Face recognition by monkeys: absence of an inversion effect.

The effect of inversion of faces upon learning visual discriminations by macaque monkeys was studied with simultaneous discriminations, concurrent discriminations, and transfer tests. In no case was performance with upright stimuli superior to that with inverted stimuli; that is, there was no obvious inversion effect. Studies of human face recognition indicate that the inversion effect is mediated by an orientation-dependent face recognition mechanism that matures within the right hemisphere during childhood. Absence of an inversion effect would indicate that monkeys may not have such a mechanism. It was hypothesized that the macaque's relatively precocious development, smaller cortex, and lack of hemispheric specialization may preclude the maturation of such a mechanism.