Forward Processing of Object-Location Association from the Ventral Stream to Medial Temporal Lobe in Nonhuman Primates.

While the hippocampus (HPC) is a prime candidate combining object identity and location due to its strong connections to the ventral and dorsal pathways via surrounding medial temporal lobe (MTL) areas, recent physiological studies have reported spatial information in the ventral pathway and its downstream target in MTL. However, it remains unknown whether the object-location association proceeds along the ventral MTL pathway before HPC. To address this question, we recorded neuronal activity from MTL and area anterior inferotemporal cortex (TE) of two macaques gazing at an object to retain its identity and location in each trial. The results showed significant effects of object-location association at a single-unit level in TE, perirhinal cortex (PRC), and HPC, but not in the parahippocampal cortex. Notably, a clear area difference emerged in the association form: 1) representations of object identity were added to those of subjects' viewing location in TE; 2) PRC signaled both the additive form and the conjunction of the two inputs; and 3) HPC signaled only the conjunction signal. These results suggest that the object and location signals are combined stepwise at TE and PRC each time primates view an object, and PRC may provide HPC with the conjunctional signal, which might be used for encoding episodic memory.