Neuroanatomical localization of the motor hand area with magnetic resonance imaging: The left hemisphere is larger in great apes

Magnetic resonance images of the brain were obtained from 2 gorillas (Gorilla gorilla gorilla), 4 orangutans (Pongo pygmaeus), 14 chimpanzees (Pan troglodytes), and 4 bonobos (Pan paniscus). The region on the motor cortex of humans identified as responsible for motor skill of the hand (the "knob") was identified in this sample on consecutive 1-mm axial scans. The shape of the knob area was traced on each scan from both hemispheres, and the area from all scans was summed to calculate the knob volume. The width of the knob was also measured and correlated highly with knob volume. A significant population-level leftward asymmetry in the volume and width of the knob was revealed (p < .05). Species differences in knob asymmetry and overall volume were not significant, but the variability in overall volume between species was substantial. Selection for the evolution of a neuroanatomical representation of the hand in primates and an evolutionary trend toward population-level right handedness are discussed.