Plasticity of ability to form cross-modal representations in infant Japanese macaques

In a previous study, Adachi, Kuwahata, Fujita, Tomonaga & Matsuzawa demonstrated that infant Japanese macaques (Macaca fuscata) form cross-modal representations of conspecifics but not of humans. However, because the subjects in the experiment were raised in a large social group and had considerably less exposure to humans than to conspecifics, it was an open question whether their lack of cross-modal representation of humans simply reflected their lower levels of exposure to humans or was caused by some innate restrictions on the ability. To answer the question, we used the same procedure but tested infant Japanese macaques with more extensive experience of humans in daily life. Briefly, we presented monkeys with a photograph of either a monkey or a human face on an LCD monitor after playing a vocalization of one of these two species. The subjects looked at the monitor longer when a voice and a face were mismatched than when they were matched, irrespective of whether the preceding vocalization was a monkey's or a human's. This suggests that once monkeys have extensive experience with humans, they will form a cross-modal representation of humans as well as of conspecifics.