Hemispheric specialization displayed by man but not macaques for analysis of faces.

Six macaques and 20 right-handed human subjects, given identical test material, were asked to choose which of two composites, one consisting entirely of the left half of a monkey or a human face, the other of the right half, appeared most like the normal face. Confirming observations of others, the group of human subjects selected the right-half composite of the face (i.e. the left-half composite of the photograph) 68% of the time, thereby demonstrating a highly significant (P less than 0.01) bias in favor of the left visual field (right hemisphere). No such bias was present when human subjects viewed faces of monkeys, despite the fact that according to limited measurements, the faces of both species are asymmetrical. And most important, the macaques exhibited no consistent bias with either monkey or human faces. On the other hand, the monkeys did respond emotionally to these colored facial images, but not to scenery, when first presented. Also, like man, they found inverted faces but not scenery more difficult to identify than when right side up. Thus, the monkeys responded emotionally and perceptually to these images as faces, yet, unlike the human observers, displayed no hemispheric preference in making their analysis.