Very comprehensive and complete summary of the second task (asymmetry and disconnection) of the block GGZ2025, with many images, figures and tables (both self-made and from the basic book). Summary contains material from Kolb & Whishaw. Now available as bundle!
The idea that the two hemispheres have separate functions (laterality), leads to the notion that
two different minds control our behavior. Typically, the left hemisphere plays a role in
producing and understanding language and controlling movement on the right side of the
body. The right hemisphere plays a role in perceiving and synthesizing nonverbal information
and controls movement on the left side of the body. Four notions on laterality:
(1) Laterality is relative, not absolute. Both hemispheres participate in nearly every
behavior; left is important for behavior, but right also has some language capabilities.
(2) Cerebral site is at least as important in understanding brain function as cerebral side.
The frontal lobes are asymmetrical, but functions are more similar to each other than
they are to those of the posterior cortex. It is best to think of many cortical functions as
localized and of hemispheric side as being but one feature of the localization.
(3) Environmental and genetic factors affect laterality; the cerebral organization of some
left-handers and females appears less asymmetrical than right-handers and males.
(4) A range of animals exhibit laterality and is not only relayed to language.
CEREBRAL ASYMMETRY
The planum temporale (Wernicke’s area) lies posterior to the primary auditory cortex
(Heschl’s gyrus) within the Sylvian (lateral) fissure. Numerous investigations have shown that
most of the persons (65 to 90%) have a larger planum temporale in the left hemisphere. The
primary auditory cortex is larger in the right hemisphere, where two Heschl’s gyri usually
reside, compared with only one in the left hemisphere.
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, GGZ2025 Neuropsychological Disorders vvanbeek
MRI scans of living brains confirm eight major anatomical differences between the two
hemispheres.
(1) The right hemisphere is larger and heavier than the left, but the left contains more
gray matter (neurons) relative to white matter (connections).
(2) The marked structural asymmetry of the left and right temporal lobes may provide
anatomical basis for their observed specialization in language and music function.
(3) The anatomical asymmetry in the temporal lobe’s cortex correlates with an asymmetry
in the thalamus. This complements an apparent functional asymmetry: the left
thalamus is dominant for language functions.
(4) The slope of the lateral fissure is gentler on the left hemisphere than on the right; the
region on the temporoparietal cortex lying ventral to the lateral fissure appears larger
on the right.
(5) The frontal operculum (Broca’s area) is organized differently on the left and right.
The area visible on the brain surface is about one-third larger on the right side,
whereas the area of cortex in the region’s sulci is greater on the left.
(6) The distribution of various neurotransmitters is asymmetrical in both cortical and
subcortical regions. Neurotransmitters: ACh, GABA, NE and DA.
(7) The right hemisphere extends farther anteriorly than does the left; the left extends
farther posteriorly than does the right; the occipital horns of the lateral ventricles are
five times as likely to be longer on the right as on the left.
(8) An overall pattern of asymmetry exists throughout the hemispheres. The largest
asymmetries favour the left hemisphere in the Sylvian and medial temporal regions
and the right hemisphere in the posterior and dorsolateral prefrontal areas.
The largest anatomical asymmetries center on the temporoparietal language areas, and these
are present before birth.
NEURONAL ASYMMETRY
One study compared the dendritic fields of pyramidal cells in Broca’s area, the left frontal
operculum (LOP), with those in the facial area of the motor cortex in the left precentral cortex
(LPC) and with homologous regions in the right hemisphere. The results show that neurons in
each region 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 graded potentials in
the dendritic tree; more branch points
allow more degrees of freedom to the
final activity of the cell.
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, GGZ2025 Neuropsychological Disorders vvanbeek
GENETIC ASYMMETRY
One study, that compared gene expression levels in perisylvian regions of left and right
hemispheres of fetal brains, found genes that are expressed differently in the two
hemispheres. The mechanism whereby differential gene expression affects anatomical and
functional asymmetry remains unknown.
SEX DIFFERENCES IN BRAIN STRUCTURE
Sex differences in gross brain anatomy
Differences favouring female brains
❖ Larger language areas
❖ Larger medial paralimbic areas
❖ Larger lateral frontal areas
❖ Greater relative amount of grey matter
❖ More densely packed neurons in temporal lobe
❖ More gyri
❖ Thicker cortex
Differences favouring male brains
❖ Larger medial frontal areas
❖ Larger cingulate areas
❖ Larger amygdala and hypothalamus
❖ Larger overall white matter volume
❖ Larger cerebral ventricles
❖ Larger right planum parietal
❖ More neurons overall
❖ Larger brain
Sex differences in brain structure have been related to differences in the distribution of
estrogen and androgen receptors during development. Women’s brain volume (purple in
figure) in prefrontal and medial paralimbic regions is significantly higher than men’s. Men
have larger relative volumes in the medial
and orbital frontal cortex and the
angular cortex (pink). Purple areas
correspond to regions that have high levels
of estrogen receptors during development,
pink to regions high in androgen receptors
during development.
ANATOMICAL ASYMMETRIES
Although women’s cerebral hemispheres are reportedly more symmetrical than men’s. this
conclusion is based largely on non-significant trend or impressions. However, several reliable
sex differences in anatomical asymmetry do exist.
❖ Asymmetry (left larger than right) in the planum temporale (Wernicke’s area) is seen
more often in men than in women. This suggests a sex-dependent decrease in
hemispheric connectivity with increasing hemispheric asymmetry.
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