Transcript UofR: Neural Basis of Cognition Lecture 2

UofR: Neural Basis of Cognition
Lecture 2
Methods, Part II
Hemispheric Specialization
25 May 2010
Electromagnetic Recording Methods
• Electromagnetic Recording Methods
– Electroencephalography (EEG)
– Event-Related Potentials (ERPs)
– Magnetoencephalography (MEG)
• Optical Recording Methods
Electroencephalography (EEG)
• Method of recording the brain’s electrical
activity
• Metal electrodes are evenly spaced on the
scalp
• Each electrode records the electrical potential,
the sum of electrical neural activity, where it is
placed, and amplifies it (units: Hertz, Hz)
Electroencephalography (EEG)
• The electrical potential under every electrode
oscillates
• Brain activity:
– Alpha: 9 – 12 Hz, predominates when relaxed
with eyes closed
• Alpha suppression: indicator of degree of activation in
the brain
– Beta: ~15 Hz, predominates while awake and alert
– Delta: 1 – 4 Hz, predominates during sleep
– Gamma: 25 – 100 Hz, unknown role
Electroencephalography (EEG)
• Normally, neurons fire in a synchronized
manner (alpha/beta/delta waves)
• Epilepsy: neurons fire in bursts at random
times
Event-related potentials (ERPs)
• Recordings of the brain’s activity that are linked
to the occurrence of some event
• Gives an idea of “when” processes occur in the
brain
• As time passes after a stimulus, the active group
of neurons changes and the EEG waveforms
change accordingly
• The waveform can be divided into components,
characteristic portions of the wave that have
been linked to certain processes
Event-related potentials (ERPs)
• ERP components are usually given names: a
letter and a subscripted number; the letter is
either P (for positive) or N (for negative)
• Exogenous components are linked to the
physical characteristics of a waveform and
occur early in the waveform
• Endogenous components appear to be
independent of stimulus characteristics and to
be driven by internal cognitive states
Event-related potentials (ERPs)
• <100ms: sensory processing; an abnormality in
early ERPs indicates a disruption in the sensory
system
• ~100ms: no longer driven solely by sensory
information, modulated by attention
• N2 (200-300ms): Mismatch negativity
• P3 (300-800ms): related to attention and updating
of memory (“oddball paradigm”)
• N4 (400-600ms): “Running out the door, Patty
grabbed her hacker, her baseball glove, her cap, a
softball, and a lamp.”
Magnetoencephalography (MEG)
• Records induced magnetic potentials instead of electric
potentials, allowing location of activity to be
pinpointed
• Uses superconductors
• Used to localize source of epileptic activity, to locate
primary sensory cortices to be avoided in surgery, and
for more general studies such as of schizophrenia
• Example: M100 stimulus generated in atypical location
in Heschl’s gyrus, supporting that paranoid
schizophrenics have difficulties in filtering early sensory
information properly
Techniques for Analyzing Behavior
• Test batteries
• Estimate of Premorbid Functioning
Computational Neuroscience
• Neural Network
– Input, hidden, output neurons
– Synaptic weights
– Use in computer science
– Hebbian processes
• Perceptron: single-layer neural network
Hemispheric Specialization
• First evidence: Broca’s area, 1860s
• Right frontal lobe: extends farther forward, is
wider
• Left occipital lobe: extends farther back, is wider
• Sylvian fissure: for most people, extends father in
the horizontal direction in the left, takes more of
an upward turn on the right
• Brodmann’s area 29 is larger on the left
• More NE in certain regions of right thalamus,
more DA/DARs in the basal ganglia on the left
Hemispheric Specialization: Methods
• Divided visual field technique
– LVF/RVF project exclusively to the contralateral
hemisphere
– Unilateral presentation
– Bilateral presentation to test perceptual
asymmetries
• Dichaptic presentation
– Analogous but for tactile modality: objects placed
in both hands simultaneously and a person is
asked to somehow identify/distinguish the items
Hemispheric Specialization: Methods
• Dichotic presentation
– Analogous but for auditory input
– Different auditory information is given to each ear
– Competition between inputs leads to suppression
of ipsilateral sensory information all\most entirely
in favor of contralateral sensory information in
each ear
Hemispheric specialization: Findings
• Visual:
– RVF/left hemisphere advantage for words
– LVF/right hemisphere advantage for faces
• Tactile:
– Right hand/left hemisphere advantage for identifying letters
drawn on the palm
– Left hand/right hemisphere advantage for identifying complex
• Auditory:
– Right ear/left hemisphere advantage for responding to words
– Left ear/right hemisphere advantage for responding to nonlinguistic stimulus
• To grossly generalize, the left hemisphere is “verbal” and
the right is “nonverbal”
Split-Brain syndrome
• Corpus callosum is critical in interhemispheric
communication
• Split-brain procedure for epilepsy
• Objects could verbally identify objects by
touch only if they were held in the right hand
• Chimeric images
• http://www.youtube.com/watch?v=ZMLzP1VC
ANo
Interhemispheric Communication
• Simple tasks are performed faster
intrahemispherically
• More complex tasks are performed faster
interhemispherically
Interhemispheric Communication
• Corpus callosum: 250m fiber topographic line
of communication between the hemispheres
• There are other subcortical commissures but
they are not as important
– For example, callosotomy patients cannot
determine whether faces presented in each of
their visual fields are the same person
– Some information can be transferred, e.g.
whether the face was old or young
– Callosal transfer time: 5-20ms
Individual Differences in Brain
Organization
• Handedness
– 90% of population is right-handed
– Population of right-handed people:
• 95% have speech output controlled by left hemisphere
• 5% by right
– Population of left-handed people:
• 70% have speech output controlled by right
hemisphere
• 15% by right
• 15% by both
Individual Differences in Brain
Organization
• Gender
– Difficult to determine gender-specific aspects of
brain function
– Little to no conclusive evidence of anything apart
from brain size