other groups) has been stimulated in the last ten years by greatadvances, notably the identification of a specific mutation in the gene encod-ing superoxide dismutase (SOD). Transgenic mice carrying this mutationhave been created, allowing elucidation of the mechanisms involved in thedisease. Now that almost all the mechanisms which underlie motor neurondegeneration are well understood, therapeutic trials can be contemplated.The mutation leads to abnormal expression of the protein which in turn leadsto motor neuron degeneration. Recent experiments have shown that the dis-ease only occurs if the gene is mutated in astrocytes as well as in the motorneurons. Experimental induction of the mutation in the target cells of motorneurons seems to be sufficient to trigger the disease.

Motor neurons of the rat spinal cord culturedin the presence of embryonic myoblasts.

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Spinal motor neurons from a rat in organ-typetissue culture. Labeling: acetylcholineesterase.

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Treatment Strategies Based on Multiple, Specific Molecules

Various cellular mechanisms have been suspected in the pathogenesis ofALS and each could point to different, new therapeutic avenues. A hypothe-sis based on oxidative stress modifying intracellular oxygen metabolismleading to increased nitrite oxide production is now considered as a margin-al possibility. In SOD transgenic mice which express the disease, bulky inclu-sions containing aggregates of the SOD protein accumulate in motor neuronsand seem to actively contribute to the death of the cell. Analysis of theseaggregates and study of the mechanisms associated with their formationcould lead to the development of compounds to inhibit their accumulation.Another hypothesis proposes that glutamate (an excitatory neuromediator)binds receptors on motor neurons, inducing a toxic influx of calcium. Thishypothesis has excited great interest for many years although the beneficialeffect of Riluzole in ALS has not been shown to be dependent on the modu-lation of glutamate metabolism. Further experiments are underway to definethis compound's mode of action. Current research, both in vitro and in vivo,is focusing on calcium-based mechanisms located in the mitochondria whichmay be among those responsible for cell death. All this research could leadto the development of pharmacological treatments based on combinations ofspecific compounds.

Better Understanding Means Better Treatment

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Depression and Schizophrenia

Several CNRS teams are using modern research tools based onfunctional brain imaging to understand the neuropathologicalbasis of mental disease. These techniques make it possible toaccurately locate the neuronal circuits involved in cognitive,emotional and motor processes in vivo. Current research isfocusing on mapping those parts of the brain which are involvedin the kinds of impaired behavioral inhibition that are seen inschizophrenia and bipolar disorder, both serious diseases whichare poorly understood in neurobiological terms.

Self-control plays a key role in governing our actions and is one of the cog-nitive processes which is more highly developed in human beings than inany other phylogenic entity. Some deficit in inhibitory mechanisms is themost common source of the kind of impulsive behavior that is so frequent-ly encountered in psychiatric pathology. Impulsive acts are not adequatelythought out beforehand, often entail risk, are inappropriate (to the physicalor social context) and are almost always prejudicial to either the patient orthird parties. The technique of fMRI can be used to identify those parts ofthe brain that are activated in the course of the execution of a task whichcalls on cerebral self control mechanisms, and thus this method can beused to compare the relevant structures in patients and healthy subjects.Cerebral activity is measured in various conditions of impulsiveness (differ-ent types and variable intensities) as evaluated using special psychometricinstruments. In this way, it is possible to define a relationship between acertain behavior pattern (impulsiveness) and a certain network of corticalstructures which are activated when a subject's capacities for self controlare solicited.

Self-Control and Cerebral Activity: From Observation …

In bipolar patients in the maniac phase who are performing a task whichwould normally elicit an inhibition response, there is considerably less acti-vation in the cortical regions than in normal subjects. Hypoactivation isparticularly marked in the prefrontal dorsolateral cortex which is known tobe involved in the decoding of instructions, and in the orbitofrontal cortexwhich is involved in decision making. These findings suggest that, althoughthese patients use the same or similar structures as healthy subjects toinhibit responses, they do so less efficiently. In schizophrenic patients, itseems that the anterior cingulate cortex is affected as well as the pre-frontal dorsolateral cortex. It is well known that impulsiveness in youngadults is predictive of subsequent psychopathy and is also, together with aneed for novel sensations, a character trait that can lead to substanceabuse. We hope one day to be able to produce maps depicting patterns ofcerebral activation which can predict susceptibility to psychopathic dis-ease. Current research could also lead to the development of more effec-tive psychotropic drugs, thereby leading to better social reintegration ofpatients.

… to Localization of the Parts of the Brain Involved

Cerebral activation patterns (fMRI image)during the performance of a task elicitinginhibitory responses.Top: healthy subjects. Bottom: bipolar patients.

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BetterUnderstandingMeans BetterTreatment

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Understanding Neurological Diseases Using TransgenicAnimals

Today's biomedical research priorities include identifying the genes responsible for neurological dis-eases and characterizing their products. Understanding the factors which regulate gene expressionin healthy subjects, as well as how regulatory mechanisms are perturbed in certain diseases whichaffect the nervous system, is indispensable if we are to develop more effective treatments; suchunderstanding may also eventually help develop gene therapy modalities based on replacing a defec-tive gene or down-regulating some gene that is being over-expressed.

Tiny Worms for Curing Neurodegenerative Diseases

Progress over the last few decades in genetics and molecular biology hasmeant that it is now possible to manipulate an animal's genome and inducethe expression of different phenotypes. Experiments based on these tech-niques have identified genes which play a major role in the pathogenesis ofvarious diseases. In this context, animals such as the fruit fly (Drosophila)and the worm Cænorhabditis elegans together with experimentally generat-ed mutants thereof have been used to identify families of genes involvedrespectively in the morphogenesis of the nervous system and the mecha-nisms controlling neuronal apoptosis. Homologs of all the Cænorhabditiselegans genes involved in neuron death have recently been found in rodentsand humans, a discovery which earned a 2004 Nobel Prize and which isopening promising avenues towards understanding and eventually curingcertain neurodegenerative diseases which represent major public healthproblems.

The fruit fly Drosophila andalousiaca.

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The worm Cænorhabditis elegans.

These two animals together with mutantsthereof are widely used in the investigation ofdiseases of the nervous system.

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Towards a Post-Genomic Science of Physiology

The ability these days to generate transgenic mice in which some specificgene has been knocked out at an early stage of development has made itpossible to define the morphological and functional characteristics of theresultant brain phenotypes in the developing animal and adult. The recent-ly developed capacity to knock genes out in an inducible fashion means thatthe consequences of de novo invalidation in adult animals can now beinvestigated. Coupled with the methods of proteomics which contributequalitative and quantitative information about the proteins being produced,transgenic animals represent an extraordinary tool for investigating notonly the effects of genetics on function in vivo but also the impact of envi-ronmental factors. These technologies have opened a new era of investiga-tion in the life sciences and in the understanding of human neurologicaldisease. Advances in this field are now making it imperative that we devel-op a post-genomic approach to physiology in order to be able to evaluatefunctional changes in transgenic animals. This approach will have to becombined with the methods of cognitive neuroscience with a view toassessing performance in whole animals. Setting up such a multidiscipli-nary strategy represents one of the CNRS' main tactics in the endeavor toexpand our understanding of diseases of the nervous system.

Better Understanding Means Better Treatment

Neurochemistry and Neuropharmacology

Novel findings from, on the one hand, molecular and cellularneurobiology and, on the other, functional investigations in inte-grated model systems, are pointing to new candidate targets forpharmacological agents and other therapeutic modalities.Above and beyond potential medical applications, such sub-stances are proving to be valuable tools in the elucidation of hownervous system function is organized, and in dissecting under-lying molecular mechanisms.

The proteomics boom has resulted in more precise molecular phenotyping.It has made it possible to define the functional specificity of key receptorsand transporters as well as of proteins involved in signal transduction andproteins which directly interact with genome-regulating factors.Automated molecular screening techniques mean that huge numbers ofcompounds can be tested on cultured cells in a reasonable time frame(although specific testing in living animals ultimately remains indispensable).

Ever-More Efficient Drug Testing

The fields of neurochemistry and neuropharmacology are benefiting frominformation obtained in transgenic animals and studies of genetic polymor-phism. Consanguineous or transgenic lineages which mimic specific dis-eases can be generated and CNRS teams are studying models like these toprobe, at the various levels of neuronal organization, the neurochemicalpathways which are specifically involved in the disease in question. At thesame time, scientists are attempting to find ways of improving the deliveryof pharmacological agents to their sites of action. In the future, the successof such attempts is going to depend on computer simulations which repro-duce the organizational and adaptive complexity of the central nervous sys-tem. In parallel, developments in the study of behavior and predictive neu-ropharmacological testing may enhance our understanding of howresponses may vary from subject to subject, and may yield important infor-mation about the impact of genetic and epigenetic factors on the activity,metabolism and distribution of synthetic compounds in the body. The daymay not be far off when personal factors are taken into account whendeciding which drug to prescribe and at what dosage.

Neurochemistry and Neuropharmacology "to Order"

Electron micrograph of a synapse in the central nervous system: the synapse represents a special target for pharmacological agents.

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In the brain, it is at synapses where information is transferred from oneneuron to another. The way the synapse works is based on cascades of bio-chemical events and in consequence, this site represents a special targetfor pharmacological agents. Studying the mechanisms which governsynaptic function is still a major research focus for neurochemists andneuropharmacologists. Studies in this field—be they in in vitro experimentsin cultured cells or studies conducted in vivo—will lead to improved phar-macological practice.

The Synapse: Highly Plastic and Constantly Turning Over

Positron emission tomography (PET). A drug interacting at benzodiazepine-specificbinding sites in vivo. Top: benzodiazepine receptors. Bottom left: competition between abenzodiazepine receptor-specific markerand the drug (at a low concentration). Bottom right: the drug at a high concentration.

Control

Triazolam0.01 mg/kg

Triazolam0.5 mg/kg

Context and Issues

Behavior is the most significant level of physiological integration, the

ultimate interface between the organism and its environment. The sur-

vival of species depends on adapted behavior and, for humans in partic-

ular, their special mastery of their environment as well. The senses,

movement, memory and communication systems constitute the areas of

analysis which allow the study of development, learning, aging, etc., as

well as of related diseases. Behavioral science thus sheds light on the

epigenetic dimension. The main foundation stone of this great flexibility

is the nervous system. This has a past—phylogenic of course but one in

which personal experience and the social environment are intimately

interwoven. The explosion of analyses of movement and the major

expansion in modeling have led to ever-increasing amounts of data of

ever-increasing accuracy and thus to many different applications,

notably in the fields of health, education and work.

Behavioral neuroscience constitutes a particularly profitable

approach to studying how the nervous system works. Combining

behavioral testing with non-invasive brain investigation modalities

(notably imaging) has deepened our understanding of important links

between structure and function, e.g. we are gaining insight into tempo-

ral dynamics in the primary areas (visual, auditory, motor, etc.) through

experimental situations proposed by behavioral science. The same is

true for genetics (transgenic notably) and behavior in which a genuine

physiological context is emerging. Although behavior and its analytical

methods are the highest level of integration for the nervous system, they

are the finest grain in other disciplines and the same is true for the

interface between ethology and ecology (e.g. individual behavior and

population biology) and in the social sciences (as illustrated by micro-

economics).

Analyzing Behavior

18 Focus The brain

Cognitive Development

What does a newborn’s brain need to develop human capacities?Understanding the mechanisms which run the genetic pro-grams dictating how the brain is constructed, defining the gen-eral rules governing how nervous information is integrated, anddecoding how they interact with the environment all constitutemajor objectives in this field.

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The discovery of the mechanisms underlying neurocognitive development ishaving substantial impact on our ideas about education and teaching. As ofthe end of the fetal period, cortical networks begin to organize according tointrinsic rules under the influence of environmental stimulation. In certainconditions, we can be witness to complete disappearance of the early effectsof the environment. The updating of mechanisms and rules of cerebral plas-ticity also make it possible to design and modify prostheses, and perfectrehabilitation programs. Research on gifted children and the factors whichdetermine exceptional talents—like those pertaining to learning difficul-ties—reveal the mechanisms of cognitive development. CNRS teams areprobing the mechanisms of development and learning of cognitive skillswhich are impaired in certain children, and the mechanisms through whichdefects in brain development (either hereditary or due to trauma) lead tospecific types of learning difficulties.

Neurocognitive Development and Society

When there are brain lesions with loss of substance, research focuses onthe causes of such lesions, on compounds which can arrest the loss of sub-stance, and on the effects of such treatment on the development of cogni-tive function in the treated region. In invasive developmental problemssuch as those observed in autism or schizophrenia, a number of teams arecollaborating to investigate the underlying abnormalities in cerebral andmental development. Abnormal interactions between two cerebral struc-tures are currently being investigated by CNRS and Inserm groups as anearly cause of schizophrenia, a disease which onsets at a later stage(towards fifteen years of age). In other diseases like autism, the investiga-tion of very elementary mechanisms of information assimilation by thebrain is revealing the existence of abnormal visual perception patterns (e.g.in very young autistic children). These abnormalities are providing a way toprobe cellular abnormalities in brain development.

Impaired Neurocognitive Development

Many of the questions being asked by cognitive scientists are the same asthose being asked by engineers designing adaptive robots: what propertiesdoes such a system need to evolve and adapt to such and such an environ-ment? For example, no machine is as effective at recognizing faces as anaverage human being. Solutions to the adaptive problems posed by natureare already inspiring machine designers.

Neurocognitive Development and Adaptive Robots

If a smooth object (A) or a rough object (B) is placed in the hand of a new-born baby and then one of the objects is shown to the baby, he will look for longer at the newobject (be it A or B) which he did not touchbeforehand. This suggests that at this age,the brain registers certain physical propertieswhatever the input route.

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Analyzing Behavior

Perception: Vision

Vision has a special place among our senses and its study has understandably excited enormous, sus-tained interest and cross-disciplinary approaches. Today in France, especially at CNRS, vision is beingresearched by a number of multidisciplinary groups spread throughout the country. Three main lines ofresearch are being pursued.

The Neurophysiology and Neurobiology of Vision

The neurons of the retina and the neural networks of the visual cortex arethe key components of cerebral visual circuits. It is now generally acceptedthat the retina represents an external projection of the brain into the sur-rounding environment. At CNRS many years of experiments in cats andmonkeys have shed light on how visual information is registered andprocessed by the retina and cortex, both in adult animals and in the courseof development. The visual cortex constitutes a superb machine for identi-fying and positioning objects in the world, and guiding actions.Understanding the architecture of the visual cortex through the methods ofneurophysiology has been advanced by neuromimetic simulations (in whichCNRS researchers are particularly skilled). The development of functionalimaging methods in humans has made it possible to capitalize on the hugeprogress made by neurophysiologists in the twentieth century through ani-mal experimentation.

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From the Psychophysics of Vision to Active Vision

Thanks to information theory, communications theory and systems theoryas well as more recent theory on the coupling of perception and action, it isnow accepted that seeing is a form of behavior in and of itself and so, thesedays, we refer to active vision. Psychophysics and the analysis of motorbehavior are the basic tools which use eye movements as the most relevantmarker for the elementary processes of vision. Perceiving and acting arethus considered as two inseparable sides of the same behavioral coin. Inthis context, it becomes clear why studying the influence of visual percep-tion on the generation and execution of movement is key. A virtual realitymovement analysis resource unit supported by CNRS has been set up inorder to probe all the specific parameters of movement in environmentalsituations with the subject in a three-dimensional space and being confront-ed by all sensory modalities.

Vision of Movement and Robotics

Finally, the visual perception of movement and the role of vision in control-ling movement are exciting great interest. This research, which is helpingus understand how we perceive the world, has mainly been spurred by thewish to improve prostheses and other corrective modalities which mightimprove the lives of the blind. Rather than studying vision itself, thismeans working out how we establish, regulate and adapt complex behav-ior patterns, in harmony with input from other sensory systems such asproprioception, the vestibular system and hearing. A deeper understand-ing of these mechanisms has led a number of CNRS teams to envisageartificial structures capable of detecting movement, recognize objects ordirect moving machines. An approach which reinforces the interfacebetween neuroscience and robotics …

Vision system for acquiring and monitoringinformation from the face and eyes of a usersitting in front of the screen, in order to determine which way he or she is looking.

Subject participating in an experiment in front of an oculometer. This apparatus is able to record the eye movements of someone reading a page on a screen.

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Miniature aerial OCTAVE robot (100 g) with a visual system which allows it to followthe relief of the terrain and land automatically.

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Smelling and Hearing

In humans, the systems responsible for emitting and receivingsound (including language) and those which mediate odor per-ception are the most important when it comes to communicationbetween individuals. Both these sets of systems are essential incultural development.

A pure sound consists of a single frequency whereas a musical sound iscomposed of a fundamental frequency plus a series of harmonic noteswhich are multiples of the fundamental frequency. Different frequencies areperceived as notes with different pitch and the brain localizes sounds inspace by comparing the information received by both ears. Local changes inair pressure cause displacement of the tympanic membrane and a series oftiny bones in the middle ear. Displacement of the fenestra ovale causes thefluid inside the inner ear to move which in turn mobilizes the ciliated cellsof the organ of Corti. Here, pressure changes are converted into movementsof the cilia and encoded as a sensory message. Recent work in this field hasfocused on the cellular and molecular mechanisms involved in translationof a mechanical signal into an electrical message. At the level of the ciliat-ed cells of the organ of Corti, calcium and potassium channels have beenshown to be important and the external and internal cells have beenassigned distinct functions. Associated with research focusing on the cen-tral pathways involved in the sensory aspects of hearing, the electrophysio-logical properties specific to the first level of auditory information (in thecochlear nucleus) have been elucidated. All this new information has con-tributed to the development of today’s more effective cochlear implants forthe hard of hearing.

Hearing

Monitoring calcium in vivo in an insect’sbrain. Areas of activity (in response toolfactory stimuli) seen in a bee’s brain.Fluorescence microscopy. The brain of alive bee is impregnated with a calcium-binding fluorescent probe so that nervousactivity can be monitored in real time,observing changes in intracellular calciumconcentrations triggered by olfactorystimuli.

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Olfaction is one of the primordial senses—odoror ergo sum. Smelling begins with the newborn’s very first breath. Olfactory pathways in the brainare in fact the first to become established in the course of ontogenesis andprovide the baby with its first sensation upong meeting the world. All thespecies’ fundamental behavior patterns, which become established subse-quently during development, are intimately linked with olfactory signals.The olfactory organ—far from being peripheral among the sensory organs—is essential for the survival of an animal. Discriminating between andselecting foodstuffs, detecting toxic substances and rotten food, attractingand recognizing potential sexual partners, establishing parental and sociallinks—common traits in the animal world which are still active in humans—all depend on special, complex neuronal organization. It is assumed thatsensory cells of the olfactory mucosa (the odor perception organ) located inthe nasal cavity are capable of projecting on specialized areas of the brain.Amazingly, these cerebral structures are able to renew themselves and aredirectly linked with emotional and memory processes. A deeper under-standing of the characteristics of olfaction would therefore not only enhanceour knowledge of the neurological basis of this unloved sense—which ismore important than its reputation would suggest—but would also open thedoor to the world of the emotions which subconsciously govern our instinctsand memories.

Smelling

Auditory nerve, sectioned near thevestibule (left), resulting in retractionwhile the cochlear portion (right) remainsintact. At the bottom, the cerebellum; atthe top, the entry of the internal auditorynerve. This operation, which is performedin patients suffering from debilitatingvertigo, has been used to show that theolivo-cochlear track is important in thefiltration of audible frequencies.

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Plasticity of Cerebral Representations of the Body

For the last two decades, evidence about the plasticity of the adult mammalian brain has been con-tradicting the dogma that the central nervous system is rigidly wired. This novel concept that thebrain is a relatively plastic entity arose from studies on cortical topological representations corre-sponding on a point-to-point basis to skin surfaces or muscles.

Immobilization of the front limb (using a plas-ter cast) of a rat for one week results in elimi-nation (white area) of the region of the corti-cal skin sensitivity map corresponding to thislimb.

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Neuromaps of the cortex display global organization features that are com-parable from one subject to another. However, the structural and function-al plasticity of synaptic connections turn such maps into neural imprints ofindividual sensory experience. In practice, the number and strength ofthese connections depend on how often they are solicited so that neu-romaps are continuously reshaped. Synaptic connections are strengthenedor weakened and the neural network architecture is modified accordingly,depending on changes in the patterns of stimuli generated by the individual’sactivities, or injury to the neural system. This property of neuroplasticityplays a key role in learning and behavior adaptation as well as in functionalrecovery after neural damage. Typical examples include professionalviolinists and Braille-reading blind people who have particularly highlydeveloped representations of their fingers.

Imprinting of Sensory Experience: Neuromaps

One team of CNRS researchers is interested in the modulatory activity ofthe cholinergic and noradrenergic systems (linked to attention, motivationand vigilance) on the activity and plasticity of neural circuits of thesomatosensory cortex during learning. Other researchers are studying howsomatosensory neuromaps change in response to modifications of envi-ronmental conditions and subjects’ experience related to tactile discrimi-nation tasks or breast feeding. These scientists are also interested in mapremodeling in the course of the recovery of function following stroke, andthe neuroprotective activity of drugs used to prevent or minimize theeffects of brain ischemia. Using fMRI, one CNRS team is collaborating withclinicians to study the reorganization of somatosensory and motor areas inamputees who have had bilateral hand allografts.

Fundamental Processes Underlying Plasticity

CNRS is actively involved (often in collaboration with other organizations) inapplication of the wealth of knowledge from basic research in the field of theplasticity of cerebral neuromaps. For example, in order to enhance surgicalabilities, researchers are working on a lingual electrostimulation guidancesystem which converts the images acquired by a camera into electrotactilesignals delivered to the tongue. A device based on such a system designedto prevent bedsores in the physically disabled was patented recently. Tactileinterfaces have been developed to help the blind to read, use the Internetand get a better sense of space. Research into cerebral plasticity and learn-ing is also contributing to the development of interfaces between humanbeings and machines or robots.

Fields of Application

Effects of nursing behavior on the neuromapof the ventrum skin in the somatosensorycortex of a lactating rat. Note the expansion ofthe neuromap and the decrease in the size ofthe receptive fields of cortical neurons result-ing in a finer grain representation of the ven-trum skin.

1. after 12 days of nursing 2. 1 month later

Integrating Vestibular Sensory Input

The inner ear houses the cochlea (the hearing organ) and thevestibule, the sensory cells of which detect the direction of thehead and its displacement in space. The vestibule functions as acenter of inertia which encodes the head’s angular and linearaccelerations. Recent advances in our understanding of the phys-iology and physiopathology of the vestibular system have poten-tial applications in both socioeconomics and health care.

Experimental apparatus to record the activity of single vestibular neurons in the brain stem of conscious rats duringlinear vestibular stimulation (possible in allthree spatial planes).

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Apparatus for measuring posturalregulation in humans consisting of a sliding platform to study displacements of the pressure center on the ground, avideonystagmographic mask for ocular movements, and a two-dimensional movement analysis system to determine the orientation of the body in space.

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Analyzing Behavior

In contrast to the other sensory systems, signals from the vestibule under-go central processing which begins at the first sensory relay. Vestibularinformation converges with other sensory signals at the vestibular nucleiand these heterogeneous signals are integrated to contribute to a three-dimensional reconstruction of the rate of displacement of the head inspace. Neuronal processing results in the sending of pre-motor messagesto the spinal medulla and the oculomotor system, and of pre-perceptivemessages to the cortex. These messages are important in regulating pos-ture, maintaining balance and directing the regard, as well as in represen-tation of the body and navigating through space. The development of tech-nical resource centers with natural vestibular stimulation systems andmodern equipment for anatomical, physiological and functional investiga-tions has resulted in the identification of areas of the parieto-temporal cor-tex in animals onto which information from the vestibule is projected. It hasbeen shown using fMRI that the human parieto-insular vestibular cortex isthe equivalent of the structures characterized in animals. Recent experi-ments have shown that this area, along with the frontal cortex, is involvedin the perception of displacement of the body in space and of verticality.

From Technical Resources …

Patients with a unilateral vestibular problem suffer serious dizziness, pos-tural and balance problems, and oculomotor and perception difficulties(spatial disorientation and poor verticality perception). Age-related bilater-al involvement is associated with profound ataxia which causes falls in theelderly. Applied research in the pharmacology of dizziness, vestibular reha-bilitation and the transfer of innovative technology are therefore respond-ing to major societal preoccupations. Experiments in animals have shownthat certain compounds lessen the sensation of dizziness and have alsorevealed their mechanism of action. In humans, the use of modern tools forthe analysis of movement and dynamic posturography together with newmathematical methods and improved signal processing software haveimproved diagnosis and rehabilitation in vestibular disease, and led to newways of preventing falls in the elderly.

… to Pharmacological Applications

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Both parietal and motor cortices are activewhen a subject is feeling that his hand is inmotion.

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Analyzing Behavior

Proprioception, the Sixth Sense

Understanding the neurological basis of the representation that everyone has of him or herself, espe-cially their own actions, is one of CNRS’ major areas of interest in the integrative and cognitive neuro-sciences. The proprioceptive system is a vast sensory organ which is constantly informing the brainabout the state of the body and its actions. If this system is impaired, the subject is deprived of all con-sciousness of his or her body as well as of any freedom to act. Progress in our understanding of thissense of action is opening important perspectives in the fields of learning, motor rehabilitation and vir-tual reality.

The body is never silent; it is constantly telling the brain about its state andall the various actions it is performing. The information underlying thismonitoring system arises from thousands of sensors located in the mus-cles, joints, tendons and skin. The subject is unaware of the intense neu-rosensory traffic which is the basis of the image that the brain constructs ofthe subject’s personal identity. This lack of awareness has been bypassed atone CNRS research unit by intercepting the relevant signals on their way tothe brain. Specific proprioceptive messages (collected using microelec-trodes inserted into superficial nerves) were identified for each of our vari-ous body postures and actions. These proprioceptive messages constitute atrue “neural bar-codes” for each of our actions.

Proprioception Tells the Brain About the Body and its Actions

The spectacular progress in our knowledge of how the brain works is inlarge part due to the recent development of functional cerebral imagingtechniques. These methods are concretizing the hope that it will one day bepossible to analyze the most sophisticated functions of the human brain ina scientific way. Recently, regions of the cerebral cortex that are at workwhen the subject becomes aware of an action were identified. Simpleawareness of an action leads to the activation of specific cortical network,not only those from which it emerges but also cerebral motor structuresresponsible for executing the action. Thus, perceiving or even simply imag-ining an action is equivalent—in a way—to performing it.

Where is the Consciousness of our Actions Located?

It is better to feed the brain with phantoms than to not feed it at all. Thishas been demonstrated by scientists who, using mechanical vibrations,generated the illusion that a limb in plaster was moving. Rehabilitationproved more or less superfluous if the brain was fed proprioceptive infor-mation while the limb was immobilized. Understanding how the humanproprioceptive system is organized has seen such progress that it is nowpossible to give a motionless subject the impression that he or she is draw-ing or writing—and he will even recognize lines that he mistakenly believeshe wrote himself. Thus, new perspectives are opening in the fields of edu-cation and therapy related to movement and language learning. Thus, thepossibilities of virtual reality—hitherto restricted to vision and touch—arebeing expanded to include movement.

Illusions of Movement are Betterfor the Brain than No Movement at all

Proprioceptive rehabilitation can maintainjoint mobility when a limb is immobilized.Illusory movement helps the brain to keep“an image of movement”.

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A recording made from a sensoryproprioceptive nerve fiber using amicroelectrode in a human being performinga writing task. Each letter generates its own proprioceptive message.

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New Approaches to Motility

Using reflexes and cortical evoked potentials, Sherrington and his stu-dents were able to define various nervous pathways as well as the sen-sory and motor centers. Since that time, many new theories—fosteredby dual advances in technology and understanding—about motor pro-gramming have led to diverse approaches to motility. At CNRS, integrat-ed analysis of motor behavior patterns and the neural networks thatgenerate them coupled with enhanced knowledge of the properties ofmembrane channels are helping us towards a more complete under-standing of the physiology of motility.

Studying Cell-Based Mechanisms in vitro

Neuron cultures, slices of nervous tissue and preparations of embryonicbrain and medullary tissue can now be analyzed directly by electrophysio-logical methods to discern the neuron’s intrinsic properties and investigatesynaptic relationships in neural networks generated in vitro. CNRS teamsanalyzing motor networks in this way are studying how motor systems areestablished in the course of development. In adults, discovery of the phe-nomena of long-term potentiation and depression is opening new perspec-tives related to the plasticity of motor systems.

Unitary Activities in Animals and Humans

Now that the electrical activity of single neurons can be measured in free-moving animals, it is possible to study how nervous information pertainingto movement is encoded. Motor behavior can be correlated to neuronalactivities in cortical and sub-cortical motor areas in rodents moving aroundin an enclosed space or in monkeys flexing their arms, and the temporaland spatial parameters of the action can be associated with one another.Multiple recordings have shown that, when performing a programmedtask, reaction times are significantly curtailed if the subject is expecting asignal to trigger the movement. In humans, analysis of deliberate wristmovements demonstrates coupling in the discharge of motor units.

From Posture to Action

In humans, motility is finalized and completely integrated into processes ofbehavioral adaptation to the environment. Motor behavior is studied atCNRS in two complementary ways which combine behavioral neuroscience,cognitive psychology and biomechanics: the first line of research focuses onthe processes underlying hand-head-eye coordination; and the second (inwhich the human body is considered as a series of articulated segments)focuses on the adaptive processes involved in posture, balance and locomo-tion. This is making it possible to establish rules linking form and, at thesame time, identifying how the processes which control posture and move-ment become established. Mental representations of actions accompanythis maturation.

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Experiment in microgravity: motor anticipation when catching a ball.

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New Frontiers in the Study of the RelationshipBetween Perception and Action

The brain acts like an active explorer of the world. To decide what action to undertake, it simultaneous-ly carries out multimodal perception and processing phenomena which configure the informationaccording to the subject’s intentions. CNRS has the interdisciplinary expertise necessary to under-standing the sensory and motor mechanisms involved in movement. Ultimately, enhanced understand-ing of the neural basis of movement will be of benefit in diagnosis and treatment, and may lead toimportant new industrial applications.

Newton’s Laws and Neuronal Processes

When someone is getting ready to catch a heavy object, the brain induces inadvance the exact amount of muscle contraction which will be necessary forthe hand not to move on impact. To anticipate the consequences of move-ment in this way, the brain uses internal models. The effects of gravity onthe body have been investigated on Earth and in space in a collaborativeEuropean program. The cerebellum and the basal ganglia are the mostextensively studied areas of the brain in this respect and the informationacquired is as relevant to sport, robotics and rehabilitation as neurology. AEuropean neurorobotics program in which CNRS is intimately involved isstimulating collaborations between neurophysiologists and roboticsexperts.

Controlling Locomotion: a Cognitive Mechanism?

Locomotion is organized on a hierarchical basis in which both automatic andcognitive mechanisms participate. The position of the walker’s head (stabi-lized by vestibular sensors) determines the locomotor path and this obeysthe same laws as those which control movement. Movements can bedetected by video cameras hooked up to computers to analyze the path.Computer specialists and mathematicians are working with neurophysiolo-gists on the geometry used by the brain to encode the space to generatesuch paths. Applications may be found in rehabilitation and robotics as wellas neurology.

Directing the Eyes, a Hierarchical System

Ocular movement is organized on the basis of a directory of sub-systemswhich stabilize the eyes (vestibulo-ocular and optokinetic reflexes), directthem and take care of accommodation. Some of these processes are auto-matic and involve sub-cortical structures such as the brain stem and thesuperior colliculus; others have more cognitive character and involve thefrontal and prefrontal cortexes as well as structures also associated withmemory. To help understand these processes, mathematical models of howthese neural networks might be working are being developed by CNRS.Direct traces of activity in the parts of the brain suspected of being involvedin controlling the direction of the eyes are being carried out in epilepticpatients undergoing preoperative examinations. These results might beexploited in ophthalmology or ear, nose and throat medicine, or even in thecontext of psychiatric disorders in which there are communication prob-lems, such as autism (a condition in which eye direction is a key parameter).

A trace of anticipatory arm muscle activity in the catching of a ball. Green: departure of the ball. Red: impact.

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Normal Language

Normal people speak and understand words with remarkablefacility and efficacy. However, accounting for the functional andstructural organization of language involves a complex analysis,as does understanding how the human brain is capable ofspeech and understanding language. Such an analysis is neces-sarily inter-disciplinary, involving linguistics, psycholinguisticsand neuroscience.

Mechanisms Involved in Understanding Language

Investigating the genetic, biochemical and even neuronal mechanismsinvolved in understanding and producing language remains difficult althoughnew brain imaging techniques can be used to study the populations of neu-rons and cerebral networks responsible for these activities. These tech-niques therefore make it possible to test the psychobiological validity of lin-guistic theories and psycholinguistic models pertaining to the understandingof language. Two contradictory hypotheses—which are still much debatedtoday—can thus be tested and compared: the first of these proposes modu-lar logistics in which the various levels of processing involved in linguisticactivities are organized on a serial, hierarchical basis; the other has it thatthe system is organized in an interactive, parallel, distributed way. Thesehypotheses can be tested using complementary imaging techniques.

Indirect Imaging Techniques

Those parts of the brain which become activated when confronting someevent or a particular linguistic treatment can be identified with an accuracyof the order of a few millimeters using fMRI and PET. It is the superiortemporal and frontal areas of the left hemisphere that are activated in thecourse of linguistic processing. However, the question of whether or not sucha network is specific to language (as assumed in the modular hypothesis)remains open.

Direct Imaging Techniques

Techniques such as EEG, evoked potentials (EPs) and MEG record changesin electrical and magnetic activity in the brain with very high temporal res-olution. This makes it possible to determine when—to an accuracy of theorder of milliseconds—the processes involved in the treatment of particu-lar events diverge. Evoked potential studies have shown that a personunderstands what a familiar word means within a few hundred millisec-onds and this method has been used to probe the extent to which the vari-ous levels of linguistic processing are independent of one another asopposed to interacting to construct the meaning in a specific context.Although these questions are far from resolved, the findings seem to sug-gest that there is extensive mutual exchange between the different levelsof linguistic processing.

Localizing language areas on a map of the brain.

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Language and Disease

In the field of language, the main contribution of neuropsychology—which is the study of the reciprocalrelationships between the brain and behavior—has been the high resolution characterization of variouslinguistic problems. The brain lesion underlying the problem may be a focal one (aphasia) or diffuse(dementia), or the problem may be developmental in origin (e.g. dyslexia). Useful information is com-ing from this field of study for the design of rehabilitation programs for patients with language prob-lems, which remain an important social and public health issue.

Understanding and Characterizing Language Problems

The characterization of language problems combines the analytical meth-ods of at least three disciplines—an eminently inter-disciplinary approach:• linguistics, to establish the structural properties of natural language (andtherefore that of the patient);• psycholinguistics, to define the cognitive processes underlying verbalbehavior (production and comprehension of the written as well as the spo-ken word);• neuropsycholinguistics, to identify cerebral correlates of language.

Functional Architecture of Language in Health and Disease

Studying the neuronal substrates of language and the cerebral mecha-nisms which are defective (and can be highly specific) is an excellent way ofdiscerning—by extrapolation—the functional architecture of language inthe brain-mind in healthy subjects as well as patients. Most of the work inthis field at this time is being done by cross-disciplinary research teams,and federated research institutions including, among others, partners ofCNRS research units. These groups have the equipment necessary for non-invasive investigations in healthy and diseased subjects as well as expert-ise in the different fields necessary for research in the field of languagepathology.

Helping Through the Implementation of Coping Strategies

Apart from the high-resolution diagnostic characterization of languageproblems, neuropsycholinguistics also addresses coping strategies, someimplemented by the victims themselves and some formulated in collabo-ration with speech therapists in the context of rehabilitation programs

Testing vocabulary. The child must choosethe image which most closely correspondsto the word being pronounced, e.g. for theword ‘chair’, the child is presented with pic-tures of a chair, hair, a bench and a stool.

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A reading test designed to evaluate spellingability. The child must choose the right oneof three words. The correct spelling is‘through’; one of the two incorrect spellingsis pronounced the same but looks quite dif-ferent (‘threw’) and the other looks similarbut does not have the same pronunciation(‘though’).

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Behavioral test in a Y-shaped labyrinth.

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Pyramidal cells of the hippocampus.

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Reasoning, Memory and Planning

Dissecting the link between brain function and thinking is a majorchallenge. Our understanding of the neural substrates for themost complex cognitive activities (e.g. those to do with memory,reasoning or the planning of actions) has benefited in recentyears from spectacular technological and methodologicaladvances in non-invasive methods for investigating brain func-tion.

Memory and Nervous Circuits

Research into memory has revealed the multiplicity of processes and sys-tems involved and has led scientists to reevaluate the specific roles of thedifferent cerebral structures and circuits implicated. With brain imagingtechniques (PET, fMRI and MEG), those parts of the human brain thatbecome activated when performing a task necessitating memory can bevisualized. Research carried out by CNRS teams has led to redefinition ofthe roles of certain regions: the hippocampus is involved in relational andepisodic memories, and the amygdala in emotional memory. Sensory andmotor information is encoded and deciphered in cortical structures.

Planning and Positioning in Space

Cerebral and calcium imaging as well as unitary, optical recordings in apopulation of neurons have been used to show, at the level of the hip-pocampus, the selectivity of neuronal activation for information on posi-tion and direction in an animal performing a behavioral task. Informationpertaining to planning is processed in the frontal cortex. The field of spa-tial cognition illustrates this approach which requires the convergence ofneuroscience, psychology and modeling. This research shows, in a gener-al way, how the various neural networks cooperate with one another,notably at the temporal level. Their functioning also involves the propaga-tion of neuronal activity in distributed networks. Such propagationdepends on neuronal plasticity and complex mechanisms insuring com-munication inside and between cells.

Reasoning

Studies of reasoning have focused on deductive reasoning in which conclu-sions are drawn on the basis of general premises. For example, recentCNRS research has shown that deductive reasoning involves a series of dis-tinct functional steps which are mediated by specific activities. In the samespirit, convergent findings in humans and monkeys suggest that the pro-cessing of mistakes is associated with the activation of certain parts of thecortex, notably the anterior cingulate cortex. These results are particularlyinteresting since error processing is an essential component of planningactions and resolving conflict. In the course of spatial learning,

the rat constructs a map from landmarks in its surroundings; this map allowsthe animal to situate itself in its environment.The molecular and cellular mechanismsunderlying this memory mechanism are being studied.

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Conceptualization and Cognitive Development

What is a concept? What conditions are necessary for a living being to formulate concepts? Does it haveto be able to speak a language? Such questions have preoccupied philosophers since Plato and Aristotle.These days, they are being addressed experimentally—in linguistics, psychology and cognitive science.

Analyzing Behavior

A concept is a component of some broader cognitive structure or mentalrepresentation, traditionally referred to as thoughts or judgments, thetruth or falseness of which could be evaluated or debated. A concept is alsothe subject of high-level cognitive processes such as recognition or per-ceptive identification, memory tasks, logical inference and the mecha-nisms whereby knowledge is acquired. The information encoded in a con-cept (e.g. the concept of “dog”) should be more stable, more abstract, moregeneral and less dense than the information contained in a visual percep-tion (e.g. the sight of a greyhound, a poodle or a bulldog): the visual expe-rience changes every time whereas the concept “dog” applies to all dogs.Do the human concepts expressed by the words “water”, “tiger”, “screw-driver”, “molecule”, “seven”, “knowledge, “democracy” or “god” all belongto a single homogenous cognitive information-processing system? Are theyacquired by children from a single innate basis selected for in the courseof the evolution of the species? Is language necessarily involved in thelearning of all types of concept? Such are the questions being asked byCNRS researchers.

What is a Concept?

Using non-linguistic methods, developmental psychologists are systema-tically studying babies’ cognitive capabilities before they acquire language.For example, in one type of experiment, babies of 4-5 months are shown aball beginning to free-fall with the bottom part of the ball’s fall hiddenbehind a screen. Then, the screen is removed. The baby is sometimesshown the ball lying on top of the table and sometimes hanging from thebottom of the table-top, each for the same amount of time. It was foundthat babies spent longer looking at the less expected scenario. The deduc-tion is that the baby was more surprised by and interested in the incongru-ous scenario than the expected one. This type of experiment has shownthat primates and babies share similar basic cognitive systems to repre-sent how physical objects behave and geometric properties of space. Theyalso apparently evaluate the number of objects in their surroundings inanalogous ways. Studies carried out at CNRS have shown that babiespossess two systems to evaluate the number of objects in a set: one ofthese systems represents with precision the size of sets containing fewerthan four components; and the other system deals with bigger sets. Howdo older children and adults shape their knowledge of arithmetic? How dothey formulate the arithmetic concept of exactly “seven”? Data fromdevelopmental studies and from the neuropsychological examination ofbrain-damaged people together with brain imaging results from adultsubjects performing calculations all suggest that language is intimatelyinvolved in harmonization of the two quantity-evaluation systems which aredisassociated in babies.

Cognitive Development and Conceptual Representation

The baby is used to seeing a ball fall freelyalthough the end of the path is hidden by ascreen.1. When the screen is removed, he is shownthe ball (a) lying on a table, (b) hanging offthe bottom of the table-top. He is surprisedwhen the ball is below the table.2. The baby is more surprised when theball is hanging with no apparent support (b)than when it is resting on the ground (a).

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Memory: Cellular and Molecular Aspects

Understanding the neural basis of memory and its diseases is one ofthe great challenges to modern science and a major focus of CNRSresearch. Our knowledge of the molecular and cellular basis of thehigher cognitive functions has exploded in recent years. This type offundamental research is enhancing our understanding of how thebrain works and is leading to the development of novel possibilitiesfor the treatment of memory problems.

Cerebral Plasticity and the Molecular Mechanisms of Memory

The ability to create memories depends on the brain’s extraordinary plastic-ity. The approaches of neurophysiology and cell imaging in animals haverevealed cerebral circuits involved in various different types of memory, andthey are making it possible to investigate the temporal dynamics of the acti-vation of these circuits when information is being encoded. They are alsobeing used to decipher some of the codes used in the neuronal activitiesthat store this information in the form of memories. Spectacular progresshas also been made in identifying the mechanisms that insure the cerebralplasticity which underlies memory storage. Some of the intracellular mech-anisms involved in the storage of past experiences have been elucidated.CNRS groups have characterized molecular pathways which lead to theactivation of genes encoding transcription factors—molecular switcheswhich mediate sustained remodeling of neural networks and memory stor-age. One of the major challenges for the future will be to understand inter-actions at the level of the brain’s organization, from the level of the moleculeand the cell, through neuronal micronetworks and neural networks, and upas far as the brain’s higher functions.

Understanding and Treating Functional Neurological Disorders

Increased knowledge of the molecular and cellular mechanisms underly-ing memory has opened unforeseen horizons onto the analysis of howimpaired function leads to memory problems in diverse mental disorders.Advances in molecular genetics, the development of animal models forvarious human diseases, and the use of neurophysiological behavioral andmolecular approaches are helping CNRS scientists to investigate the spe-cific molecular and cellular mechanisms affecting neuronal plasticity thatare disrupted in selective memory deficits observed in aging, and variousneurological, psychiatric and neurodegenerative diseases. Future researchshould identify endogenous markers and make it possible to investigatenovel therapeutic avenues, such as molecular pharmacology, gene thera-py, stem cell transplantation, central nervous system stimulation, andenvironmental effects. The neuroscientific stakes in the post-genomic eraare immense, concerning not only improving our understanding of how thebrain works in relation to mental processes but also opening new perspec-tives in the biomedical field and reacting to the increased economic andsocial costs incurred as a result of the rising incidence of cognitive deficitand mental disease.

Some synapses are activated duringlearning, triggering molecular events that lead to the transcription of genes in the neuronal nucleus. These mechanismsconsolidate the activated neural networkand lead to memory storage.

Sectioned brain tissue showing activation ofthe transcription factor Zif268 in neurons ofthe hippocampal dentate gyrus on inductionof synaptic plasticity (top) compared with theother side as control (bottom).

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Decoding Nervous Information

Context and Issues

The nerve cell—the fundamental building block of the nervous sys-

tem—has certain morphological characteristics related to its very spe-

cial functional properties. Its tree-like, dendritic structure means that it

is able to receive information coming from neighboring cells via synaps-

es formed along its various projections.

The synapse is where nervous signals are transmitted, a process

which involves both electrical and chemical mechanisms: when a com-

pound (referred to as a neurotransmitter) is released, it binds to spe-

cific membrane-bound proteins on the target neuron and induces a

change in the ionic balance across the target cell’s plasma membrane.

The resultant local change in electrical potential generates a current

which is conducted to the neuron’s cell body where the various currents

resulting from all the cell’s synapses which are being activated are

added together. If a certain threshold is attained, an action potential is

sent along the axon to a terminal where there are synapses with the

dendrites of another cell, possibly another neuron or a muscle cell, an

endocrine cell, etc.

The neuron’s shape and the distribution and position of its synapses

and neurotransmitter receptors are key when it comes to its intrinsic

electrical properties. Since the properties of the neuron as a cell must

be intimately associated with how information is processed, one of the

main priorities in today’s functional neuroscientific research is under-

standing how networks of neurons are organized and their molecular,

cellular, metabolic and electrical workings—with the ultimate aim of

identifying the key components of the neural code.

This line of fundamental research aimed at deciphering nervous

information is a major focus at a large number of CNRS laboratories.

32 Focus The brain

Decoding the Nervous System

33

Top: the amphibian Xenopus metamorphos-ing, a model for the study of plasticity asso-ciated with changes in body morphology.Bottom: a new-born rat is used to study the establishment and maturation of spinal(locomotor), bulbar (respiratory) and cortical networks.

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Neural Networks: Development andFunctional Plasticity

Functional plasticity in neural networks is essential for thedevelopment of the nervous system which involves not onlyforming new circuits and synaptic connections but also modu-lating the strength of connections and the bioelectric propertiesof neurons in existing neural networks. Understanding the sys-tem’s dynamic properties and the mechanisms underlying itsplasticity is going to require input from a variety of experimentaldisciplines.

Maturation of Motor Rhythm–Generating Networks

During the developmental process, the neural networks which generaterhythmic movements such as respiration or locomotion provide excellentmodel systems to study plasticity. The behaviors that they control are easyto measure and their functional maturation can be accurately dissected. Invitro preparations of tissue from the amphibian or rodent central nervoussystem contain components which mediate plasticity. Although the spinallocomotor networks of rats and mice are operational at birth, they subse-quently undergo remodeling. This ontogenic maturation is controlled bypathways in the brain stem which modulate the properties of the cells andsynapses in embryonic medullary neurons, temporarily repressing certainintraspinal inhibitory pathways. CNRS teams have also profitably usedgenetically modified animals to study neural network development, e.g. inmurine respiratory networks. Apart from information on how genetics con-trols the formation of such circuits, these studies have shed light on theroles of neurotransmitters such as serotonin and norepinephrine in func-tional remodeling during maturation.

Neuron Activity and the Development of Networks

Activity-dependent mechanisms are critical in the development of neuralnetworks, both before and after birth, e.g. in the hippocampus of the devel-oping rat, spontaneous discharge by these neurons contributes to the for-mation and maturation of GABAergic networks. Early in the developmentalprocess, the synapses formed by these future inhibitory interneurons areexcitatory so the GABA released by them (the only ones which are prema-turely active) plays a “pioneering” role in establishing cortical networks.Spontaneous neuronal activity seems to be a universal determinant in thedevelopment of neural networks throughout the brain of the new-born ratduring the time that the somato-sensory cortex is developing. The net-work’s endogenous activity as well as sensory input contributes to estab-lishing coordination between the sensory and the motor systems. Suchfundamental research is a necessary preliminary to understanding devel-opmental problems and diseases affecting the central nervous system inadults.

GABAergic neurons in a rat’s brain.

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Modeling and Interdisciplinary Work in the Neurosciences

Major progress has been made in recent years at the interface between physics and neuroscience as aresult of creating CNRS interdisciplinary centers, which bring together experimentalists and theoreti-cians to study brain function. Theoretical approaches attempt to formalize the experimental resultsand, on the basis of predictions derived from models and simulations, are key to designing experi-ments. They also make it possible to investigate the effects of fundamental mechanisms at differentlevels of integration which might be difficult to observe directly. The synergy between theory and exper-iment is benefiting from advances in information technology with increased processor speed meaningthat now, the interactions of simulated neurons as well as biological neurons can be followed in realtime with an iteration period of under a millisecond.

Decoding the Nervous System

At the microscopic level, current, realistic electrical models of biologicalneurons which take into account the spatial distribution of intrinsic con-ductances and synapses in the system can be used to study: • the genesis and retrograde propagation of neuronal discharge in the den-drite; • the compartmentalization of cellular integration processes and theirnon-linear features; • and the importance of the chronological order of pre- and post-synapticpotentials in synaptic plasticity. At the macroscopic level of integrated systems, advances in neural net-work theory are improving our understanding of how collective statesemerge. In particular, interdisciplinary approaches at CNRS have shownthat recurrent connections in the cerebral cortex play a crucial role in theemergence and amplification of functional sensitivity in sensory systems,and in the generation of persistent activity in memory and decision-mak-ing. Analytical studies are identifying the conditions in which populations ofneurons can generate a diverse repertoire of synchronized oscillations,from the slow waves of sleep to ultrarapid fluctuations.

New Tools to Study Complexity

In the technique of dynamic clamping (developed through collaborationbetween CNRS and Inserm), a cell is subjected to an artificial intracellularcurrent simulating synaptic stimulation by a virtual network. It is used in invitro systems with no spontaneous biological activity to diminish the phe-nomenological gap between highly reductionist approaches (tissue slicesand cultures) and real physiological networks in which synaptic recurrenceis intact. An in vivo application is the real-time grafting of artificial supervi-sors into functional networks, e.g. the “parler-neurone” which is indistin-guishable from that of the biological assembly. This method coupled withour enhanced understanding of sensory and motor coding in cortical net-works should lead to new possibilities in the field of interfaces betweenman and machine (e.g. artificial sensors and sensory substitution or brain-assisted command of implants).

Hybrid Technologies in Functional Neuroscience

Propagation of visual signals evoked alonghorizontal connections in a cat’s primaryvisual cortex. a. Intracellular recording of a simulatedvisual signal in a neuron of the primaryvisual cortex. b. One-off simulated visual signal meas-ured using a voltage-sensitive dye (coloredscales on the right).

Retinotopy of the human visual cortex.Mathematical models applied to fMRI dataunfold the anatomy of the occipital lobe, mak-ing it possible to compare different subjectsand reconstruct activation profiles accordingto retinal eccentricity (scale given by the col-ored rings in the central disk).

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Molecular modeling of hanatorin which is apotassium channel blocker.

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Calcium channels on the dendrites of motorneurons.

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Cerebral Ion Channels: Identification andPharmacological Applications

Ion channels in nerve cell membranes play a special role in trans-ferring information within neural networks. Identifying these ionchannels in the brain and elucidating their mode of action areindispensable if we are to understand how neurons process infor-mation—ultimately with a view towards developing specific phar-macological modalities.

Ion channels are characteristic of excitable cells, notably neurons. In thesynapse, their intimate association with neurotransmitter receptors leadsto the exchange of ions between the extracellular and intracellular envi-ronments, and to the generation of synaptic potentials. In some cases, theion channel is an integral part of the receptor in which case binding of therelevant neurotransmitter molecule induces it to open. The permeability ofother channels depends on the degree of polarization of the neuron’s plas-ma membrane. Over the last few decades, many types of channel havebeen discovered in nervous tissue with various specificities, notably sodi-um, potassium and calcium. CNRS research units contributed to isolatingthe genes encoding these various receptor families.

One Channel … Several Channels …

The patch clamp technique has been used to characterize the various typesof ion channel and determine their specificity. To immobilize their prey,certain species in the animal world (including various spiders, mollusks,scorpions and snakes) produce toxins which specifically block ion chan-nels. These compounds provide excellent pharmacological tools which helpus understand how ion channels and their inhibitors work. Several CNRSresearch units are engaged in this type of research which, apart from itsrelevance to fundamental processes, is discovering new industrial applica-tions, notably in the form of more selective pesticides which neither induceresistance nor damage the environment.

Characterizing the Activity of Ion Channels

Calcium channels are unique in that they are the mediator of cellular exci-tation and also provide means of entry for calcium which is itself a directmediator of many physiological functions. Very recently, a new family ofgenes encoding calcium channels was discovered. Members of this T-typefamily of calcium channels have a very low threshold and a CNRS teamworking in collaboration with Inserm has shown that the product of theCaV3.2 gene is involved in pain perception. Calcium channels are particu-larly dense at nociceptors and T-channel antagonists could have strongpain-killing potential.

Calcium Channels: Recently Discovered Functions

A trace of the electrical activity of a singlepotassium channel.

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Synaptic Plasticity, Potentiation and Depression

Cellular models for learning and memory have been capturing neurobiologists’ attention for more thanthirty years. For a long time, long-term potentiation (an increase in the efficiency of synaptic transmis-sion) was the only explanation for why certain changes in neuronal circuits persist through develop-ment and why various forms of plasticity depend on behavioral experience. These days, other media-tors of plasticity, involving not only synaptic receptors but also ion channels modulating neuronexcitability, are thought to participate in the storage of neuronal information in the brain.

Registering Information

Long-term potentiation (LTP) at glutamate-dependent synapses was dis-covered more than thirty years ago but the mechanisms underlying how itworks were not understood in depth until fifteen years ago. It is induced bya transient increase in the frequency of stimulation at the synapse or bycombining repeated synaptic activation with post-synaptic depolarization.More accurately (according to Hebb’s hypothesis), synaptic information rel-evant to the neuron (i.e. productive of an action potential) has to be consol-idated. In both cases, LTP results from the activation of calcium-permeableNMDA (N-methyl D-aspartate) glutamate receptors which act like detectorsof coincidence. The rise in the cytoplasmic calcium concentration induces acomplicated cascade of catalytic events which can lead to enhanced trans-mission efficiency or an increase in the densities of certain types of gluta-mate receptor at the synapse. A CNRS team recently showed that the later-al mobility of these receptors on the surface of the dendrite depends on theactivity of the neuron and on certain protein kinases known to be involved inLTP.

Depression of Synaptic Responses: Enhanced Plasticity?

The registration of information at the synapse can, paradoxically, lead to aloss of plasticity because, once potentiation has been induced, the synapseis saturated. The validity of the concept of reversibility (proposed thirty yearsago by theoreticians) was not experimentally demonstrated until much laterwith the discovery of protocols able to “erase” in a reproducible manner anyinformation that is registered but that is no longer useful to the host.According to Hebb’s principle, synaptic information that is irrelevant to theneuron must be suppressed. Long-term depression (LTD) is induced at glu-tamatergic synapses if the neuron’s response (the action potential) system-atically precedes the synaptic response.

Beyond the Synapse …

Functional plasticity is not confined to effects on synaptic transmission andit is now accepted that synaptic changes are associated with modificationsof neuronal excitability, the two being functionally synergistic. The currentchallenges are to understand the rules and identify the ion channelsinvolved together with the mechanisms that regulate them. This line ofresearch is being pursued by several CNRS laboratories.

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Decoding the Nervous System

Electron micrograph of a cryofracturedsynapse.

An action potential.

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Likelihood of an action potential being trig-gered after long-term potentiation (LTP) andlong-term depression (LTD) of a hippocam-pal neuron. LTP is associated with enhancedsynaptic integration whereas DTP is associ-ated with depressed integration.

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Biorobotics

Nature shows us novel ways of dealing with complicated prob-lems, which can lead to innovative applications. Evidence of thisare the many and various bionic constructs which have takeninspiration from biological models: in 1894 the shape of the air-plane wing was taken from that of the stork; in 1948, burdockinspired Velcro; and in 2002, the lotus leaf was used as a model inthe design of the self-washing window.

Reproducing for a Better Understanding

The field of biorobotics takes its inspiration from biology, touching upon theareas of bionics and biomimetics. Nevertheless, it is not so much dependenton biological structure as on animal behavior and the underlying nervous cir-cuits. Whether living or inanimate, independent machines have to confrontmajor challenges in the real world—challenges which the sensory and neu-rological systems of animals have learned to cope with remarkably well.Biorobotics is therefore a transdisciplinary science which is informed by theneurosciences and animal behavior but also depends on many of the con-cepts of physics and chemistry as well as on the methods used by engineersfor analysis, formalization and simulation. The fruits of biorobotics comefrom a dual approach:•To take some principle from the complex world of living organisms thentranslate it into a technology that is accessible to humans (e.g. electronics)and implement it in the form of an intelligent machine such as a sensor, aswitch or a vehicle (independent, terrestrial, aerial or spatial). The many andvarious sensory and motor systems encountered in animals (often especial-ly effective in arthropods which have been evolving for one hundred timeslonger than Man) have been particularly inspirational.• In return, to shed new light on the original biological principle in questionas a result of the physical realization (e.g. its strengths and weaknesses) andto help design new experiments to test the system and improve it as well asenhance our understanding of the underlying physiology.

Intelligent Machines for Many Applications

For twenty years already, CNRS has been engaged in biorobotics research—in the context of the life sciences, engineering and more recently throughinformation and communications science and technology. CNRS scientistshave built neuromimetic, terrestrial and aerial robots, all inspired by senso-ry and motor principles elucidated in flies or humans which have enrichedour understanding of physiology as well as giving rise to a whole series ofpatents for innovative sensors and automatically piloted vehicles. Recently,the sustained support of the Robea Program has allowed CNRS groups todevelop the Rabbit, Psikharpax, SimBioMan, RobocCoa, EcoVia and robot-needle projects. All of these are inspired by biological models to a greateror lesser extent and although most are still in the computer simulationphase, the formalization of problems and confrontation of the findings withneuroethological data are already proving rich materials for biologists.Biorobotics is a novel field of development at CNRS which is expanding ourunderstanding of animal and human behavior and of the underlying mech-anisms that govern nervous processing—above and beyond its potentialindustrial applications.

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Arrangement of electronic componentsdesigned to insure that a robot-fly missesobstacles in its path. This regular networkresembles those seen in the visual cortexof vertebrates and insects.

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By virtue of its eye which mimics the retinalscanning system discovered in the eye of thefly, this aerial robot OSCAR (100 g) is able tofix contrast and follow the eye direction.

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This robot-fly (12 kg) can move at a speed of50 cm/s through an obstacle course bydetecting optical changes due to its ownmovement.

Context and Issues

The intrinsic properties of adult brain tissue as well as behavior pat-

terns in adult humans and animals are all dependent on processes

occurring during prenatal and postnatal development. Correct execu-

tion of the programmed specifications governing the construction of an

animal’s brain and the maturation of its fundamental elements is key

when it comes to how well the adult brain will function. Early perturba-

tion—in the fetus or the perinatal period—can lead to nervous system

disorders in adulthood. Experiments on brain development being per-

formed today will no doubt provide new information about the mecha-

nisms involved in the acquisition of the fundamental properties of neu-

rons and their networks, and on the impact of such processes on func-

tion. Such developmental research brings together the various disci-

plines of the molecular, cellular, functional and cognitive neuro-

sciences.

From the simple reflex to the most complex action, each modality

underpinned by the activity of one or more neural networks implies pro-

gressive implementation in the course of brain development. This stag-

gering of the maturation process results from interactions between

genetic factors and the environmental factors which regulate gene

expression. Our understanding of the molecular and cellular processes

involved in the formation, maturation and differentiation of neural net-

works is far from complete. It was once believed that neurons could not

divide any more after birth, but then neural stem cells and progenitors

were discovered in certain parts of the adult brain, focusing fresh atten-

tion on the study of brain development. These cells, which have kept the

differentiation potential of embryonic cells, are without doubt a promis-

ing source of cells for treatment based on substitution and compensa-

tion, especially in neurodegenerative disorders. Nevertheless, extensive

study and animal experimentation by neurobiologists will be required

before this type of cell can be routinely used for nervous tissue grafts in

adult patients.

Understanding Brain Development

38 Focus The brain

Understanding Brain Development

Architect Genes, Brain Stem Development and Neural Network Function

Are some aspects of behavior determined genetically and passeddown through the generations? One of the answers to this ques-tion about the relationship between genetics and behavior almostcertainly lies in the mechanisms that lead to the emergence of thefirst cerebral activities in the embryonic neural tube.

The Rhombencephalon, a Good Model to Study these Activities

The rhombencephalon is one of the structures that appears before the neu-ral tube does. It is where the “reticular” neuronal systems that controlbreathing and digestion, cardiovascular regulation, attention cycles andmany other vital functions develop. This part of the nervous system is prob-ably the most completely characterized in terms of architect genes whichmediate regionalization of the neural tube during the early stages of embry-onic development. Knocking out the Hox genes has shown that they areresponsible for the anteroposterior regionalization of the rhombencephalon.Further down in the network of transcriptional interactions in the rhomben-cephalon, expression of the genes involved in the production of serotoniner-gic neurons and functional units, such as the autonomic reflex arc, has beendetected at very early stages of the human embryo. These genes have beenshown by neurobiologists to be involved in the maturation of neural networksand to have repercussions on postnatal behavior despite the remodeling andplasticity which characterizes every aspect of fetal development.

Information Learned from Knock-Out

By knocking out specific genes, several groups of CNRS researchers havebeen able to identify various species involved in the construction of neuralnetworks with specialized functional properties. Hox gene products and thetranscription factor Krox-20 are important in maturation of the parafacialrespiratory module whereas the preBötzinger respiratory module resultsfrom MafB-determined specifications at a later time. Knock-out technology isalso contributing to our understanding of postnatal respiratory disease, inhumans as well as animals.

Selection, Conservation and Suppression of Neural Networks

In the course of development, the expression of architect genes is believedto help determine the functional organization of neuronal circuits. If thishypothesis is correct, a change in the embryonic transcriptional systemcould disrupt an existing network or generate some novel neuronal compo-nent. Knocking out the Hox 1 gene (which is being studied by CNRS scien-tists) leads to the appearance of a supernumerary neuronal system, thecells of which are not eliminated during fetal development and which, afterbirth, control breathing rate. Thus, the Hox gene system may be involved inselecting, conserving and suppressing functional neural networks in devel-opment (and through evolution). Deeper understanding of early abnormali-ties in gene expression could help us to understand and enable us to treatcertain diseases which onset in adulthood but result from perturbations inthe embryo.

39

Adult neurons at the pontomesencephalicjunction. Despite the anatomical complexityof the various populations of cerebralneurons (in red, dopaminergic groups A8and A9; in green, cholinergic groups of thepontine tegmentum), embryonic architectgenes influence the functional organizationof adult circuits.

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Segment of the chick rhombencephalon sur-gically excised from the rest of the embry-onic brain and developing in ovo. Architectgenes expressed in this segment, especiallyKrox-20 (shown in brown), are sufficient formaturation of the parafacial neuronal respi-ratory module.

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Top: activity of a “reticular” neuron respon-sible for respiratory behavior. Bottom: each breath corresponds to oneactivation of the diaphragm. The starting up of these neurons is largely dependent on events that occur early in neural tubedevelopment.

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Understanding Brain Development

Neural stem cells and the progenitor cells that they generate when theydivide have been detected in the hippocampus and the subventricular zoneof the brain using markers for cell division. The daughter cells—neuronsand glial cells—become assimilated into the hippocampal dentate gyrusand the olfactory bulb. Physiologists and behaviorists are trying to under-stand what role this neurogenesis plays in cognitive and olfactory func-tions—and more generally in its contribution to nervous system plasticity.This work is informed by transgenic models generated by geneticists, andrelies on coordinating the methodologies of cell biology, electrophysiology,imaging and behavioral analysis.

Neurogenesis and Cerebral Plasticity in Adults

Many diseases of the central nervous system are marked by the degenera-tion of neurons or oligodendrocytes so replacement could constitute aneffective treatment modality. Two approaches are being contemplatedbased on our expanding knowledge in this field. The first focuses on how todeliver adult stem cells to the site of damage and insure their assimilationinto existing functional networks. The second involves growing stem cells upin culture and then transplanting them into nervous tissue in the form of“neurospheres” (small clusters of cells): this approach has been used suc-cessfully in animal models of certain diseases, including multiple sclerosis,stroke and Parkinson’s disease.

Cell Therapy

A challenge in fundamental biological research which will determine thefeasibility of using neural stem cells is the elucidation of the genetic pro-grams and environmental signals that regulate their number, how they areactivated and the conditions in which they migrate and become assimilatedinto existing networks. This endeavor is very much underway with collabo-rative projects between neurobiologists and developmental biologists build-ing up information about the mechanisms of developmental neurogenesis indiverse biological models (from invertebrates to monkeys).

Genetic and Epigenetic Factors

Neurogenesis and Morphogenesis in Adults

Particular neural cells have been characterized recently in the central nervous systems of adult rodentsand humans. Referred to as stem cells, they are multipotential cells which means that, given the correctconditions, they can differentiate to form any of the three main cell types found in the central nervoussystem, namely neurons, oligodendrocytes or astrocytes. This discovery raises the question of what theyare doing in the brain and opens therapeutic perspectives which might lead to novel ways of repairingdamaged nervous tissue.

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Frontal section through the subventricularzone (SVZ) showing many cycling cells(same method as that described above).Most of the stem cells in the adult brain arefound in the SVZ.

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Frontal section through the hippocampal den-tate gyrus. Labeled with BrdU—an analog ofthymidine which is incorporated into replicat-ing DNA—showing cells (arrows) in the adulthuman brain which are continuing to divide togenerate new neurons. Counter-stain: Cresyl Violet.

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Conclusion

Neuroscientific research is changing its outlook. Advances in cell

imaging, improvements in functional and metabolic imaging tech-

niques, progress in the analysis of electrical signals, and contributions

from genomics and proteomics are all adding new dimensions to our

understanding of how the brain works, enabling us to establish firm

correlations—in both time and space—between the activation of differ-

ent parts of the brain and the processes implemented when carrying

out simple tasks or performing more complex behaviors.

It is in this technological context that the integrated neurosciences

are going to be developing over the next couple of decades although

experiments in primates will continue to contribute important data

about how information is processed in the human brain. The major

challenges in neuroscientific research in the third millennium will be

deducing the general rules that govern nervous system function against

the background of all the complexity of the living organism, and open-

ing new avenues to identify the physiological basis of thought and con-

ceptualization.

Better understanding means better treatment and fundamental

research is going to be essential if we are to discover new pharmaco-

logical agents and devise effective treatment protocols for patients suf-

fering from diseases involving the nervous system. This quest is partic-

ularly urgent in the area of neurodegenerative disease which can be so

devastating for its victims and their families.

In this endeavor, inter- and multidisciplinary work will certainly

guarantee progress in both the fundamental and applied neuro-

sciences.

This booklet is published by the CNRS Délégation à l’information scientifique et technique (DIST).

Acknowledgments to all the scientists who helped produce this booklet.

Head of Institutional Publications: Stéphanie Lecocq (+33 1 44 96 45 67)Coordinators: Anne-Solweig Gremillet, Stéphanie Lecocq and Jean-Pierre TernauxIconography: Marie Bacquet and Christelle PineauGraphic design: Laura SlawigGraphics: Laser GraphieTranslation: Amaïa Traduction

Printed by CaractèreDecember 2005

FOCUS DEC.2005

www.cnrs.fr