Anatomical organization of forward fiber projections from area TE to perirhinal neurons representing visual long-term memory in monkeys.

A number of studies have shown that the perirhinal (PRh) cortex, which is part of the medial temporal lobe memory system, plays an important role in declarative long-term memory. The PRh cortex contains neurons that represent visual long-term memory. The aim of the present study is to characterize the anatomical organization of forward projections that mediate information flow from visual area TE to memory neurons in the PRh cortex. In monkeys performing a visual pair-association memory task, we conducted an extensive mapping of neuronal responses in the anteroventral part of area TE (TEav) and area 36 (A36) of the PRh cortex. Then, three retrograde tracers were separately injected into A36 and the distribution of retrograde labels in TEav was analyzed. We focused on the degree of divergent projections from TEav to memory neurons in A36, because the highly divergent nature of these forward fiber projections has been implicated in memory function. We found that the degree of divergent projection to memory neurons in A36 was smaller from the TEav neurons selective to learned pictures than from the nonselective TEav neurons. This result demonstrates that the anatomical difference (the divergence) correlates with the physiological difference (selectivity of TEav neurons to the learned pictures). Because the physiological difference is attributed to whether the projections are involved in information transmission required for memory neurons in A36, it can be speculated that the reduced divergent projection resulted from acquisition of visual long-term memory, possibly through retraction of the projecting axon collaterals.