Rat hippocampal neurons fire in a spatially selective fashion [1]. We show that place fields enlarge (by 75%) and shift (by 1.4cm) in a direction opposite to the direction of movement of the rat, within a few traverses of a route, even if the environment has been experienced extensively on previous days. The expansion is not a result of locomotion or neural activity per se because it reoccurs when the rat runs on a different track immediately after running on the first one. This provides an evidence for systematic changes in neuronal firing properties due to and during experience. The results are consistent with the predictions of models [2, 3] of learning of sequences via Hebbian [4] synaptic potentiation. Thus, these data provide an evidence for Hebbian synaptic enhancement during behavior, and show that such learning occurs even when a rat enters a highly familiar environment after a day's absence.
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