Hippocampal neurons in the monkey with activity related to the place in which a stimulus is shown.

In order to analyze the functions of the hippocampus in the primate, and to advance the understanding of amnesia, the activity of 994 single hippocampal neurons in the monkey was analyzed during the performance of a task known to be affected by hippocampal damage in which both an object, and its position in space, must be remembered. The serial multiple object-place memory task required a memory for the position on a video monitor in which a given object had appeared previously. It was found that 9.3% of neurons recorded in the hippocampus and hippocampal gyrus had spatial fields in this and related tasks, in that they responded whenever there was a stimulus in some but not in other positions on the screen. We found that 2.4% of the neurons responded to a combination of spatial information and information about the object seen, in that they responded more the first time a particular object was seen in any position. Six of these neurons were found that showed this combination even more clearly, in that, for example, they responded only to some positions and only if it was the first time that a particular stimulus had appeared there. It is concluded that there are neurons in the primate hippocampus which (1) respond to position in space and (2) in some cases combine information about stimuli and their position in space, responding to a stimulus only the first time it is seen in a position in space, for example. Thus, not only is spatial information processed by the primate hippocampus, but it can be combined with information about which stimuli have been seen before. The ability of the hippocampus to form such conjunctions may be an important property for its role in memory.