Abstract
Rhesus monkeys gather much of their knowledge of the social world through visual input and may preferentially represent this knowledge in the visual modality. Recognition of familiar faces is clearly advantageous, and the flexibility and utility of primate social memory would be greatly enhanced if visual memories could be accessed cross-modally either by visual or auditory stimulation. Such cross-modal access to visual memory would facilitate flexible retrieval of the knowledge necessary for adaptive social behavior. We tested whether rhesus monkeys have cross-modal access to visual memory for familiar conspecifics using a delayed matching-to-sample procedure. Monkeys learned visual matching of video clips of familiar individuals to photographs of those individuals, and generalized performance to novel videos. In crossmodal probe trials, coo-calls were played during the memory interval. The calls were either from the monkey just seen in the sample video clip or from a different familiar monkey. Even though the monkeys were trained exclusively in visual matching, the calls influenced choice by causing an increase in the proportion of errors to the picture of the monkey whose voice was heard on incongruent trials. This result demonstrates spontaneous cross-modal recognition. It also shows that viewing videos of familiar monkeys activates naturally formed memories of real monkeys, validating the use of video stimuli in studies of social cognition in monkeys.
Highlights
Many primate species have complex social repertoires that require individual recognition [1,2,3,4,5,6,7]
Even when we tested our subjects with videos from substantially different views and context in a final transfer test, they showed significant transfer of matching performance. These results show that monkeys extracted invariant features from a subset of videos that allowed them to generalize selection of the appropriate still image across the considerable variation in the sample videos. While such successful generalization suggests that the monkeys recognize the familiar monkeys depicted in the videos as those they live with, it is possible that performance is based strictly on similarity among the videos, with no reference to memories of the familiar monkeys formed outside the context of the experiment
Subjects continued to perform well above the level expected by chance in very challenging generalization tests with videos collected from a new view and in a different context (Figure 2)
Summary
Many primate species have complex social repertoires that require individual recognition [1,2,3,4,5,6,7]. Controlled laboratory tests, in which learning is experimentally manipulated, are required to address these important questions in social cognition. To date, few such experimental studies of social recognition have been conducted; far more effort has been devoted to understanding how primates perceive, process, and remember nonsocial stimuli (see a recent review [13]). The ability to keep track of the social relations of conspecifics is critical for survival in many species [5,14] and individual recognition is a fundamental cognitive requirement for such mental tracking of the social environment. Monkeys and apes do discriminate and identify specific faces (e.g. discrimination: [16,17,18,19,20,21,22], ‘‘identification’’ using symbols: [23,24]), and recent studies have begun to characterize the underlying perceptual mechanisms for face recognition in nonhuman primates [25,26,27,28,29]
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