Dominant Hemisphere Identification

• Handedness – tells us likelihood of LH being dominant (i.e.,

location of speech center)• 96% in right handers• 70-85% in left handers

• Behavioral tests

• Functional neuroimaging

• Clinical tests – Wada, TMS

Language Processing

• Speaking a written word involves at least five neocortical areas. Each area performs certain functions

Brain areas involved in Language

Visual Pathway

Lateralized Eye Movements

• Three synonyms for walking or intelligence

• Define impish or prudish

• Which direction does Thomas Jefferson face on the nickel?

• Which states share a border with North Carolina?

Lateralized Eye Movements – Interpreting LEMs

• Leftward movement from viewer’s perspective indicates LH activation (RVF squashed as LH taxed)

• Rightward mvt = RH location of function

Gross Laterality Tests

• Comparative (primary) tasks– Differences to lateralized

presentations• Accuracy and reaction time to emotional

matching, abstract words, etc

• Competitive (secondary) tasks– Finger tapping during math,

emotion, language tasks– Dowel balancing task

Tapping during nursery rhyme

Laterality of Auditory Processing

Selectively deliver to one hemisphere but suppressing ipsilateral pathway

Monoaural vs Dichotic Listening

•With dichotic input the ipsilateral ear’s input is suppressed.

Left ear advantage for melodies, right ear advantage for shadowing spoken letters

Dichotic Listening Results

• Right Ear Adv– Digits– Words– Nonsense syllables– Morse code– Pitch changes in Thai by

Thais– Voicing & Place– Difficult rhythms– Ordering temporal

information– Backward speech

• Left Ear Advantage– Melody– Musical chords– Environment Sounds– Emotional Sounds– Prosody– Complex Pitch changes

• No advantage

– Rhythms– Vowels

Dichotic Listening in Unusual Cases

• Genie (neglected/linguistically deprived) shows a left ear (RH) advantage for words

• Right hemispherectomy show normal right ear (LH) advantage for syllables

• Split brains show normal right ear advantage for digits

Hemisphericity

• Does one hemisphere dominate individual’s cognition or cognitive style?

Street Test of Right Hemisphere Dominance

Mooney (1957) – ID age & gender

Left hemisphere dominance

Similarities Test (selected items)

• Orange

• Coat

• Wagon

• Wood

• Egg

• Poem

• Fly

• Banana

• Dress

• Bicycle

• Alcohol

• Seed

• Statue

• Tree

Thompson, Bogen, Marsh, 1979

• Industrial cultures LH>RH

• Non-industrial cultures RH>LH

Gazzaniga’s Interpreter Model

LECTURE 7Homotopic Callosal Connections

Equipotentiality hypothesis vs homotopic principle

EEG site pairings

Callosal Connections

Principle of Callosal Homotopy

• The general principle of callosal homotopymthat the corpus callosum unites "corresponding and identical regions" (Meynert, 1872, p. 405), was initially proposed by Arnold (1838-1840) in his anatomy tables and later popularized by Meynert (1872).

• Bruce (1889-1890) criticized Meynert's endorsement, calling it speculation and opinion, ungrounded in physiological fact.

• Bremer (1958), however, continued to advance this principle, based on the anatomical and electro-physiological research of his day (Curtis, 1940a,b).

Principle of Callosal Homotopy• CITATIONS• Arnold, F (1838-1840). Tabulae anatomicae. London: Black &

Armstrong. • Bremer, F. (1958). Physiology of the corpus callosum. Research

Publications for the Assessment of the Nervous and Mental Disability, 36, 424-428.

• Bruce, A. (1889-1890). On the absence of the corpus callosum in the human brain, with description of a new case. Brain, 12, 171-190.

• Curtis H.J. (1940a). Intercortical connections of > corpus callosum as indicated by evoked potentials. Journal of Neurophysiology, 3, 407-413

• Curtis H.J. (1940b). An analysis of cortical potentials mediated by the corpus callosum. Journal of Neurophysiology, 3, 414-422.

• Meynert T (1872). The brain of mammals. In S. Stricker (Ed.) Manual of human and comparative histology, Vol II, (pp 367-537). London: The New Syndenham Society.

Principle of Homotopy

Four types of cortico-cortical projections:

1) homotopic, 2) homoareal, 3) heterotopic, and 4) ipsilateral

Arnold (1838-1840) – Anatomical tables – first mention of callosal homotopic connectivity

Myers (1850s) – popularized homotopic principle

Bremer (1956) – “general principle of homotopy” based on Curtis (1940;1944) electrophysiological studies

Reciprocity in Callosal Connections

• Representation of the reciprocity of callosal connections: strong homotopic connectivity, and wherever there is heterotopic connections, there is normally ipsilateral connections to the same areas.

Callosal Function Models

• 1. Transfer of information

• 2. Inhibition of opposite side processing

• 3. Homotopic inhibition, generating complementary percepts

Conduction Time

Conduction Time in Split Brains

Anatomical asymmetry

• LH contains – more gray matter

• Larger cells and greater cell density (but not all areas), more nonmyelinated fibers esp. frontally, suggesting more localized, more serial processing

• RH contains– More white matter

• more myelinated axons to link different brain regions

– slightly larger and heavier than LH

Cell Density differencesHuman SMG (posterior language areas)

Larger LH pyramidal cells in Superior Temporal Gyrus

• HOWEVER asymmetry is not found in nearby angular gyrus

Homotopic inhibition theory

• Priming explained

LECTURE 8Functional Dichotomies

Aphasia by handedness & hemisphere damaged

• Right handed: LH 60%, RH 2%

• Left handed: LH 32%, RH 24%

Split Brain PatientsSplit Brain Patients

PREOPERATIVE

POSTOPERATIVE

RIGHT HANDLEFT HAND

RH superiority onBlock design

RH superiority onDrawing tasks

Local-global stimuli used to investigate hierarchical representation

Facilitatory effect for global-local stimuli

STUDY INTACT BRAINS

Visual Laterality Method

• Tachistoscopic presentation (less than 200 ms)– Lateralized stimulus exposure– Compare performance LVF vs. RVF presentations.– Dependent variables: reaction time, accuracy

Methodology Issue

• 2 @ 2 = 3 or 4

• 2 @ 3 = 6 or 8

• 2 @ 4 = 8 or 16

• Analysis of correct responses only

Functional DichotomiesBlackburn Intellectual Sensuous

Oppenheimer Time, History Eternity, Timelessness

Levy, Sperry Analytic Gestalt

Bogen Propositional Appositional

Luria Sequential Simultaneous

Semmes Focal Diffuse

I Ching The Creative: Heaven,Masculine, Yang

The Receptive: Earth,Feminine, Yin

Many sources Verbal Spatial

Many sources Intellectual Intuitive

Jung Causal Synchronicity

Bacon Argument Experience

• Left Hemisphere• Verbal• Sequential, temporal,

digital, routinized• Logical, analytic• Familiar• Propositional

• Right Hemisphere• Nonverbal,

visuospatial• Simultaneous, spatial,

analogical, parallel, integrative

• Gestalt, holistic, synthetic

• Novelty• Appositional