Download Outline for cognitive neuroscience Chapter 1 Introduction to Method

Outline for cognitive neuroscience
Chapter 1 Introduction to Method
 Outline and page number
 Experimental methods in cognitive neuroscience
 1. Animal experiment (P106-112): single cell recording, lesion, *genetic
manipulation
 2. *neurology
 3. Converging methods
 A. cognitive deficits following brain damage(P124-126): single vs double
dissociation
 B. TMS (P128)
 C. Functional imaging: ERP(P129-134), MEG(P135), PET and fMRI(P136-143)
 Gist
 Single-cell recording
 This method enable researchers to describe response characteristics of individual
elements(precise and causal).
 Single-cell recording is done extracellularly in order not to damage the neuron
measured.
 Background noise vs action potential: neurons in different areas and circumstance
have different background firing rate, that is the neuronal firing rate when no
stimulus is presented. This renders the exact firing rate of neurons meaningless,
what counts is the change of firing rate in different conditions.
 Single-cell recording is essentially a correlational approach. Stimuli-firing rate ->
response characteristics;
 Receptive field of neuron: the function of neuron firing rate that depends on the
spatial-temporal variables of stimuli. In visual system, the RF is smallest in the
primary visual cortext and become larger toward higher visual cortex.
 Topographic representation: orderly relation between the RF properties of
neurons and the external world.
 Limitation: being a correlational approach, it is hard to establish casual
relationship.
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Lesions
 Basic method: remove one or more parts of the brain and measure the change of
behavior.
 Underlying logic: if a neural structure contributes to a task, then rendering the
structure dysfunctional should impair the performance.
 Problems with lesion study:
 Lesion may force the animal to alter the operation of intact structure
 Lesion may cause the animal to develop a compensatory strategy to minimize
the consequences of the lesion.
 Newer method:
 Neurochemical lesions
 Cooling
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Cognitive deficits following brain damage
 Why we study patient with brain lesion?
 The complex cognitive task require the integrative activity of many
component operations. Patient with specific brain lesion may lost the ability
of one particular operation.
 Study dysfunctional behavior can help identify the component operations
that underlie normal cognitive performance.
 Keep in mind: the challenge for the cognitive neuroscience is to determine
whether the observed behavioral problem results from damage to a particular
mental operation or is secondary to a more general disturbance.
 Single vs double dissociations
 Single dissociation: two groups (patients with a particular lesion & control),
two task(one highly depend on the interested cognitive operation while the
other relatively irrelevant to that operation). The performance of
experimental group is significantly worse than the control group. But the
difference may result from more general factors such as the relative difficulty
of the two tasks.
 Double dissociation: two (or more) groups(patients with a particular lesion &
patients with another type of lesion), two task (one highly depend on the
interested cognitive operation while the other on another operation). The
performance of Group A is worse than that of Group B in task A, whereas the
performance of Group B is worse than that of Group A in another task. Strong
evidence that a group of patient has a selective deficit in a certain operation.
TMS
 Temporarily disrupt the function of a selective brain region(1-1.5 cm2)->
degeneration of a certain behavioral performance: causal relationship between
the function of that region and that performance. Also can investigate the
time-course of a cognitive process: disrupt certain brain area at different intervals
after on-set of that process.
 Limitation of TMS:
 only able to study the simple and straightforward processes.
 Only able to study the function of cortical areas that are superficial surface of
the brain.
ERP(EEG)
 Principle: a particular cognitive process will induce a convergent brain activity and
thus a convergent scale potential pattern. This convergent pattern is superposed
onto the background activity, which is random over time. thus when averaging the
EEG waveform caused by many same process will render the noise vanishing and
the waveform that reflects the specific cognitive process salient.
 Advantage of ERP: Since the brain potential follows neuronal activity immediately,
ERP provides a precise temporal record of underlying neural activity.
 Problem with ERP: poor spatial resolution: neural generator is difficult to localized
from the potential distribution on the scalp.
ERF(MEG)
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Principle: active neurons also produce small magnetic fields that propagates to the
scale without distortion. Record the time course and distribution of magnetic
fields time-locked to specific event-> event-related field.
 Advantage of ERF: similar temporal resolution with ERP but higher spatial
resolution.
 Problems with MEG
 It is only possible to detect current flow that is oriented parallel to the surface
of the skull;
 Too expensive.
PET and fMRI
 Fundamental principle: measure the changes in metabolism or blood flow in the
brain while the subject is engaged in cognitive tasks. Therefore, these methods are
correlational.
 PET
 Underlying assumption: there will be increased blood flow to the brain
regions that engaged in cognitive processes. Measured index: regional
cerebral blood flow(rCBF)
 Inject tracer to the blood. detect the gamma ray emitted by the isotope in the
blood.
 Spatial resolution: 5-10 mm3 ; temporal resolution: tens of second.
 fMRI
 underlying assumption: the active neurons require more oxygen to fulfill their
energy consumption-> increased blood flow carry much more oxygenated
hemoglobin than the neurons can consume immediately-> increase of the
ratio of oxygenated to deoxygenated hemoglobin-> oxygenated and
deoxygenated hemoglobin have different magnetic properties that can be
detect by the scanner. Measured index: blood oxygenation level dependent
effect, BOLD.
 The reason for the prevalence of fMRI(comparison with PET):
 MRI scanner is more available than PET
 Methodological advantage:
 Relatively high spatial and temporal resolution of fMRI
 Same subject can be measured repeatedly->statistical analysis of
single subject
 BOLD can be time-lock to specific event, allowing more flexible
experimental design.
 Problem with PET and fMRI
 Correlational approach: correlation does not imply causation.
 Poor temporal resolution in comparison with ERP