3D spatial coding in the monkey hippocampus

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Since the discovery of hippocampal place cells by O'Keefe, there have been many studies on the neural basis of spatial cognition in the rodent hippocampus, but knowledge on primate hippocampal place cells is limited. The purpose of this study was to record neuronal activity from the hippocampus of monkeys moving in a 3D environment, and to clarify the relationship between the animal's direction of movement (horizontal and vertical) and place responsiveness. Conventional recording of neuronal activity via signal transmission cables is problematic because it interferes with the movement of monkeys in a 3D environment with complex structures. Previously we trained the monkeys in a 3D environment consisting of horizontal and vertical moving spaces connected in an L-shape, and found the movement of the monkey was restricted at the L-shaped ends, which could make it difficult to analyze neuronal activity. Therefore, this fiscal year we completed a square doughnut-shaped 3D environment. In this fiscal year, we first and trained a rhesus monkey to move in clockwise and counter-clockwise directions in this 3D environment. We also developed an AI-based monkey behavior monitoring system capable of identifying the monkey's position and pose. Then, a cranioplastic head cap was surgically attached to the head of the monkey; evoked potential mapping is currently being conducted to accurately estimate the optimal location for recording neuronal activity.