Space,Time and Memory in the Hippocampal Formation

In primates as well as rodents, the posterior parietal cortex maps spatial relationships having both egocentric and external frames of reference. In this chapter, the form in which rat posterior parietal cortex neuronal activity maps position within trajectories through the environment is considered in detail and compared to the forms of spatial mapping observed for neurons of the hippocampus and entorhinal cortex. Evidence is presented to indicate that posterior parietal neurons simultaneously map positions both within and across segments of paths through an environment. It is suggested that the specific nature of posterior parietal cortex mapping of space serves, in part, to transition knowledge of position in the environment, given by hippocampus and entorhinal cortex, into efficient path-running behavior via projections to primary and secondary sensory and motor cortices. Posterior parietal cortex activity is also hypothesized to play a role both in driving trajectory dependence of hippocampal place cells and in anchoring spatially specific hippocampal and entorhinal cortical activity to the boundaries of the observable environment.