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PSYC 3530 Chapter Notes -Temporal Lobe, Lateral Sulcus, Commissurotomy

9 pages114 viewsFall 2013

Department
Psychology
Course Code
PSYC 3530
Professor
Guy Proulx

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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.
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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
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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.
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