Histological asymmetries of primary motor cortex predict handedness in chimpanzees (Pan troglodytes)

Like humans, chimpanzees display robust and consistent hand preferences during the performance of certain tasks. Although correlations have been demonstrated between gross anatomic measures of primary motor cortex asymmetry and handedness in captive chimpanzees, the relationship between histological architecture and behavioral lateralization has not yet been investigated. Therefore, we examined interhemispheric asymmetry of several different microstructural characteristics of the primary motor cortex in the region of hand representation from 18 chimpanzees tested on a coordinated bimanual task before death. At the population level our data showed leftward bias for higher layer II/III neuron density. Of note, however, there was no population-level asymmetry in the areal fraction of Nissl-stained cell bodies, a finding that differs from previous studies of this cortical region in humans. Nonetheless, we found that asymmetry in the density of layer II/III parvalbumin-immunoreactive interneurons was the best predictor of individual hand preference. These results suggest that histological asymmetries are related to handedness in chimpanzees, while overall patterns of asymmetry at the population level might differ from humans.