encyclopedia of neuroscience || cognitive neuroscience: an overview

Cognitive Neuroscience: An Overview 1117

Cognitive Neuroscience: An Overview
C M Wessinger and E Clapham, University ofNevada, Reno, NV, USA
ã 2009 Elsevier Ltd. All rights reserved.

Understanding how brain enables mind is the goal ofcognitive neuroscience. More broadly stated, cogni-tive neuroscience is concerned with understanding theneurobiological basis of mental processes – in otherwords, thinking about how the brain is involved inthinking. Thinking about thinking. There is littledoubt that human brains can engage in the task ofthinking about thinking and likely have been able todo so for some time. Unfortunately, throughout mostof human history, more practical concerns necessarilyoccupied human thought. Humans were moreconcerned with thinking about and acting on beha-viors that were related to their survival: storingenough food to enable them to survive the winter,killing the bear before the bear killed them, andfinding shelter. They had to concentrate on under-standing and working within their environment inorder to survive to struggle another day. Anythingthat did not specifically contribute to their survivalwas a luxury. Thus, thinking about thinking wouldhave been a luxury – a luxury that humans couldnot afford. However, as soon as civilization developedto a point that day-to-day survival did not occupyevery waking thought, humans could afford to thinkabout thinking. They began to think about how othersthought and why they did what they did. OedipusRex, the ancient Greek play that deals with parent–child conflicts, as well asMesopotamian and Egyptiantheories designed to explain religion and the universeare excellent early attempts to understand why peopleact the way they do. Unfortunately, there are signifi-cant pieces of the equation missing. These thinkersand philosophers did not systematically explore therelationship between brain, behavior, and mind. Thisis where modern-day science and scientific methodol-ogy come into play. The notion that we must observe,manipulate, and measure the environment in order tounderstand the world and our place within it is centralto scientific methodology. When we apply such tech-niques to understanding how the brain enables mind,we call that cognitive neuroscience.Certainly, one could make a case that cognitive

neuroscience has been around ever since scientists indifferent disciplines began exploring the commongoal of understanding how brain enables mind. Suchearly explorations were quite illuminating and many

have stood the test of time. Some early theories thatspecific areas of the brain gave rise to specific func-tions were put forth by the phrenologists FranzJoseph Gall and J G Spurzheim, who declared thatthe brain was organized around 35 distinct functionsranging from such basic cognitive functions as lan-guage and color perception to more philosophicalconcerns such as hope and self-esteem. Each of theseareas was supported by a specific brain region, andwith increased proficiency in one of these areas, thecorresponding brain representation would grow insize, in turn causing a corresponding bump on theskull. However, once subjected to strict scientificmethodology, phrenology was shown to be untrue.It took the great neurologist John Hughlings Jacksonto again substantiate that a structure–function rela-tionship between the brain and behavior existed.Rather than being associated with bumps on theskull, his hypotheses were based on careful clinicalobservations of patients. While seizing, some of hisepileptic patients would move in characteristic pat-terns, almost with a systematic rhythm. This led himto propose a somatotopic organization – that is, amap of the body in the brain. Also at approximatelythis time, the French neurologist, Paul Broca, reportedperhaps the most famous neurological case – one ofhis patients had suffered a stroke and nowwas unableto speak but was still able to understand speech.Postmortem examination showed damage in the leftfrontal area, which is now known as Broca’s area andstill believed to play a major role in language output.In contrast, German neurologist Carl Wernickereported a patient who babbled incessantly followinga stroke, but that is all it was – babbling – the wordskept coming but made no sense. Wernicke’s patienthad a lesion in a more posterior region in the lefthemisphere, which has become known as Wernicke’sarea, and it is still believed to play a major role inunderstanding language. Neuroanatomists were rea-lizing similar divisions of the brain, but on a morecellular level. Perhaps the most famous is KorbinianBrodmann, who analyzed cellular organization of thecerebral cortex using a Franz Nissl developed stain tovisualize the different layers of cortical neurons,dividing the cerebral cortex into more than 50 distinctregions. This method of looking at cellular differencesin cortical layers is known as cytoarchitectonics.Many others contributed to this work, includingConstantin Von Economo, Gerhardt von Bonin, andPercival Bailey. More recently, similar techniqueshave been used to demonstrate more than 30 differentcortical areas in the visual system alone. Such early

1118 Cognitive Neuroscience: An Overview

investigations still guide cognitive neurosciencetoday, particularly with regard to helping the fieldunderstand structure–function relations within thebrain and how these combine to enable mind.The formal birth of the field known as cognitive

neuroscience was relatively recent. One story as tohow the field developed involved a taxi ride sharedby two eminent brain scientists – biopsychologistMichael S Gazzaniga and behavioral-turned-cognitivepsychologist George Miller – on the way to a dinnermeeting in the early 1970s. The dinner was being heldso scientists from many different disciplines coulddiscuss how to pursue a common goal: understandinghow the brain enables the mind. Such a joining offorces deserved an appropriate name – not just arenaming of biopsychology or cognitive neuropsy-chology, well-respected disciplines already strivingto understand the structure–function relations of thebrain–mind problem, but a name that incorporatedthe investigations of information processing akin tocognitive science: one that reflected a merging ofall of these many disciplines and more. After muchdiscussion, the term ‘cognitive neuroscience’ seemedideal. Another story concerning the birth of thefield involves a similar meeting with other eminentbrain scientists such as Larry Squire, a neuroscientistrenowned for his research exploring the biologicalbasis of memory, and Stephen Kosslyn, a cognitivepsychologist well-known for his mental imageryresearch. However, the exact events that led to theadoption of the term cognitive neuroscience are notimportant. What is important is the fact that manyscientists using quite variedmethodologies areworkingtogether, and making headway, toward the commongoal of cracking the code of the brain and understand-ing how this approximately 6-pound lump of soft,squishy, neuronal tissue allows us to think and feeland emote and cry and to dream – how the brainenables mind. Many of the varied methodologies thatcontribute to this exciting and rapidly expanding fieldare described here.In order to help us understand the inherently mul-

tidisciplinary approach of cognitive neuroscience, weneed to understand some of the key methodologies.Techniques ranging from exploring the molecularbasis of the brain to exploring consciousness are uti-lized, combined, dissected, and manipulated byresearchers in the quest to understand the neurobio-logical underpinnings of mental operations. How-ever, cognitive neuroscience is more than a simplerecombination of various methodologies and tech-niques. The field is extremely innovative in that itborrows the best techniques from the best disciplinesin order to provide the best innovative insights intohow brain enables mind. Such innovation can be

more fully appreciated by exploring some of the keymethodologies and techniques.

One key avenue in understanding how brainenables mind is grounded in understanding the rela-tionship between brain anatomy and cognition.Many studies have tried to relate differences in in-telligence to differences in brain size and structure.Differences in neuronal type, size, density, andconnectivity have also been explored in trying tounderstand structure–function relations betweenbrain anatomy and cognition. These explorations ofbrain structure can be loosely grouped under the termneuroanatomy. Formally, neuroanatomy is the studyof structure of the nervous system. Primary questionsof interest to neuroanatomy are identifying the vari-ous parts and components of the nervous system, aswell as describing how these parts and componentsare interconnected. To further complicate matters, itis necessary to use specialized techniques and meth-ods to ask these questions at multiple levels. Not onlyis it important to know the size and shape of a neuronbut also it is important to know where that neuronconnects. Where does the input come from and wheredoes the output go? This very debate was truly a keydevelopment in the birth of neuroscience, and neuro-science is key to cognitive neuroscience. Two brilliantneuroanomists were debating the interconnectivity ofthe nervous system. Camillo Golgi invented a silverstain that permitted visualization of single neurons.Using Golgi’s stain, Santiago Ramon y Cajal demon-strated that the neurons were discretely bound, indi-vidual entities that did not touch. This view wascontrary to that of Golgi, who believed the wholebrain was a continuous mass of tissue sharing a com-mon cytoplasm, or syncytium. Cajal extended thefield by being the first to identify the unidirectional(from axon to dendrite) nature of electrical transmis-sion within neurons. These findings were elucidatedwith early microanatomical techniques. Microanat-omy or fine anatomy has now developed to the pointof exploring the organization of neurons and theconnections. This is opposed to gross anatomy,which is concerned with the general structures andconnections visible to the naked eye. Gross andmicroanatomical techniques continue to inform thefield of cognitive neuroscience by providing essentialknowledge concerning brain and neuronal structureand connectivity.

Another key technique employed by cognitive neu-roscientists that is grounded in neuroscience is physi-ology. This is a branch of neuroscience that deals withthe functions and processes of life, as well as thephysical actions and reactions associated with theseprocesses. Simply stated, physiology measures bodilyresponses such as blood pressure, heart rate, and


neuronal electrical activity to the environment. Onephysiological measure of great import is neuronalelectrical activity. Despite the fact that the electricalpotential is extremely small, when large numbers ofneurons are active simultaneously, they produceenough electrical signals that when summed can bemeasured by electrodes placed on the scalp. Electro-encephalography (EEG) involves recording a contin-uous stream of overall brain activity while the personperforms a variety of tasks, sometimes in an experi-mental setting and sometimes as part of everyday life.These continuous recordings of continuous activityhave proven extremely useful clinically and havehelped with epilepsy, sleep disorders, and anxiety.EEG can only provide limited information when

exploring cognitive processes because the recordingstend to reflect global brain activity, as opposed tospecific cognitive or perceptual processes and func-tions. However, by synchronizing EEG collectionwith specific cognitive and perceptual events, it ispossible to disentangle the brain waves and seechanges in electrical activity associated with changesin cognitive and perceptual processes. This techniqueis called event-related potential (ERP) because theneuronal potentials are related to specific events.Given that neurons produce unidirectional electricaltransmission and constraints of Farady’s law of induc-tion (which states that a changing electric field pro-duces a magnetic field orthogonal to the direction offlow of electricity), they produce miniscule magneticfields.Magnetoencephalography (MEG) records thesesummed magnetic potentials at the scalp in a mannersimilar to ERPs.One great advantage of these techniques is that

millisecond accuracy is possible in terms of linkingsummed electrical signals specific to an event; thus,cognitive neuroscientists can understand exactlywhen and how the brain is responding to a specificevent. One major drawback of these techniques is thevery notion of summed electrical signals. Such sum-ming results in limited spatial resolution. Further-more, because the summed signals must be collectedat the scalp, localization of depth is also problematic.It is difficult to determine where in the brain thesignals occur.This problem can be overcome by taking advan-

tage of the inherently multidisciplinary nature of cog-nitive neuroscience by bringing other techniquesto bear. One technique that can help inform EEG,MEG, and a variety of other cognitive neurosciencetechniques is magnetic resonance imaging (MRI).Formally, MRI is a branch of neuroimaging that is aclinical specialty that uses noninvasive techniques toproduce images of the brain. Interestingly, MRI alsotakes advantage of Faraday’s law in that it uses a

supercooled, superconducting electromagnet to pro-duce a strong electrical field. This magnetic field isthen systematically manipulated to produce images ofthe body. These images currently provide great detailconcerning the gross anatomy, and even microanat-omy, of the nervous system. Such information, whencoupled with ERP and/or MEG techniques, can pro-vide essential structure–function relations withinthe human brain. Such techniques can help surgeonsunderstand not only tumor location but also functionscompromised by the tumor and weigh them againstfunctions that may be compromised by the surgery.

A recent interesting use of MRI in conjunction withother techniques involves trying to understand thebiological basis of autism, an increasingly more prob-lematic developmental disorder. Hutsler and collea-gues showed that increases in dendritic spine densitycould be related to increases in autistic symptoms.Certainly, these data do not indicate that dendriticspine density is responsible for autism, just thatthere is some connection, and this connection needsto be investigated further. This is an excellent exam-ple of the utility of cognitive neuroscience: by com-bining neurology, neuropsychology, radiology, andmicroneuroanatomy, researchers are making tremen-dous strides in understanding the biological basis ofautism.

Within the past 15 years, MRI has been extendedto include a functional component. By taking advan-tage of what from a radiologist’s viewpoint is noise,cognitive neuroscientists are able to see the brain inaction – almost. It is well-known that changes incognitive and perceptual activity result in localizedchanges in neuronal activity, which result in locali-zed changes in metabolic needs, which result in loca-lized changes in blood flow, which result in localizedchanges in blood oxygenation. The end of this cas-cade, the localized changes in blood oxygenation, canbe visualized with functional magnetic resonanceimaging (fMRI) in order to infer localization of func-tion. Also, when MRI signals are time locked tocognitive and perceptual changes, localization offunction, as well as localization of time, can beinferred. This is similar to positron emission tomog-raphy (PET), another neuroimaging technique thatcan image the blood flow change portion of thecascade to infer localization of function, except thatPET uses ionizing radiation and fMRI does not.Another advantage that fMRI has over PET is greaterspatial and temporal resolution. PET provides reso-lution in terms of minutes and centimeters for tempo-ral and spatial properties, respectively. However,fMRI, particularly event-related fMRI, can providetemporal accuracy in seconds and spatial accuracy inmillimeters.


So far, we have reviewed several biologically basedtechniques and have a greater understanding of theneuroscience part of cognitive neuroscience. But whatabout the cognitive part? Is there really a need formore when trying to understand how brain enablesmind? Certainly. Without the ‘cognitive,’ there wouldbe no true understanding of the mind portion of theequation.Cognition can be defined as any form of informa-

tion processing, mental operation, or intellectualactivity such as thinking, reasoning, remembering,imagining, or learning. Thus, cognitive psychology,a major contributor to the field of cognitive neurosci-ence, is the study of behavior related to cognition.The principles and goals of cognitive psychology reston two key concepts. First, mental operations dependon internal representations. Second, mental opera-tions undergo transformations.The notion that mental operations depend on inter-

nal representations is not that far-fetched; after all, bydefinition, if the operations are mental and takingplace in the mind, then they are necessarily internal.It is also easy to accept and understand the notionthat mental operations have to undergo transforma-tions. After all, we do not directly perceive and act inthe world; rather, our perceptions, thoughts, andactions depend on what our sensory systems aredesigned to detect within our environment, and oursenses are not capable of translating everything in ourenvironment into useable information. For example,we do not readily see infrared light or directly sensechanging electrical and magnetic fields, but thesephysical properties do exist in the environment –they are just not detectable by our sensory systems.We need to use other physical-detection tools specifi-cally designed to detect these aspects of our environ-ment. Nonetheless, we still perceive, think about, andact within our environment reasonably well. This isbecause our brains take the information provided byour sensory systems and transform it into signals thatcan be understood, interpreted, and acted upon.Michael Posner and Richard Mitchell first conducteda classic study illustrating both of these key principlesin the mid-1970s. Using a series of letters, they wereable to clearly demonstrate that mental processeswere internal and that these processes were necessar-ily manipulated and transformed when acting onthem. The experiment is quite simple: it requiresonly that one present letter pairs to participants andask them if the letters are the same or different. Whyis it a classic experiment? Because of how the letterswere paired and how the question of same or differentwas asked. In the simplest form, four different lettersare used – two consonants and two vowels – and theyare displayed in either upper or lower case. Using

these stimuli, different levels of sameness can beasked. The lowest level is based on the physicality ofthe letters: are they physically the same? AA and aaare two examples of stimuli that would be judges asphysically the same, Aa and aA are examples thatwould be correctly judged not the same. The nextlevel of question involves identity, or the name ofthe stimuli. The previous four examples would all bejudged the same based on name identity. AB and AEare examples of stimuli that would be judged as dif-ferent. The next higher question involves judging ifthe letters are the same or different based on moredetailed information concerning the letters – whetherthe letters are consonants or vowels. Reaction timedata were collected during this experiment andclearly show that the mental processes cannot neces-sarily be overtly witnessed (they are internal) andvarious mental processes take various amounts oftime, depending on the level of complexity involvedin the process (or number of transformations neces-sary to perform the task). That is, the physical judg-ment was the easiest and fastest, the type judgmenttask was the most difficult and slowest, and the iden-tity task (naming) was somewhere in between.

Another key technique that is often consideredunder the cognitive domain of cognitive neuroscienceis neuropsychology. This is a science that seeks tointegrate psychological observations of behavior andmind with neurological observations of the nervoussystem and the brain. This field is closely related toneurology, a branch of medicine concerned withunderstanding and treating disorders of the nervoussystem. One difference is that neuropsychology isspecifically concerned with understanding and treat-ing cognitive, perceptual, and other psychologicaldeficits related to nervous system compromise. Suchdeficits can come in the form of vascular disorderssuch as strokes or bleeds, tumors, trauma, anddisease.

These deficits of structure can also cause deficits offunction, which is where cognitive neuroscience canbenefit. The real strength of cognitive neuroscience isthe ability to take a variety of seemingly disparatetechnologies and bits of data and integrate theminto a coherent explanation of how brain enablesmind. One of the most fruitful paradigms in cognitiveneuroscience that depends on converging datainvolves the effects of brain lesions on behavior. Fun-damental concepts, such as the notion that languageresides primarily in the left hemisphere and the factthat the left hemisphere controls the right side of thebody (and vice versa), are grounded in such a combi-nation of techniques. The logic is pretty straightfor-ward. If mental processes and behavior depend onprocessing within a certain brain region, then damage


to this region should impact the processes and behav-ior. Unfortunately, it is not that straightforward. Justbecause behaviors are disrupted following brain dam-age does not necessarily mean that the region dam-aged is responsible for the disrupted process, onlythat it is involved in the processes.This is why cognitive neuroscientists often design

research paradigms that involve at least two tasks, andtwo different types of brain compromise. Such a com-bination of techniques provides the most informationbecause it is useful in dissociating structure–functionissues. This notion of separating or dissociating vari-ous mental processes into relatively independentlyfunctioning units is quite attractive. A classic exampleof this approach can be illustrated by considering thefindings of Broca and Wernicke. Broca’s patient hadtrouble with speech output, but his language compre-hension was intact. Wernicke’s patient has problemswith language comprehension, but his speech outputwas intact. These were two different language disor-ders in two different people. Behaviorally, these twolanguage functions are dissociable. By adding thecomponent of neuroanatomy – what part of thebrain is compromised – the dissociation is strength-ened. A modern example of a similar double dissocia-tion can be illustrated with the phenomenon ofblindsight. Blindsight is extremely interesting becauseit allows cognitive neuroscientists to pursue what isperhaps the highest level of cognition possible by thehuman brain – consciousness – in an establishedpatient model. Blindsight comes about followingdamage to cortical regions of the primary visual path-way. Such damage results in corresponding regions ofblindness in the visual field. Under certain testingconditions, some patients demonstrate residual visionwithin this area of blindness – residual vision withoutawareness. That is, they can respond correctly to somestimuli while denying actually having ‘seen’ the sti-muli. Hence the term blindsight.Interestingly, few patients with such damage dem-

onstrate blindsight, and those who do so demonstratethe phenomenon differently. A study by Wessingerand colleagues showed this in terms of a dissociation:one participant (FN) can detect direction of motionbut cannot detect differences in shape, whereasanother participant (CLT) can detect differences inshape but not differences in motion. These data sug-gest a dissociation of motion and shape processing inblindsight. Interestingly, MRIs demonstrate differ-ences in the location and extent of their damage. FNhas greater damage in his inferior occipital lobe andCLT has greater damage in his parietal lobe – bothhave limited sparing around the calcarine fissure.These data suggest that it is not the damage toprimary visual cortex that results in differences in

blindsight; rather, it is compromise of higher ordervisual processing (i.e., extrastriate) modules. CLT’smotion module is damaged, and thus does not havemotion blindsight, but it does have shape blindsightbecause his form regions are less damaged. The oppo-site is true for FN: his form processing capabilities aretoo damaged to provide any useful form processingdata, but his motion processing capabilities still pro-vide some limited information – information that issufficient to drive motion blindsight. Overall, thesedata indicate that blindsight may depend on survivingremnants of primary visual cortex and vestiges ofconnections to surviving, albeit severely compro-mised, extrastriate visual modules.

Blindsight is not the only interesting exploration ofconsciousness undertaken by the multidisciplinaryapproach of cognitive neuroscience. In an extremelyinteresting combination of philosophy and neurosci-ence, Benjamin Libet has made a career of investigat-ing consciousness.

In a groundbreaking and often controversial seriesof experiments, Libet investigated the neural timefactors in conscious and unconscious processing.These experiments are the basis for his ‘backwardreferral hypothesis.’ Libet and colleagues concludethat awareness of a neural event is delayed �500 msfollowing the onset of the stimulating event, andmore important, this awareness is referred back intime to the onset of the stimulating event. Thus, weare not aware of the event until after it occurs,although we often think we are aware of the eventfrom the onset. Fortunately, backward referral of ourconsciousness is not so delayed that we act withoutthinking. The actual beginning of the act occurs suffi-ciently after the awareness of the intent to act, givingus time to override inappropriately triggered behav-ior. This ability to detect and correct errors is whatLibet believes is the basis for free will. Cognitiveneuroscience is investigating consciousness and freewill and making headway.

Certainly, this is not an exhaustive review andexplanation of the methods that converge to formthe field of cognitive neuroscience. In fact, there area whole host of animal techniques not covered. Bydefinition, the field of cognitive neuroscience – amultidisciplinary science that borrows from manyseemingly disparate fields in order to develop coher-ent explorations of the neurobiological underpin-nings of cognition – is dynamic. The field is inconstant flux, always expanding and growing andexploring new questions related to furthering ourunderstanding of how brain enables mind. To some,such constant change may seem disconcerting, butto others it is what makes cognitive neuroscienceexciting. Such constant change is in fact why cognitive


neuroscience came about in the first place. By com-bining the best of many disciplines, cognitive neuro-science is asking and answering interesting questionsabout how the brain enables mind, putting the field atthe forefront of helping humans understand theirplace within the world.Given that scientists from many seemingly dispa-

rate fields were already pursuing the common goal ofunderstanding how brain enables mind, why wasit necessary to formally define a multidisciplinaryapproach that has now become known as cognitiveneuroscience? It is hoped that by raising excitingissues ranging from the development of the neurondoctrine to the use of cytoarchitectonics to relatebrain morphology to autism, from the collection ofbehavioral data in cognitive paradigms to the integra-tion of converging data in dissociation paradigms,and the combination of philosophy and neuroscienceto understand consciousness, this article helps youunderstand why cognitive neuroscience emerged as anew multidisciplinary field of study. In sum, it was toaccurately represent the merging of many differentbrain sciences, ranging from a neuroscientific andstructural approach to a cognitive and functionalapproach. A neuroscientific approach is concernedwith understanding the basic mechanisms of neuralaction that result in processing and communicationof information within neuronal assemblies. It is alsonecessary to consider the anatomical substrates orstructures that are composed of these neuronal assem-blies. A cognitive approach is concerned with charac-terizing the cognitive processes that lead to specificfunctions or behavior. Cognitive neuropsychology isconcerned with similar characterizations with theadded dimension of trying to understand what struc-tures are necessarily involved in various cognitiveprocesses. Similarly, cognitive science is concernedwith characterizing the computational algorithms nec-essary for such cognitive processes. By incorporatingthe concerns of these wide-ranging disciplines, cogni-tive neuroscience is more than just a sum of its parts;rather, it is a stronger, more comprehensive approach

to understanding how the brain enables the mind –one that begins at the molecular level and continuesto the problem of understanding human consciousexperience.

See also: Cerebral Cortex; Cognition: An Overview of

Neuroimaging Techniques; Cognitive Control and

Development; Contextual Interactions in Visual

Perception; Electroencephalography (EEG); Executive

Function and Higher-Order Cognition: Neuroimaging;

fMRI: BOLD Contrast; History of Neuroscience: Early

Neuroscience; Neuroanatomy Methods in Humans and

Animals; Neuroimaging; Phrenology; Positron Emission

Tomography (PET).

Further Reading

Banich MT (2004) Cognitive Neuroscience and Neuropsychology,2nd edn. Boston: Houghton Mifflin.

Farah MJ and Feinberg TE (eds.) (2000) Patient Based Approachesto Cognitive Neuroscience. Cambridge: MIT Press.

Finger S (1994) Origins of Neuroscience: A History of Explora-tions into Brain Functions.New York: Oxford University Press.

Gazzaniga MS (2000) (editor-in-chief) The New Cognitive Neuro-sciences. Cambridge: MIT Press.

GazzanigaMS, Ivry RB, and MangunGR (2002)Cognitive Neuro-science: The Biology of the Mind, 2nd edn. New York: Norton.

Hutsler JJ, Love T, and Zhang H (2007) Histological and magnetic

resonance imaging of cortical layering and thickness in autism

spectrum disorders. Biological Psychiatry 61: 449–457.

Kandel ER, Schwartz JH, and Jessell TM (1991) Principles ofNeural Science, 3rd edn. New York: Elsevier.

Kosslyn SM and Koenig O (1992) Wet Mind: The New CognitiveNeurosciences. New York: Free Press.

Libet B (2005)Mind Time: The Temporal Factor in Consciousness.Cambridge, MA: Harvard University Press.

Posner MI and Mitchell RF (1967) Chronometric analysis of clas-

sification. Psychological Review 74(5): 392–409.

Rains GD (2002) Principles of Human Neuropsychology. Boston:McGraw-Hill.

Wessinger CM, Fendrich R, and Gazzaniga MS (1999) Variability

of residual vision in hemianopic subjects. Restorative Neurol-ogy and Neuroscience 15: 243–253.

Wessinger CM, Fendrich R, and Gazzaniga MS (2005) Cognitive

neuroscience: What is it and why? In: Adelman G and Smith BH

(eds.) Encyclopedia of Neuroscience, 3rd edn. (on CD-ROM).Oxford: Elsevier.

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