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Chapter 11&12

Chapter 11 & 12 .pdf

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Strang Burton

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Chapter 11 Cerebral AsymmetryInability to produce or to understand language is called global aphasia The single most curious feature of human brain organization is cerebral asymmetry the left and right cerebral hemispheres have partly separate functions As described in the preceding Portrait cerebral asymmetry was especially apparent in M S s loss of language skills but maintenance of musical skills Anatomical Asymmetry in the Human BrainLaterality the idea that the two cerebral hemispheres have separate functions The left hemisphere plays a special role in producing and understanding language and in controlling movement on the right side of the body whereas the right hemisphere specializes in perceiving and synthesizing nonverbal information including music and facial expression And the right hemisphere controls movement on the left side of the body Four variables complicate the research on laterality 1Laterality is relative not absolute Both hemispheres play a role in nearly every behaviour thus although the left hemisphere is especially important for the production of language the right hemisphere also has some language capabilities 2Cerebral site is at least as important in understanding brain function as cerebral side The frontal lobes are asymmetrical but their functions are more similar to each other than they are to those of the posterior cortex on the same side 3Laterality is affected by environmental and genetic factors 4Laterality is exhibited by a range of animals Cerebral AsymmetryAlso called Wernickes area the planum temporale lies just posterior to the primary auditory cortex Heschls gyrus within the Sylvian or lateral fissure On average in 65 of the 100 brains studied by Geschwind and Levitsky the planum temporale in the left hemisphere was nearly 1 cm longer than that in the right hemisphere In contrast the neighbouring primary auditory cortex of Heschls gyrus is larger in the right hemisphere because there are usually two Heschls gyri in the right hemisphere and only one in the left MRI scans of living brains confirm eight major anatomical differences between the two hemispheres 1The right hemisphere is slightly larger and heavier than the left but the left contains more gray matter relative to white matter 2The temporal lobes display a marked structural asymmetry that may provide an anatomical basis for the observed specialization of the left and right temporal lobes in language and in music functions 3The asymmetry in the cortex of the temporal lobes is correlated with a corresponding asymmetry in the thalamus This anatomical asymmetry complements an apparent functional asymmetry in the thalamus the left thalamus being dominant for language functions 4The slope of the lateral fissure is gentler on the left hemisphere than on the right5The frontal operculum Brocas area is organized differently on the left and right The area visible on the surface of the brain is about one third larger on the right than on the left whereas the area of cortex buried in the sulci ridges of the region is greater on the left than on the right 6The distribution of various neurotransmitters is asymmetrical in both the cortical and the subcortical regions The particular asymmetries in the distribution of acetylcholine gamma aminobutyric acid GABA norepinephrine and dopamine depend on the structure under consideration 7The right hemisphere extends farther anteriorly than does the left the left hemisphere extends farther posteriorly than does the right and the occipital horns of the lateral ventricles are five times as likely to be longer on the right as on the left 8The details of anatomical asymmetry are affected by both sex and handednessNeuronal AsymmetryArnold Scheibel and his colleagues compared the dendritic fields of pyramidal cells in Brocas area the left frontal operculumLOP with those in the facial area of the motor cortex in the left precentral cortexLPC and with homologous regions in the right hemisphere Their results show that the neurons in each of these regions have distinct patterns of dendritic branching The degree or pattern of branching is important because each branch is a potential location for the enhancement or suppression of the graded potentials in the dendritic tree Thus more branch points allow more degrees of freedom with respect to the final activity of the cell Genetic Asymmetryresearchers suggest that some of the genes may regulate the production of growth factors that would in turn facilitate the development of specific regions in one hemisphere or the other A provocative idea is that the asymmetrical expression of genes may account for functional properties such as handedness which to date has no known basis Asymmetry in Neurological PatientsPatients with Lateralized LesionsIn the strongest experimental method for demonstrating the lateralization of function called double dissociation by Hans Leukas Teuber two areas of the neocortex are functionally dissociated by two behavioural tests Each test is affected by a lesion in one zone but not in the other Lesions in the left hemisphere of right handed patients consistently produce deficits in language functionsspeech writing and reading that are not produced by lesions in the right hemisphere Thus the functions of the two hemispheres are dissociated However performing spatial tasks singing playing musical instruments and discriminating tonal patterns are more disrupted by right hemisphere than by left
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