The role of the entorhinal cortex in object recognition memory and spatial memory in the Japanese monkey

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1 : Connections of the entorhinal cortex (EC) with the hippocampal formation (HF) were examined using retrograde and anterograde treats in Japanese monkeys. The EC had heavy projection fibers to the dentate gyrus (DG) and CA1-CA3 fields. Those in the DG and CA1-CA3 fields were distributed in the molecular layer. Main inputs from the HF to the EC were originated from the pyramidal layer of the subiculum (Sub) and CA1 field. The CA1 field was superior to the Sub in the number of projection cells. All these projections were topographically organized ; the medial part of the EC was connected with the rostral portion of the HF (the DG, CA1-CA3 fields and Sub), while the lateral part of the EC was with the caudal portion. These results suggest that the EC had two main outputs to the HF (to the DG and CA1 field) and two main inputs from the HF (from the CA1 field and Sub). 2 : Effects of bilateral EC lesions in Japanese monkeys were examined on delayed matching to location (DML), to color (DMC), and to photographic stimuli (DMP) with 2 foils, and transverse patterning problem (TPP). For DML a mild impairment was produced only in relearning, while no impairment was seen in delay test (20, 30 and 40 sec. delays). For DMC, no deficit was seen on relearning or delay test (30, 60 and 120 sec. delays). For DMP, a mild impairment was produced in long-term delay (15 min.) of delay test alone, while no impairment was seen in relearning or short-term delay conditions (30, 60 and 120 sec. delays). In TPP, the EC monkeys learned as quickly as the controls. These results suggest that the EC might be involved at least in retention of spatial learning and recognition of stimuli with longer delay (15 min.).