The ability to follow eye gaze and its emergence during development in macaque monkeys

The ability of monkeys to follow the gaze of other individuals is a matter of debate in many behavioral studies. Physiological studies have shown that in monkeys, as in humans, there are neural correlates of eye direction det ection. There is little evidence at the behavioral level, however, of the p resence and development of such abilities in monkeys. The aim of the presen t study was to assess in juveniles and adult pig-tailed macaques (Macaca ne mestrina) the capacity to use eye cues only to follow the gaze of an experi menter. Biological stimuli (head, eye, and trunk movements) were presented by an experimenter to 2 adult monkeys with their heads restrained (Experime nt 1) and to 11 monkeys of different ages, free to move in their home cages (Experiment a). A nonbiological stimulus served as a control. Results show ed that macaques can follow the gaze of the experimenter by using head/eye and eye cues alone. Trunk movements and nonbiological stimuli did not signi ficantly elicit similar reactions. Juvenile monkeys were not able to orient their attention on the basis of eye cues alone. In general, gaze following was more frequent in adults than in juveniles. Like in humans, however, su ch abilities in macaques dramatically improve with age suggesting that the transition to adulthood is a crucial period in the development of gaze-foll owing behavior.