BRAIN LATERALIZATION LANGUAGE AND COGNITION CEREBRAL LATERALIZATION u

BRAIN LATERALIZATION LANGUAGE AND COGNITION

CEREBRAL LATERALIZATION u Significant Events in History • Marc Dax (1836) u Dax was the first to report left hemisphere involvement in speech disorders caused by brain damage. • Paul Broca (1860’s) u Unaware of Dax’s work, Broca made key discoveries regarding left hemisphere involvement in aphasia. • Broca’s area, Broca’s aphasia • Hugo-Karl Liepmann (1900’s) u Apraxia is almost always caused by left hemisphere damage.

CEREBRAL LATERALIZATION u Assessments of Lateralization • Sodium Amytal Test • Dichotic Listening Test • Functional Brain Imaging

CEREBRAL LATERALIZATION u Speech lateralization and handedness • The left hemisphere is dominant for speech in majority, both right- and left-handed, although greater variability among left-handed individuals. u Neurological studies of aphasics (Russell & Esper, 1961). • Right handed aphasics: 60% left, 2% right hemisphere damage • Left handed aphasics: 30% left, 24% right hemisphere damage u Results of Sodium amytal tests (Milner, 1974). Left-hemisphere dominance for speech in: • 92% of right-handed individuals • 69% of left-handed or ambidextrous individuals • 30% of left-handed or ambidextrous individuals with early left hemisphere damage

CEREBRAL LATERALIZATION u Sex Differences and Lateralization • Some evidence suggests that the male brain is more lateralized than female brain. • e. g. , Mc. Glone’s (1977, 1980) studies of unilateral stroke victims and WAIS subscore tests • Some f. MRI studies show that females tend to use both hemispheres in languagerelated tasks more so than males.

SPLIT-BRAIN STUDIES u Meyers’ and Sperry’s work in cats u Split-Brain Patients • Commisurotomy to reduce seizure spreading. Tests involving verbal identification of stimuli presented to the left or right hemisphere. u Tests involving spatial stimuli presented to L or R hemisphere. u

Myers and Sperry (1953)

Testing Split-Brain Patients

HEMISPHERIC SPECIALIZATIONS u Examples of Lateralization • Left Hemisphere Specializations Language u Controlling ipsilateral movement u • Right Hemisphere Specializations Spatial ability u Emotion u Musical ability u • See table 16. 1 in Pinel for more examples

NEUROANATOMICAL ASSYMETRY u Planum Temporale • Larger in left hemisphere in most individuals Geschwind and Levitsky (1968) study u Witelson (1983) study • Asymmetry is present at infancy • Asymmetry of planum temporale in chimps and other apes u u Left planum temporale and perfect pitch

APHASIA u Definition: Acquired disorders of language secondary to brain damage u Common Subtypes • Broca’s aphasia • Wernicke’s aphasia • Global Aphasia • Conduction Aphasia

LANGUAGE AREAS

BROCA’S APHASIA u Characteristic symptoms • labored and poorly articulated speech • agrammatism (telegraphic speech) • anomia • agraphia (writing impairment) u Region of brain damage • left inferior frontal cortex, 3 rd frontal gyrus, anterior to face region of motor cortex (Broca’s area)

WERNICKE’S APHASIA u Characteristic symptoms • poor comprehension of spoken and written language • fluent and spontaneous speech, but incoherent • paraphasia (sound and word substitutions) • alexia (reading impairment) u Region of brain damage • left superior temporal gyrus (Wernicke’s area)

GLOBAL APHASIA u Characteristics • Total loss of comprehension and expressive abilities, involving both spoken and written language. • Some automatic speech, such as emotional exclamations retained u Damage is extensive • involves both B. and W. areas, large portions of frontal, temporal and parietal cortex.

CONDUCTION APHASIA u Characteristics • fluent speech, comprehension only slightly impaired • repetition primarily impaired (esp. novel or nonwords, or sentences) u Brain regions damaged • arcuate fasciculus area) (connection between B and W • or primary auditory cortex

APHASIAS: SUMMARY

WERNICKE-GESCHWIND MODEL Connectionist Model for the anatomical analysis of aphasias

DYSLEXIA u Developmental Dyslexias • Some controversy in categorizing this disorder • Sensory-processing problem? • Memory disorder? u Acquired Dyslexias • surface dyslexia (whole word reading impaired) • deep dyslexia (phonological dyslexia)

NEURAL ABNORMALITIES IN DYSLEXIA u Anomalies in cortical cell arrangement • Ectopias: unusual groupings of cells in outer layers • Micropolygyria: excessive cortical folding • Disoriented cells u These abnormalities probably occur during neural migration during fetal development

BRAIN IMAGING DYSLEXIA u f. MRI studies show different patterns of brain activation in dyslexics and nondyslexics. u Dyslexic subjects: • showed less activation in posterior regions (e. g. Wernicke’s area) and overactivity in anterior regions compared to nondyslexics. • showed less activation of visual cortex in response to written words.