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.