The hippocampus and space: Are there “place neurons” in the monkey hippocampus?

Since the findings by Scoville and Milner (1957), the hippocampus has been presumed to have an important central function in memory. As they reported, amnesic patients with medial temporal lobe damage did not show deficits in every kind of memory or learning ability. It is now accepted by many investigators in the field of memory and learning that memory can be divided into several subtypes, and damage of the hippocampus does not impair all kinds of memory. One critical aspect of this forum seems to be the determination of suitable memory subtypes. As Nadel described in the lead article, which is based on the cognitive map theory, involvement of spatial factors is a strong candidate for this rule because spatial memory is reported to be severely impaired by hippocampal damage, not only in the rat, but also in the monkey (Parkinson et al., 1988; Gaffan and Harrison, 1989) and human (Smith and Milner, 1981; 1989). The existence of “place neurons” in the rat is presumed to provide strong evidence for foundation of the cognitive map theory (O’Keefe and Nadel, 1978). Thus, if one is to discuss whether spntitrl cognitive mapping is also applicable to the primate hippocampus, it becomes critical to determine if there are neurons that change their firing rates in particular locations in the environment. Although investigation of this is very rare in humans, because it must be limited to clinical examination of the results of accidental damage, more complete evaluation is possible in the monkey. The first study of this subject has recently been reported (On0 et al., 1991). Although there are still far less data than there are for the rat, we believe it will be useful for us to describe some of our results here.