UofR: Neural Basis of Cognition Lecture 2 Methods, Part II Hemispheric Specialization 25 May 2010

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 N 2 ( ms): Mismatch negativity P 3 ( ms): related to attention and updating of memory (“oddball paradigm”) N 4 ( ms): “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 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