Last Lecture History of disconnection syndromes History of disconnection syndromes The structure of the corpus callosum The structure of the corpus callosum Independent minds in the same head? Independent minds in the same head?

REMINDER THIS WEEK’S DISCUSSION SECTION READING AT EXCEL

This Lecture Hemispheric differences in the Bisected Brain Hemispheric differences in the Bisected Brain Functions of the corpus callosum Functions of the corpus callosum Hemispheric Asymmetries in the Normal Brain Hemispheric Asymmetries in the Normal Brain Methods and theories Methods and theories The corpus callosum and resource allocation The corpus callosum and resource allocation

Right Hemisphere Spatial Abilities Revealed PRE OPERATIVE POST OPERATIVE RIGHT HAND LEFT HAND RH superiority on Block design RH superiority on Drawing tasks

Language LH Dominance: Note: Language abilities vary greatly among patients Speech production Speech production single word by RH in some patients single word by RH in some patients speech/reading comprehension: speech/reading comprehension: RH vocab ~ age 10 RH vocab ~ age 10 grammar comprehension: grammar comprehension: RH fails at more difficult grammatical constructions. RH fails at more difficult grammatical constructions. Active: The horse kicked the cow. Passive: The cow was kicked by the horse.

Hemispheric Differences in Matching Letters and Locations Compare verbal vs. spatial matching by LH and RH Compare verbal vs. spatial matching by LH and RH Task: Present target items (letters or location) followed by a probe Task: Present target items (letters or location) followed by a probe Does probe match a target item? Does probe match a target item? (Reuter-Lorenz & Miller 1998)

Hemispheric Asymmetries on Basic Visual Tasks VerbalSpatial percent correct LH RH

Corpus Callosum permits access to specialized processing LH dominance Language (comprehension/ production, reading) Language (comprehension/ production, reading) fine motor skills (writing, coordinated movements/gestures) fine motor skills (writing, coordinated movements/gestures) RH dominance Visuospatial/constructive abilities Visuospatial/constructive abilities manipulo-spatial tasks manipulo-spatial tasks Face processing Face processing Emotion perception Emotion perception

Why did previous studies miss these disconnection effects? Sperry’s lab ensured lateralized input Sperry’s lab ensured lateralized input Sperry’s patients had complete callosal section** Sperry’s patients had complete callosal section** This success inspired study of laterality in the normal brain. This success inspired study of laterality in the normal brain.

Hemispheric Differences in the Normal Brain

Major Dichotomies LEFT VERBAL VERBAL ANALYTIC ANALYTIC SEQUENTIAL SEQUENTIAL LOCAL LOCAL RATIONAL RATIONALRIGHT NONVERBAL/SPATIAL NONVERBAL/SPATIAL HOLISTIC HOLISTIC PARALLEL PARALLEL GLOBAL GLOBAL IMPRESSIONISTIC/ EMOTIVE IMPRESSIONISTIC/ EMOTIVE

Examples of Global/Local

Behavioral Asymmetries in Normal Subjects Visual Laterality Method Tachistoscopic presentation (less than 200 msec) Tachistoscopic presentation (less than 200 msec) lateralized stimulus exposure lateralized stimulus exposure Compare performance for left visual field (LVF) vs. RVF presentations. Compare performance for left visual field (LVF) vs. RVF presentations. Dependent variables: reaction time, accuracy Dependent variables: reaction time, accuracy

Visual Field Asymmetries Task: Does display contain H or L ? Task: Does display contain H or L ? Independent variables: Independent variables: 1) target level: local or global. 2) visual hemifield: LVF or RVF Dependent variable: RT Dependent variable: RT XXXX X X X X X X X XL L L L LL L L L Sample displays

Split versus Together...

Why do performance asymmetries emerge with an intact corpus callosum?

Info is processed by the hemisphere specialized for the material or task. input transferred to specialized hemisphere input transferred to specialized hemisphere VF Differences reflect interhemispheric transfer time and info degradation. VF Differences reflect interhemispheric transfer time and info degradation. Callosal relay model

Direct access model The hemisphere that gets the stimulus (directly) processes it VF differences reflect relative ability/efficiency of hemispheres. VF differences reflect relative ability/efficiency of hemispheres. Both hemispheres do everything more or less well Both hemispheres do everything more or less well

Attention is directed contralateral to the more active hemisphere. Engaging LH verbal processes biases attention rightward --> RVFadvantage Engaging LH verbal processes biases attention rightward --> RVFadvantage Engaging RH spatial/nonverbal processes biases attention leftward--> LVF advantage Engaging RH spatial/nonverbal processes biases attention leftward--> LVF advantage Activation-orienting model (Kinsbourne) bog + bog bog bog