Abstract
Crossmodal integration of audio/visual information is vital for recognition, interpretation and appropriate reaction to social signals. Here we examined how rhesus macaques process bimodal species-specific vocalizations by eye tracking, using an unconstrained preferential looking paradigm. Six adult rhesus monkeys (3M, 3F) were presented two side-by-side videos of unknown male conspecifics emitting different vocalizations, accompanied by the audio signal corresponding to one of the videos. The percentage of time animals looked to each video was used to assess crossmodal integration ability and the percentages of time spent looking at each of the six a priori ROIs (eyes, mouth, and rest of each video) were used to characterize scanning patterns. Animals looked more to the congruent video, confirming reports that rhesus monkeys spontaneously integrate conspecific vocalizations. Scanning patterns showed that monkeys preferentially attended to the eyes and mouth of the stimuli, with subtle differences between males and females such that females showed a tendency to differentiate the eye and mouth regions more than males. These results were similar to studies in humans indicating that when asked to assess emotion-related aspects of visual speech, people preferentially attend to the eyes. Thus, the tendency for female monkeys to show a greater differentiation between the eye and mouth regions than males may indicate that female monkeys were slightly more sensitive to the socio-emotional content of complex signals than male monkeys. The current results emphasize the importance of considering both the sex of the observer and individual variability in passive viewing behavior in nonhuman primate research.
Highlights
Successful integration into complex social environments requires humans and nonhuman primates to recognize, manipulate, and behave according to the immediate social context
On the congruent stimulus video (Figure 3A), they spent more time looking to the eye region than the mouth region (F(1,4) = 69.115, p = 0.001), and looked longer to the eye and mouth regions than to the rest of the video (eyes > other F(1,4) = 45.672, p = 0.003; mouth > other F(1,4) = 21.927, p = 0.009)
There was a weak trend for a sex difference in the relative looking to the eye and mouth regions (ROI x Sex interaction: F(1,4) = 5.262, p = 0.084), with females exhibiting a larger differentiation than males (Figure 3A inset)
Summary
Successful integration into complex social environments requires humans and nonhuman primates to recognize, manipulate, and behave according to the immediate social context. Key elements of this task are building representations of relations between self and others, and flexibly using these representations to guide social behavior [1,2]. The remarkable behavioral [3-6] similarities between humans and nonhuman primates include the use of species-specific facial expressions and vocalization [7-9] For both species, decoding the specific “message” of a social display relies on crossmodal integration. The rhesus communicative system is comprised of a small repertoire of relatively fixed calls characterized with distinct facial expressions, postures, and gestures and associated with particular social contexts. This repertoire has been successfully used to explore the evolutionary basis and neural mechanisms of visual speech perception (reviewed by [10])
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