Functional microcircuit recruited during retrieval of object association memory in monkey perirhinal cortex.

The primate temporal cortex implements neural mechanisms for memory retrieval from visual long-term storage, and memory neurons have been identified at the single-neuron level whose activities following cue presentation encode the presented object ("cue-holding" neurons) or to-be-recalled target ("pair-recall" neurons). Although these two types of neurons can potentially interact during the target recall, little is known about information flow among these neurons. We conducted simultaneous recordings of multiple single units in macaque perirhinal cortex while they performed a pair-association memory task. Granger causality analysis revealed the emergence of directed couplings during the delay period predominantly from cue-holding neurons to pair-recall neurons. Moreover, these interactions coincided with unidirectional signal flow from the recipient recall neuron to another recall neuron, implying cascade-like signal propagation among the memory cell assembly. These results suggest that directed interactions among perirhinal memory neurons are dynamically modulated to implement functional microcircuitry for retrieval of object association memory.