Specialized face processing and hemispheric asymmetry in man and monkey: Evidence from single unit and reaction time studies

Experimental and clinical studies have generally shown that the neural mechanisms for face processing in man are (1) designed to deal with the configuration of upright faces and (2) located predominantly in the right cerebral hemisphere. Monkeys would seem to process faces in a different manner to humans since they appear to show no hemispheric asymmetry and to treat upright and inverted faces equivalently. We re-examine these claims. Our reaction time studies reveal that monkeys do behave like human subjects since they process facial configuration faster when stimuli are presented upright as compared with horizontal or inverted. Single unit studies in the monkey reveal patches of neurones responsive to faces in the upper bank and fundus of the left superior temporal sulcus (STS). Recording from the right hemisphere also reveals cells responsive to faces but in this hemisphere such cells appear less numerous. These cells process upright faces faster than inverted faces. Face processing in monkeys and man appears to utilize qualitatively similar mechanisms, but the extent and/or direction of cerebral asymmetry in these mechanisms may not be similar.