Recognition of facial expressions in a Japanese monkey (Macaca fuscata) and humans (Homo sapiens)

Recognition of facial expressions by a Japanese monkey and two humans was studied. The monkey subject matched 20 photographs of monkey facial expressions and 20 photographs of human facial expressions. Humans sorted the same pictures. Matching accuracy by the monkey was about 80% correct for both human and monkey facial expressions. The confusion matrices of those facial expressions were analyzed by a multi-dimensional scaling procedure (MDSCAL). The resulting MDS plots suggested that the important cues in recognizing facial expressions of monkeys were ''thrusting the mouth'' and ''raising the eyebrows.'' Comparison of the MDS plots by the monkey subject with those by human subjects suggested that the monkey categorized the human ''happiness'' faces. This may suggest that the monkey has an ability to recognize human smile face even though it is learned. However, the monkey did not differentiate the human ''anger/disgust'' faces from the human ''sad'' faces, while human subjects clearly did. This may correlate with the lack of eyebrow movement in monkeys.