Representation of place by monkey hippocampal neurons in real and virtual translocation

The hippocampal formation (HF) is hypothesized as a neuronal substrate of a cognitive map, which represents environmental spatial information by an ensemble of neural activity. However, the relationships between the hippocampal place cells and the cognitive map have not been clarified in monkeys. The present study was designed to investigate how activity patterns of place-selective neurons encode spatial relationships of various environmental stimuli; to do this, we used multidimensional scaling (MDS) for hippocampal neuronal activity in the monkey during the performance of real and virtual translocation. Of 389 neurons recorded from the monkey HF and parahippocampal gyrus (PH), 166 had place fields that displayed increased activity in a specific area of an experimental field and/or on a monitor (place-selective neurons). The MDS transformed relationships among the 16 places in the experimental field and the monitor, expressed as correlation coefficients between all possible pairs of two places based on the 166 place-selective responses, into geometric relationships in a two-dimensional MDS space. In the real translocation tasks, the 16 places were distributed throughout the MDS space, and their relative positions were well correlated to real positions in the experimental laboratory. However, the correlation between the MDS space and real arrangements was significantly smaller in virtual than real translocation tasks. The present results strongly suggest that activity patterns of the HF and PH neurons represent spatial information and might provide a neurophysiological basis for a cognitive map.