Cross-modal representation of identity in primate hippocampus

Faces and voices are the dominant social signals used to recognize individuals amongst human and nonhuman primates. Yet, evidence that information across these signals can be integrated into a modality-independent representation of individual identity in the primate brain has been reported only in human patients. Here we show that, like humans, single neurons in the marmoset monkey hippocampus exhibit invariant neural responses when presented with the faces or voices of specific individuals. However, we also identified a population of single neurons in hippocampus that was responsive to the cross-modal identity of multiple conspecifics, not only a single individual. An identity network model revealed population-level, cross-modal representations of individuals in hippocampus, underscoring the broader contributions of many neurons to encode identity. This pattern was further evidenced by manifold projections of population activity which likewise showed separability of individuals, as well as clustering for family members, suggesting that multiple learned social categories are encoded as related dimensions of identity in hippocampus. The constellation of findings presented here reveals a novel perspective on the neural basis of identity representations in primate hippocampus as being both invariant to modality and comprising multiple levels of acquired social knowledge.