Brain size: a possible source of interindividual variability in corpus callosum morphology

Abstract

The corpus callosum (CC) is the main fiber tract connecting the cerebral hemispheres, and it has been estimated that about 200-350 million fibers run through this structure in the human brain (1,2). The CC seems to be important in the transfer and facilitation of associative information between the hemispheres. It is thought that the cross-sectional size of the CC may indicate the number of fibres crossing through (1), implying that a larger callosal area may indicate a higher capacity for interhemispheric transfer. Because the midsagittal CC size is so easy to measure either in post mortem material or on magnetic resonance images (MRI) it is one of the human brain structures to receive particular attention. There is some evidence suggesting that the morphology of the CC may be related to language dominance (13,14), gender (19), handedness (71,73-75), Down`s syndrom (68), dysphasia (51), schizophrenia (76), and dyslexia (32). The sometimes conflicting results in CC morphology were interpreted in two ways. The common interpretation has been that a larger CC midsagittal area (total CC or CC subarea) reflects increased interhemispheric connectivity resulting in (or due to) increased ambilaterality (73). This interpretation is at variance with the interpretation by Clarke and Zaidel (13) that the CC size indicates the amount of fibers inhibiting or interfering processes located in the dominant hemisphere. In the light of these controversies and with respect to the enormous variability in CC size across the subgroups tested, we employed whole-brain in-vivo magnetic resonance morphometry in order to investigate the anatomical relationship between CC midsagittal size and forebrain volume (FBV). This approach may also provide an empirical evaluation of the recently suggested relationship between brain size and lateralization (61) and it might help to explain the large interindividual variability in callosal size. In particular, we were interested to answer the following questions: (i) Is there an allometric relation between callosal size and brain size; (ii) If there is a relation between callosal size and brain size does this relation follow a geometrical rule?; (iii) Is there a true influence of gender? and ( iv); Is handedness or brain lateralization related to callosal size?