Abstract
Magnetic resonance images (MRI) were collected in a sample of 23 apes, 14 Old World monkeys, and 8 New World monkeys. The total area or volume of the anterior and posterior cerebral regions of each hemisphere of the brain was measured. The results indicated that a rightward frontal and leftward occipital pattern of asymmetry was present at a population level in the great ape sample. Population-level cerebral asymmetries were not revealed in the sample of New or Old World monkeys. The total area or volume of the planum temporale, which was localised only in the great apes, was also measured in both hemispheres. A leftward planum temporale asymmetry was evident at the population level in the great apes. It was hypothesised that the rightward frontal and leftward occipital asymmetries would correlate with leftward planum temporale asymmetries. This hypothesis was based on the assumption that, similar to development of the human brain, the nonhuman primate brain ''torques'' during development due to a growth gradient which progresses anterior to posterior, ventral to dorsal, and right to left. The results of this study confirmed the predicted relationship between cerebral volume and the planum temporale asymmetries. This supports the hypothesis that the great ape brain may develop in a ''torquing'' manner, producing similar anatomical asymmetries as reported in humans.