IMPORTANT: This is a detailed summary of the literature relevant for the Cognitive Neuroscience Exam. It covers the required chapters from the book "Principles of Cognitive Neuroscience" as well as the additional assigned readings.
Emphasis is placed on concepts that the lecturer finds interestin...
Brain Perturbations That Elucidate Cognitive Functions
1. Perturbations imposed by stroke, trauma, or disease: correlating a patient’s signs,
symptoms and behavior during life with the location of brain lesions discovered during
autopsy
→ Limitations: brain damage is the result of many factors that are not under
experimental control; distribution of brain regions supporting cognitive functions
varies among individuals making it difficult to generalize results (can be
addressed by combining information about locus of damage across group of
patients)
→ Surgical lesions (in animals): problems with interpretation + ethical concerns
→ Diaschisis: if one area of the brain is lesioned other areas innervated by the
damaged area may form the loss of input cease to function normally
→ Damage to cortical area can damage nearby fiber tracts, thereby disrupting the
function of more distant areas
2. Pharmacological Perturbations: administering drugs that interfere with/augment release
of/ response to neurotransmitters at synapses (e.g. caffeine, cocaine, antidepressants)
→ Examine influence of chronic drug (ab)use on cognitive processes; critical in
advancing understanding of brain regions and cellular mechanisms involved in
normal sense of reward
→ Administering a drug acutely and monitoring effects on cognitive functioning
(disadvantage: relative lack of specificity of the effect bc whole brain is exposed
to drug)
3. Perturbation by intracranial brain stimulation: direct electrical stimulation of specific brain
region; electrodes are placed onto or into the brain of human or animal
→ Electrodes placed transiently (during surgery) or chronically
→ Chronically implanted electrodes allow assessment of function of individual
neurons as the animal performs a cognitive task it has been trained to do
→ Altering strength of stimulation can vary response of the neuron
4. Perturbation by extracranial brain stimulation: less invasive than intracranial stimulation
→ Transcranial magnetic stimulation (TMS): strong but transit and rapidly changing
magnetic field is generated over a region of the scalp by passing an intense,
rapidly varying electrical current through a set of coils → induces a rapidly
changing electrical field in underlying brain tissue resulting in extraneous flow of
current that transiently interacts with local neural processing (creates a reversible
brain “lesion”; weaker stimulation sometimes activates underlying area)
, → Repetitive TMS (rTMS): series of TMS pulses over several minutes is applied →
effect on cognitive function is tested by behavioral tests administered during and
after the TMS application
→ TMS can either impair or improve performance on tasks involving stimulated
area, allowing for inferences about the role of that area in performing the task
→ Delivering single TMS pulse to brain area at specific times during the course of a
task trial, then study its influence on task performance on that trial (advantage:
provides greater temporal resolution)
→ Drawbacks of TMS: affects relatively large brain area which limits spatial
resolution; can only delivered to superficial brain regions; can result in concurrent
stimulation of scalp and head muscles (painful twitching); stimulation entails risks
(e.g. might trigger a seizure)
→ Benefits of TMS: relatively good temporal resolution, non-invasive
→ Direct current stimulation (tDCS): constant, low-amplitude, electrical current is
applied directly to the scalp with two electrodes (anodal/positive stimulation
thought to increase cortical excitability; cathodal/negative stimulation thought to
decrease excitability); low spatial resolution; can be used to treat chronic pain
5. Optogenetics: combines genetics with use of laser light to activate specific neural circuits
or neuronal cell types
→ High neuronal selectivity and high temporal resolution
→ Incorporating ion channels that open/close in response to light of certain
wavelength into neurons of interest (so they can be turned on/off with light)
→ Genetic material that codes for light sensitive ion channels comes from algae and
is inserted into a virus which is injected into particular brain region → infects brain
area and leads to production of these ion channels
→ Enormous potential for cognitive neuroscience as well as for treatment of certain
clinical disorders (e.g. parkinson’s)
Measuring Neural Activity during Cognitive Processing
1. Direct electrophysiological recordings from neurons: measuring action potentials
produced by individual neurons (generally done in monkeys or mice for less complex
cognitive functions)
→ Extracellular recording: electrodes are inserted into extracellular space in
cerebral cortex or in deeper brain structures → monitor electrical activity
associated with action potentials generated by one/more neurons near the tip
(gives information about behavior of small group of nerve cells)
→ Intracellular recording: electrodes with a finer tip inserted in single neuron →
records electrical activity from within the cell as action potentials and
post-synaptic potentials are generated (provides more detailed information about
how single neurons behave during cognitive functions)
→ Peristimulus time histogram (PSTH): stimulus is presented several times;
neuron’s responsiveness to stimulus determined by temporally aligning
, responses following each of the trials and then summing number of action
potentials across trials into a histogram time-locked to the stimulus (shows neural
firing pattern across time in response to stimulus)
● Averages out random background firing, giving clearer picture of average
response specifically related to stimulus processing
→ Time curves: stimulus is varied along particular dimension and strength of
response is plotted as a function of the stimulus parameter being varied (curve
defines selective sensitivity of the cell to some values of the stimulus parameter
relative to others)
→ Multielectrode recording arrays: evaluates concurrent responsiveness to set of
neurons in given brain area; single electrode has multiple recording points along
its length which enables simultaneously recording multiple neurons across brain
region of interest
2. Electroencephalography (EEG): electrical potentials are recorded at different positions
over the scalp relative to the voltage at a reference electrode
→ Noninvasive
→ EEG signal derives from summed dendritic field potentials of groups of neurons
that are varying together (not action potentials from single cells)
→ Excitatory synaptic input to the dendrites near the cell body depolarizes the
voltage across membrane of the underlying neuronal dendritic trees → resulting
separation of charge causes intracellular current flow along inside of the dendrite
and conduction of the return current through the tissue outside the dendrite
→ Scalp electrodes detect fluctuating voltages associated with dynamically
changing return currents (local field potentials/LFPs)
→ LFPs reflect more of the integrative processing of large cortical neurons rather
than the output of firing of the cell to other regions that is reflected in spike
recordings
→ Ongoing EEG signals measured over time used to assess various aspects of
brain function; typically analyzed in terms of the power in various frequency
bands at each electrode location (major bands of interest are delta, theta, alpha,
beta, gamma, and high gamma)
→ Relative power of frequency bands along with other aspects of EEG is useful for
assessing overall state of the brain (e.g. arousal level, sleep stages); detecting +
monitoring abnormal activity in epilepsy; however limited usefulness for
investigating specific cognitive functions
3. Event-related Potentials (ERPs): small voltage fluctuations in an ongoing EEG triggered
by sensory and cognitive events
→ reflect the summed electrical activity of neuronal populations specifically
responding to those events
→ High temporal resolution (milliseconds); esp. Useful for studies in which timing
and sequence of brain activity are important (e.g. attention to sensory input)
, → Because ERPs are smaller than EEG signal it is necessary to average multiple
trials to extract ERPs from background noise
→ ERPs are extracted by averaging those epochs of the EEG signal that are
time-locked to repeated occurrences of a specific sensory, motor, or cognitive
event
→ Measure of average evoked voltage changes over time, where time zero is the
time of occurrence of the event
→ Average trace obtained comprised series of negative and positive peaks (named
according to their electrical polarity → N or P; and their latency → N100=
negative peak 100 ms after stimulus onset)
→ Inverse problem: given distribution of electrical activity recorded at the scalp
could have been produced by any one of a number of different sets of generators
inside the head → especially for more complex activity distributions and/or for
studies without additional source-related information, must be viewed with
caution
4. Magnetoencephalography (MEG): used to extract time-locked responses, called
event-related magnetic-field responses (ERFs), from ongoing MEG signals
→ Both MEG/ERF and EEG/ERP signals arise from current flow triggered by
depolarizations in dendritic trees of cortical neurons oriented perpendicularly to
the cortical surface
→ Key difference: MEG measures magnetic fields produced by these currents
rather than associated voltage fluctuations
→ If the field strengths are measured at different points on the surface of the head
with a magnetometer, the distribution of these values over space and time can be
obtained, including whether they are positive (coming out of the head) or
negative (going into the head)
→ MEG is sensitive mainly to neuronal activity in cortical valleys (sulci) and
relatively insensitive to activity in gyri
→ Less complete than EEG but easier to localize MEG signals (because EEG
currents are distorted by skull and other tissue, and EEG signals are more
complex)
5. Positron emission tomography (PET) imaging: measures changes in concentration of
radioactive molecules in the bloodstream in relevant brain areas
→ Molecules labeled with radioactive probe atoms (e.g. 15O) are injected into the
bloodstream → travel to areas of increased neural activity because of increased
metabolism and thus blood flow to those areas
→ Radioactive decay (collision of positron with electron) emits two “annihilation
protons” (gamma rays) in opposite direction from collision which are detected by
sensors surrounding the head of the subject laying in the PET scanner
→ Location of active regions can be imaged by reconstructing collision lines using
computer algorithms
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