Are planum temporale and sylvian fissure asymmetries directly related? A MRI study in great apes

In humans and great apes, both the planum temporale (PT - part of Wernicke's area) and the sylvian fissure (SF) in the left cerebral hemisphere have been consistently shown to be larger than the corresponding structures in the right hemisphere. The greater length of the SF in the left hemisphere is commonly thought to be a direct consequence of the larger expansion of the PT in the same hemisphere. However, there is a lack of studies that have attempted to directly assess the tenability of this hypothesis. To address this lack of data, we collected magnetic resonance images (MRI) of the brain in 28 apes. The surface area of the PT and the length of the pre- and post-central SF were measured in each hemisphere using image acquisition and analysis software. In accordance with previous findings, the PT was markedly larger in the left hemisphere than in the right, and there was also a leftward asymmetry of the SF, particularly of its post-central section. However, we found no statistically significant correlation between asymmetry of the PT and of the post-central SF, whereas we did find evidence of a positive association between asymmetry of the post-central SF and of the inferior parietal lobe. These results are congruent with those of a recent study with human subjects [Neuropsychiatry, Neuropsychology and Behavioral Neurology 12 (1999) 1]. Overall, this converging evidence leads us to question the widely accepted notion of a direct relationship between PT and SF asymmetries and to consider possible implications of this finding for the study of the evolutionary origin of PT asymmetry in primates.