Long-term visual recognition of familiar persons, peers, and places by young monkeys (Macaca fuscata).

Five-year-old Japanese monkeys were tested on long-term visual recognition memory. The objects used were those that they had encountered daily during their first 2 years of life as to-be-remembered stimuli: persons, peers, and places. After a 3-year delay, we conducted a recognition test using the preferential looking paradigm where the monkeys were presented with pictures of these familiar stimuli in combination with those of novel stimuli. The monkeys looked relatively longer at pictures of familiar stimuli than at those of novel stimuli, while participants in the control group, who had never seen the stimuli in question, showed no discriminative preference. The monkeys thus recognized the familiar stimuli through the pictures, suggesting retention of visual information on real objects encountered in their lives even after a 3-year delay. Our present findings confirmed long-term visual recognition in monkeys, which might be essential to ecologically and socially significant behaviors such as individual identification.