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Sex Differences in Brain Structure & Asymmetry in Healthy College Students
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Leonard ,
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Towler ,
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Halderman ,
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Welcome ,
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Otto
Christiana M.
Stephen
Laura K.
Suzanne
Ron
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Christine Chiarello , University of Florida, Gainesville , University of California, Riverside & Diagnostic Imaging Center, Riverside, CA
Introduction
Results
It is frequently claimed that women have a more bilateral organization for language
and reduced brain asymmetry compared to men. Experimental support for these
claims have been mixed. Positive findings are less often seen in large scale studies
(Watkins et al., 2001; Good et al., 2001).
The current study, part of the Biological Substrates for Language Project, affords the
opportunity to explore this issue in another large-scale investigation. In this project,
selected brain structures and asymmetries were measured in 200 individuals whose
behavioral asymmetries had been assessed with the divided visual field procedure.
This interdisciplinary approach enables a powerful test of the hypothesis that
reduced brain asymmetry is associated with more more bilateral language
lateralization in women. The size of the sample also allowed us to address questions
about the relative contribution of brain size to reported sex differences in white
matter and relative corpus callosum area.
Although the strength of the relationship
between white matter proportion and
cerebral volume is stronger in women,
hierarchical multiple regression revealed no
significant quadratic contribution from
cerebral volume and the interaction between
sex and volume did not approach
significance. Cerebral volume contributed
25% with sex contributing an additional
unique 3.4% of the variance to the
relationship (model significant, p < .0001).
We had three questions:
1. Are there any sex differences in the size or asymmetry of perisylvian regions
thought to be associated with language function?
2. Is the reported sex difference in relative corpus callosum size due to sex
differences in brain size (as Jancke and colleagues have reported, Jancke et al.,
1996).
3. Do women have a relatively smaller proportion of white matter (Gur et al.,
1999; Allen et al., 2003) or is this difference also attributable to differences in brain
size?
Conclusions
This large-scale investigation provided little evidence for sex differences in the asymmetry
of perisylvian structures. There were, however, significant sex differences in the coefficient
of asymmetry of cerebral white matter, as well as the asymmetry of white matter
proportions. Men had .7% more white matter in the right hemisphere than women (p <
.001). Our findings on gray/white matter relations exactly replicate the report of Allen et al.
(2003) using a different segmentation algorithm in a smaller sample.
Method
PARTICIPANTS:
• 100 male, 100 female native English speakers
• 18-34 years of age
• 28 (14%) are not right-handed
Past reports that the planum temporale is less asymmetrical in women may be due to
inadequate sample sizes that are unrepresentative of the wider population. Based on the
results shown here, it appears more likely that sex differences in behavior are due to the
very large differences in brain volume than the extremely modest differences in asymmetry.
BRAIN MEASURES: SIZE & ASYMMETRY
• Volume: gray and white matter of the cerebral hemispheres
• Area: corpus callosum (seven subdivisions)
• Average length: planum temporale, planum parietale, Heschl’s gyrus, pars
triangularis, pars opercularis
PROCEDURE:
• Volumetric MRI scans (1.2 mm thick sagittal images) on 1.5 GE Scanner
• Brain tissue extracted, reoriented, and segmented into isometric voxels with
FSL software http://www.fmrib.ox.ac.uk/
• Surface and volume measurements made with scripts written in PVWave
http://www.vni.com/ (Leonard, et al., 2007)
• At least two raters made each measurement, blind to hemisphere and subject
characteristics. Differences were resolved by consensus.
• Reliability was > .99 for gray and white matter measures and > .85 for
structure measures.
•Analyses were conducted with PC-SAS (SAS Institute, Cary NC).
Although the shape of the function between
proportional corpus callosum size and white
matter volume differs for men and women,
hierarchical multiple regression revealed
that no significant quadratic contribution
from brain volume and the interaction
between sex and brain volume did not
approach significance. Cerebral volume
contributed 15% with no additional unique
contribution from sex (model significant, p <
.0001)
References
Allen, J. S., Damasio, H., Grabowski, T. J., Bruss, J., & Zhang, W. (2003). Sexual dimorphism and asymmetries in the
gray-white composition of the human cerebrum. Neuroimage, 18, 880-894.
Results of t tests on the effect of sex. Significant results (p < .01) are highlighted. Degrees of freedom = 198 except for
gray matter asymmetry (df = 178) and left cerebellum (df = 187), where the variances were significantly different in men
and women. The denominator for white matter proportion is total cerebral gray and white matter. The denominator for
proportional corpus callosum size is total cerebral white matter.
Acknowledgments
This research was supported by NIDCD grant 5R01DC6957. Great thanks are due to Matthew Manry, Natalie Anne
Hammond, Caroline Anderson, Dawn Joseph, Michelle Long, Gabrielle Messmer and Stephanie Demian.
Good, C. D., Johnsrude, I., Ashburner, J., Henson, R. N., Friston, K. J., & Frackowiak, R. S. (2001). Cerebral
asymmetry and the effects of sex and handedness on brain structure: a voxel-based morphometric analysis of 465
normal adult human brains. Neuroimage, 14, 685-700.
Gur, R. C., Turetsky, B. I., Matsui, M., Yan, M., Bilker, W., Hughett, P., et al. (1999). Sex differences in brain gray and
white matter in healthy young adults: correlations with cognitive performance. J Neurosci, 19, 4065-4072.
Jancke, L., Staiger, J. F., Schlaug, G., Huang, Y., & Steinmetz, H. (1997). The relationship between corpus callosum
size and forebrain volume. Cerebral Cortex, 7, 48-56.
Leonard, C., Eckert, M., Givens, B., Berninger, V., & Eden, G. (2006). Individual differences in anatomy predict reading
and oral language impairments in children. Brain, 129, 3329-3342.
Watkins, K. E., Paus, T., Lerch, J. P., Zijdenbos, A., Collins, D. L., Neelin, P., et al. (2001). Structural asymmetries in
the human brain: a voxel-based statistical analysis of 142 MRI scans. Cereb Cortex, 11, 868-877.