THE RELATIONSHIP BETWEEN CORPUS-CALLOSUM SIZE AND FOREBRAIN VOLUME

Using high-resolution in vivo magnetic resonance morphometry we measur ed forebrain volume (FBV), midsagittal size of the corpus callosum (CC ) and four CC subareas in 120 young and healthy adults (49 women, 71 m en). We found moderate linear and quadratic correlations, indicating t hat the CC and all CC subareas increase with FBV both in men and women (multiple r(2) ranging from 0.10 to 0.28). Allometric equations revea led that these increases were less than proportional to FBV (r(2) rang ing from 0.02 to 0.30). Absolute CC measurements, as well as CC subare as relative to total CC or FBV (the latter measures termed the CC rati os), were further analyzed with regard to possible effects of handedne ss. gender, or handedness by gender interaction. Contrary to previous reports, left-handers did not show larger CC measurements compared to right-handers. The only apparent influence of gender was on the CC rat ios, which were larger in women. However, smaller brains had larger CG ratios which were mainly independent of gender, a result of the less than proportional increase of callosal size with FBV. We suggest that the previously described gender differences in CC anatomy may be bette r explained by an underlying effect of brain size, with larger brains having relatively smaller callosa. This lends empirical support to the hypothesis that brain size may be an important factor influencing int erhemispheric connectivity and lateralization.