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12. Variations in Cerebral Asymmetry.docx

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York University
PSYC 3530
Guy Proulx

12. Variations in Cerebral Asymmetry Monday, October 28, 2013 4:57 PM Handedness and Functional Asymmetry  Brains (and hemispheres) differ in size, gyral patterns, distribution of gray and white matter, cytoarchitectonics, vascular patterns, and neurochemistry, among other things  Sex and handedness influence the organization of the cerebral hemispheres and ultimately the effects of cerebral injury  Anatomical Studies o Hand preference is correlated with differential patterns of right–left asymmetry in the parietal operculum, frontal cortex, occipital region, vascular patterns, and cerebral blood flow o In comparison with right-handers, a higher proportion of left-handers show no asymmetry or a reversal of left and right anatomical asymmetries o Handedness may appear more closely related to anatomical anomalies because left- handers display more variation in the lateralization of speech o Male right-handers have a significantly deeper Sylvian fissure on the right than on the left, but there was no difference in left-handers o The cross-sectional area of the corpus callosum was 11% greater in left-handed and ambidextrous people than in right-handed people  Functional Cerebral Organization in Left-Handers o In left-handers, language is represented in the left hemisphere in 70%, in the right hemisphere in 15%, and bilaterally in 15% o Henri Hécaen and Jean Sauguet suggested that left-handers can be subdivided into two genetic populations differing in cerebral organization: familial left-handers, who have a family history of left-handedness, and nonfamilial left-handers, who have no such family history  The performance of nonfamilial left-handed patients with unilateral lesions is like that of right-handed patients on neuropsychological tests  Theories of Hand Preference o Environmental Theories  Behavioral-- men held shields in left hand to protect the heart, women held baby in the left hand to be near the heartbeat, leaving right hand open to perform skilled tasks  Environmental reinforcement  Environmental accident-- left-handedness develops through a cerebral deficit caused by accident o Anatomical Theories  The first theory attributes right-handedness to enhanced maturation and ultimately greater development of the left hemisphere  The second theory reminds us that many animals have a left-sided developmental advantage that is not genetically encoded o Hormonal Theories  Brain plasticity can modify cerebral asymmetry significantly early in life, leading to anomalous patterns of hemispheric organization  Differences in testosterone level might influence cerebral asymmetry  Testosterone’s inhibitory action takes place largely in the left hemisphere, thus allowing the right hemisphere to grow more rapidly, which leads to altered cerebral organization and, in some people, to left-handedness o Genetic Theories  Most genetic models for handedness postulate a dominant gene or genes for right-handedness and a recessive gene or genes for left-handedness  But the model that best predicts the actual number of left-handers in the population, by Annett, rejects this idea in favor of a dominant gene (rs+) responsible for the development of speech in the left hemisphere Sex Differences in Cerebral Organization  Women tend to be more fluent than men in using language, and men tend to perform better than women in spatial analysis   Sex Differences in Behavior o o Research has found many more mathematically “gifted” boys as girls o Women are more sensitive to all forms of sensory stimulation, except for vision o Sex-related differences are often argued to be related to life experience, but Kimura argues compellingly that this relation is unlikely for the cognitive behaviors summarized in Table 12.3  Sex Differences in Brain Structure o o The male brain is larger than the female brain, and differences in body size cannot account for this difference o Men have a small (4 points) advantage in IQ scores, on average o Female brains appear to have larger volumes in regions associated with language functions, in medial paralimbic regions, and in some frontal-lobe regions o Male brains tend to have more neurons (gray matter) and female brains more neuropil (that is, dendrites and axons and thus connections) per neuron o The Luders group found increased cortical thickness in females throughout much of the cortex o Influence of Sex Hormones  The Goldstein team proposes that a large part of the observed sex differences in cerebral organization is related to differences in the distribution of receptors for gonadal hormones during development o Established Asymmetries  Asymmetry (left larger than right) in the planum temporale is seen more often in men than in women  Male brains have a larger asymmetry in the Sylvian fissure than do female brains  The asymmetry in the planum parietale, which favors the right hemisphere, is about twice as large in men as in women  The posterior part of the callosum (the splenium) is significantly larger in women than in men  Women have a larger anterior commissure than do men  The ridges in our fingerprints, which are established early in fetal life, are asymmetrical, with the fingers on the right hand having more ridges than do fingers on the left hand; Kimura found that most people have the asymmetry, but women are far more likely to show an atypical pattern  Whereas the right testicle tends to be larger than the left testicle, breasts tend to be larger on the left than on the right  The Homosexual Brain o Homosexual men outperform all groups on verbal fluency, a test in which heterosexual women outperform heterosexual men, whereas homosexual women had the lowest scores o Parts of the hypothalamus of homosexual men differ from those of both heterosexual males and females, who also differ from one another in this regard  Sex Differences Revealed in Imaging Studies o Not only are there differences in the anatomical organization of the male and female brain, but there are differences in the functional activity of the brains as well o  Research with Neurological Patients o Two types of lesion-related differences are possible:
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