11. Cerebral Asymmetry
Tuesday, October 22, 2013
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
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-
4. Laterality is exhibited by a range of animals
Was once believed to be uniquely human & related to language
o Wernicke’s area = planum temporale
o MRI scans of living brains confirm 8 major anatomical differences between the two
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.