UofR: Neural Basis of CognitionLecture 2

Methods, Part IIHemispheric 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 non-linguistic

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=ZMLzP1VCANo

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