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Abstract:
In previous studies, cell size asymmetries have been reported
in several cortical locations including Broca's area (Hayes &
Lewis, 1993; Hayes & Lewis, 1995) and auditory and
language-associated regions within the temporal lobe (Hutsler &
Gazzaniga, 1996). In both cortical regions larger pyramidal cells
are found within the left hemisphere, and this asymmetry is
restricted to the largest subgroup of layer III pyramidal
cells--the magnopyramidal neurons. This finding may reflect
connectional differences between the two hemispheres or differences
in functional activation (Pasic & Rubel, 1989). Here we report
a limited number of cases showing the early presence of these
asymmetries in the human brain. Cell sizes within layer III were
measured from multiple locations of several perinatal human cases
and data collection was performed blind to the hemisphere and
location of the cortical region. Cell size asymmetries were readily
apparent at 18 months postnatal during the early stages of language
acquisition, and preliminary evidence indicates that the size
asymmetry may be present even earlier. The early presence of this
asymmetry suggests that magnopyramidal cell asymmetries may
represent a structural precursor for later functional asymmetries.
It remains to be established whether these asymmetries co-occur
with language lateralization and whether they are reversed or
absent in individuals who are not left hemisphere dominant for
language or who have impairments related to language use and
acquisition.
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