A TEST OF THE REVERBERATORY SHORT-TERM MEMORY HYPOTHESIS FOR HIPPOCAMPAL 'PLACE' CELLS

E.I. Moser¹*; M. Moser¹; F.P. Houston²; M. Newton²; P. Lipa²; J. Meltzer2; C.A. Barnes2; B.L. McNaughton2

1. Ctr Biology Memory, Trondheim, Norway
2. NSMA, Univ Arizona, Tucson, AZ, USA

The cellular and network level dynamics underlying place specific firing remain poorly understood. According to one hypothesis, external information triggers the initial firing as the rat enters the place field, but subsequent firing represents some form of reverberatory short-term memory, with a capacity for short sequences of items which are replayed each theta cycle. As the rat moves in space, new information would be added to the top of the reverberation stack and old information would drop off the bottom, resulting in the observed, gradual phase shift known as "phase precession". This hypothesis predicts that, if the hippocampus is transiently inactivated while the rat is in a place field, but beyond the beginning of it, the memory will be lost, and so the place field will be truncated. Stimulation of the cortical input to the hippocampus causes widespread, strong activation of inhibitory interneurons, via both feed-forward and feedback connections. During the IPSP (about 30 msec), synaptically evoked discharge is suppressed. Bilateral perforant path stimulation delivered at 4 sec intervals while rats traveled around a track typically resulted in complete suppression of all CA1 cell activity recorded from multiple tetrodes for 100-300 msec. Paradoxically, electrically-evoked population spikes are substantially enhanced for a period of about 100 msec after a short inhibitory phase following perforant path stimulation, yet spontaneous activity is strongly suppressed. Irrespective of where in a place field the stimulation occurred, there was no evidence (in over 120 pyramidal cells) for place field truncation. Nor did the inhibition appear to affect the relationship between firing phase and position.Thus, reverberation of activity within the hippocampus does not appear to provide any contribution to place field dynamics.

Support Contributed By: MH46823, MH01565 & Norwegian Research Council

hippocamus, temporal coding, phase precession, learning and memory