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Guy Proulx (13)

11. Cerebral Asymmetry.docx

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PSYC 3530
Guy Proulx

11. Cerebral Asymmetry Tuesday, October 22, 2013 11:29 AM Anatomical Asymmetry in the Human Brain  Laterality-- 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  Four variables complicate the research on laterality: 1. Laterality is relative, not absolute 2. Cerebral site is at least as important in understanding brain function as cerebral side 3. Laterality is affected by environmental and genetic factors  Male and right-handed brains tend to be more symmetrical than female or left- handed brains 4. Laterality is exhibited by a range of animals  Was once believed to be uniquely human & related to language  Cerebral Asymmetry o Wernicke’s area = planum temporale o MRI scans of living brains confirm 8 major anatomical differences between the two hemispheres: The right hemisphere is slightly larger and heavier than the left, but the left contains more gray matter relative to white matter. The 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, respectively. The 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. The slope of the lateral fissure is gentler on the left hemisphere than on the right. The region of the temporoparietal cortex lying ventral to the lateral fissure therefore appears larger on the right. The frontal operculum (Broca’s 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. This anatomical asymmetry probably corresponds to the lateralization of the regions, the left side affecting the production of grammar in language and the right side possibly influencing tone of voice. The 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. The 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. These asymmetries presumably correspond to some gross difference in cerebral organization that has yet to be identified. The details of anatomical asymmetry are affected by both sex and handedness. o Overall, anatomical asymmetries center on the language areas, with most of the frontal and parietal lobes showing little gross asymmetry o  Neuronal Asymmetry o There are far more dendritic branches in neurons in Broca's area (left frontal operculum-- LOP) than in other areas  Genetic Asymmetry o There are genes that express differently in each hemisphere o The asymmetrical expression of genes may account for functional properties such as handedness, which to date has no known basis Asymmetry in Neurological Patients  Cerebral asymmetry was first established by studying patients with neurological disease, such as epilepsy, that is lateralized to one hemisphere  Patients with Lateralized Lesions o Unilateral lesions in the left hemisphere of right-handed patients can produce aphasias that do not develop from lesions in the right hemisphere o Double dissociation-- experimental technique by which two areas of neocortex are functionally dissociated by two behavioral tests, each test being affected by a lesion in one zone but not the other o Performing spatial tasks, singing, playing musical instruments, and discriminating tonal patterns are more disrupted by right-hemisphere than by left-hemisphere lesions o Left temporal lobe lobectomy resulted in impaired verbal tests, whereas right temporal lobe lobectomy resulted in impaired non-verbal tests  Patients with Commissurotomy o To prevent the spread of a seizure when medication has failed to impose control, commissurotomy, the surgical procedure of disconnecting the two hemispheres by cutting the 200 million nerve fibers of the corpus callosum, is sometimes performed o Because the functions in these separate cortexes, or split brains, are thus isolated, sensory information can be presented to one hemisphere, and its function can be studied, without the other hemisphere having access to the information o Cortical connections with the sensory and motor systems and their bodily receptors and effectors are unaffected o o The right hemisphere has a special role in recognizing faces  Brain Stimulation o Stimulation of the left hemisphere can block the ability to speak, whereas stimulation of the right hemisphere seldom does so o Applying an electrical current to the cortex of a conscious patient has 4 general effects-- three excitatory and one inhibitory: 1. Stimulation can produce localized movements, localized dysthesias (numbness or tingling in the skin), light flashes, or buzzing sensations. These effects are normally evoked from primary motor, somatosensory, visual, and auditory areas and pathways, respectively, and are produced by the stimulation of either hemisphere with about the same frequency, a result that illustrates the often overlooked fact that the brain has symmetrical as well as asymmetrical functions. 2. Stimulation can produce what Penfield called “interpretive” and “experiential” responses. These uncommon but often highly reliable phenomena include alterations in the interpretation of the patient’s surroundings, such as deja vu, fear, and dreaming states, and the reproduction of visual or auditory aspects of specific earlier experiences. That is, patients report specific “memories” in response to specific stimulation. These phenomena usually arise from tissue showing epileptogenic discharge, but their occurrence reveals an asymmetry: stimulation of the right temporal lobe produces these phenomena more frequently than does stimulation of the left temporal lobe, suggesting that the right hemisphere has perceptual functions not shared by the left hemisphere. 3. Stimulation of the left frontal or temporal regions may accelerate speech production. Ojemann suggested that this acceleration may result from a type of “alerting response” and may occur in other cognitive processes, especially memory, although this possibility is difficult
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