Effects of self-locomotion on the activity of place cells in the hippocampus of a freely behaving monkey.

The activity of hippocampal cell assemblies is considered to function as the neural substrate for a cognitive map in various animal species. The firing rate of hippocampal place cells increases when an individual animal reaches a specific location in an environment. Although cumulative views on place cells have been revealed by studies using rodents under free-behavior conditions, few studies have addressed the spatial representation provided by hippocampal neurons in primates. Moreover, although previous work in rats has demonstrated the importance of locomotion velocity and direction in the spatially selective discharge of hippocampal neurons, it remains unknown whether a corresponding phenomenon exists in the primate hippocampus. In the present study, we recorded the activity and investigated the spatial representation of the hippocampal neurons in a freely behaving monkey performing a shuttle-movement task. We observed increased activity in a subset of hippocampal neurons (place cells) when the monkey crossed a particular location. Many of the monkey place cells exhibited sensitivity to locomotion velocity rather than to locomotion direction. These results suggest the existence of primate hippocampal place cells comparable to those in rodents, with the exception that, in primates, velocity information has a stronger impact on place cell activity than directional information.