Neuroeconomics || Experimental Methods in Cognitive Neuroscience
Post on 15-Dec-2016
Embed Size (px)
C H A P T E R
Experimental Methods in CognitiveNeuroscience
Christian C. Ruff and Scott A. Huettel
O U T L I N E
Introduction 77Measurement Versus Manipulation 78Strengths and Limitations of Different Methods 78
Measurement Techniques 79Invasive Neurophysiology: Single-Unit Recording 79Non-Invasive Neurophysiology 81Metabolic Neuroimaging 84
Manipulation Techniques 92Brain Stimulation 92Lesion Studies 101
Conclusion: Convergence across Methods 105
The growth of cognitive neuroscience as an aca-demic discipline has been inextricably tied to thedevelopment of its research methods. These methodsnow provide unprecedented access to brain structureand function. When combined with theoretical per-spectives from psychology, economics, and other disci-plines, they allow us to generate new models offunctions like memory, attention, and decision making.As these methods have become more refined, theyhave also become more accessible to the research com-munity. Experiments that would have been impossiblea decade ago are now readily conducted as graduatestudent projects. The increased power, flexibility, andaccessibility of these techniques have had unques-tioned benefits for scientific progress: each year,several thousand scientific articles are published usingthe core methods of cognitive neuroscience.
However, the growth of cognitive neuroscience hascarried an unexpected cost. It has become possible forinexperienced researchers to design and carry out cog-nitive neuroscience experiments without having adeep understanding of the underlying brain functionor of what they are recording. This accessibility canhave undesirable consequences. When data are being
collected or analyzed, errors may go undetected, lead-ing to inaccurate results. Research results may lead tooverstated or implausible claims or may be reinter-preted to fit a previously held view. A poor under-standing of methods can alter the very direction ofnew research. Researchers who become too focused ona single technique may apply that technique indiscrim-inately, regardless of whether it is appropriate for theirspecific research question. Paradoxically, the advancesin cognitive neuroscience methods have made it easierfor researchers to make mistakes!
Any consideration of neuroscientific methodsshould begin with a fundamental observation: differ-ent techniques address different aspects of neural func-tion. This simple fact is often lost in populardescriptions of neuroscience, which often refer generi-cally to activity in a brain region that predicts somebehavior or trait. However, the interpretation of agiven result may strongly depend on what is beingmeasured: neuronal firing, brain metabolism, neuro-transmitter levels, or some other brain property.
Providing a comprehensive introduction to all of thediverse methods of cognitive neuroscience would gowell beyond the scope of this chapter. An in-depthunderstanding of any particular method would requirebackground knowledge of the neurophysiological
77Neuroeconomics. DOI: http://dx.doi.org/10.1016/B978-0-12-416008-8.00006-1 2014 Elsevier Inc. All rights reserved.
processes underlying the measured signals, the con-nections between brain structure and functionrecorded by the method, the biophysics and signalprocessing associated with the corresponding experi-mental hardware, and the statistical methods used totranslate raw data into inferences. This chapter intro-duces these topics at an elementary level and refersthe reader to excellent textbooks and primary researcharticles for more in-depth coverage. It focuses primar-ily on the conceptual issues involved in selecting aresearch technique and evaluating the data obtainedusing each technique. As such, it is primarily intendedfor those who are new to cognitive neuroscience andwho seek guidance on how to evaluate the strengthsand limitations of published work. Accordingly, eachtechnique is introduced in conjunction with specificexamples drawn from recent neuroeconomic studies.
Measurement Versus Manipulation
Cognitive neuroscience techniques can be dividedinto two main categories. Measurement techniques, asthe name implies, measure changes in brain functionwhile a research participant (human or animal)engages in some cognitive activity. A typical neuroeco-nomic experiment using a measurement techniquemight require the participant to make a series of sim-ple decisions while the researchers record changes inneuronal firing or metabolic activity that might differbetween, say, higher-value or lower-value choices.Measurement techniques are often described (some-times derisively) as being correlational because theycan show that signals from a brain region co-occurwith a function of interest, but they cannot show that aregion is necessary for that function.
Manipulation techniques, in contrast, examine howperturbations of the brains function either bytransiently changing neuronal firing rates or neurotrans-mitter levels or by permanently damaging tissue change cognitive functions or behavior. Accordingly,manipulation techniques are sometimes called causalapproaches. Neuroeconomists have used manipulationtechniques to disrupt processing in specific regions,which in turn alters the choices people make (e.g., ininteractive games).
This chapter follows this basic division, first intro-ducing techniques that measure changes in brain func-tion which track the variables within decision models,then considering techniques that change neural proces-sing and also decision behavior. It is important to rec-ognize that measurement and manipulation techniquesprovide distinct and complementary information aboutbrain function. Cognitive neuroscience research pro-gresses more quickly when measurement techniquesestablish links between brain structure and cognitive
function and then manipulation techniques probe thatrelationship to improve inferences and models.
Strengths and Limitations of Different Methods
How do neuroscientists determine which researchmethod to apply to a given research question? Broadlyconsidered, three factors have primary importance:temporal resolution, spatial resolution, and invasive-ness (Figure 6.1). Temporal resolution refers to the fre-quency in time with which measurements ormanipulations can be made. Techniques that recordneuronal activity directly through electrophysiologicalmeans tend to have very good temporal resolution(e.g., millisecond precision); techniques that measureindirect metabolic correlates of neuronal activity tendto have intermediate temporal resolution (e.g., secondsto minutes); and techniques that manipulate brainfunction through drug effects or brain lesions tend tohave the poorest temporal resolution (e.g., minutes todays). Spatial resolution refers to the ability to distin-guish adjacent brain regions that differ in function.Techniques that position electrode sensors directlywithin the brain have the highest spatial resolution(e.g., individual neurons or better); techniques of func-tional neuroimaging have intermediate spatial resolu-tion (e.g., millimeters to centimeters); and techniquesthat measure electrical signals that spread diffuselytend to have the lowest spatial resolution (e.g., centi-meters to the entire brain).
Finally, neuroscience techniques differ with respectto whether they can make measurements withoutdamage to or disruption of the brain (or other body tis-sue). Non-invasive techniques record endogenous brainsignals using sensors outside the body. Thus, thesetechniques can be conducted repeatedly in human vol-unteer participants, with no appreciable risk in partici-pation. Invasive techniques introduce a chemical orrecording device into the body. While some such tech-niques can be used in human volunteers (albeit withsignificant attention paid to issues of participantsafety), other invasive techniques can only be used inhuman patients (e.g., prior to neurosurgery) and/ornon-human animals.
This brief summary conveys the critical point that nosingle technique provides a comprehensive account ofbrain function. Different techniques provide comple-mentary information, some giving detailed spatial mapsof functions and others indexing very rapid changes inactivity when those functions are engaged. Every deci-sion process identified in this book has been exploredusing a range of neuroscience techniques, and converg-ing evidence from different techniques and researchparadigms has enabled more powerful conclusions thancould be obtained from any one approach in isolation.
78 6. EXPERIMENTAL METHODS IN COGNITIVE NEUROSCIENCE
The measurement techniques used within cognitiveneuroscience measure information transmission by neu-rons, either directly or indirectly. This section will considerfive such techniques that are organized by the aspect ofneural function they measure: single-unit recording,electroencephalography (EEG), magnetoencephalography(MEG), positron emission tomography (PET), and func-tional magnetic resonance imaging (fMRI).
As introduced in the previous chapter (Chapter 5),there are two types of neuronal information proces-sing: axonal signaling and dendritic integration. Whena neuron fires, it sends a signal called an action poten-tial down its axon to one or more other neurons. Theaction potential is evident as a small change in thevoltage of the axons membrane, and thus it can bemeasured with electrodes that are positioned immedi-ately adjacent to that neuron a technique known assingle-neuron or single-unit recording.
The action potential evokes the release of neurotrans-mitters at the synapse; when those neurotransmittersbind to receptors on the dendrites of a post-synapticneuron, they cause its membrane potential to becomemore positive