Hippocampal function was analyzed by making recordings from hippocampa
l neurons in monkeys actively walking in the laboratory. ''Spatial vie
w'' cells, which respond when the monkey looks at a part of the enviro
nment, were analyzed. To assess quantitatively the information about t
he spatial environment represented by these cells, we applied informat
ion theoretic techniques to their responses. The average information p
rovided by these cells about which location the monkey was looking at
was 0.32 bits, and the mean across cells of the maximum information co
nveyed about which location was being looked at was 1.19 bits, measure
d in a period of 0.5 s. There were 16 locations for this analysis, eac
h being one-quarter of one of the walls of the room. It also was shown
that the mean spontaneous rate of firing of the neurons was 0.1 spike
s/s, that the mean firing rate in the center of the spatial held of th
e neurons was 13.2 spikes/s, and that the mean sparseness of the repre
sentation measured in a 25-ms period was 0.04 and in a 500-ms time per
iod was 0.19; (The sparseness is approximately equivalent to the propo
rtion of the 25- or 500-ms periods in which the neurons showed one or
more spikes.) Next it was shown that the mean size of the view fields
of the neurons was 0.9 of a wall. In an approach to the issue of how a
n ensemble of neurons might together provide more precise information
about spatial location than a single neuron, it was shown that in gene
ral the neurons had different centers for their view fields. It then w
as shown that the information from an ensemble of these cells about wh
ere in space is being looked at increases approximately linearly with
the number of cells in the ensemble. This indicates that the number of
places that can be represented increases approximately exponentially
with the number of cells in the population. It is concluded that there
is an accurate representation of space ''out there'' in the primate h
ippocampus. This representation of space out there would be an appropr
iate part of a primate memory system involved in memories of where in
an environment an object was seen, and more generally in the memory of
particular events or episodes, for which a spatial component normally
provides part of the context.