Behavioural Brain Research, 29 (1988) 245-258 245 Elsevier BBR00650 Specialized face processing and hemispheric asymmetry in man and monkey" evidence from single unit and reaction time studies* D.I. Perrett, A.J. Mist!in, A.J. Chitty, P.A.J. Smith, D.D. Potter, R. Broennimann and M. Harries MRC Cognitive Neuroscience Research Group, Psychological Laboratory, University of St. Andrews, Fife (U.K.) (Received 28 August 1986) (Revised version received 24 November 1986) (Accepted 25 November 1986) Key words: Visual processing; Cortical organization; Face orientation; Cerebral asymmetry; Configurational encoding; Reaction time; Macaque monkey 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 ofneurones 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. 1. INTRODUCTION Clinical studies of face recognition indicate that in man the neural mechanisms involved in the processing of faces are lateralized predominantly (but not exclusively) in the right cerebral hemi- sphere. Evidence for this claim comes from both clinical and experimental studies. Neuropsychological examinations reveal that patients with brain damage in the right hemi- sphere are more impaired on tasks involving face memory and face perception than patients with equivalent damage in the left hemisphere8,4°. The complete loss of face recognition capacities (prosopagnosia) may depend on bilateral brain damage6"22, though there are several reports of prosopagnosic patients with suspected unilateral (fight) brain damage7,42. Experimental work with normal human subjects reveals consistent visual field differences in the processing of facial material. Presentation of faces to the left visual field (and hence initially to the right hemisphere) leads to faster processing than presentation to the right visual field (left hemisphere) 16,31.This asym- * First presented at the IBB S workshop on Hemispheric specialization and interhemispheric communication, held in Rotterdam on 20-21 March 1986. Correspondence: D.I. Perrett, MRC Cognitive Neuroscience Research Group, Psychological Laboratory, University of St. Andrews, Fife, KYI6 9JU, U.K. 0166-4328/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)