Proportional anterior commissure area in humans and monkeys.

Behavioral testing following division of the corpus callosum has shown that the anterior commissure can transfer information related to visual tasks interhemispherically in nonhuman primates but not in humans. We have found that the ratio of anterior commissure cross-sectional area to total neocortical commissural area is significantly greater for rhesus monkeys than for humans. This difference persists when the proportions are weighted for these structures' known differences in axon density. We hypothesize that this interspecies dissimilarity reflects a difference in the distribution of interhemispheric communication and may explain the behavioral differences observed after callosal section.