Rapid updating of the hippocampal representation of space.

Abstract

Several studies suggest that rats solving spatial tasks in relation to stimulus arrays fail to use novel cues to find a goal. This is analogous to a learning phenomenon called ‘blocking’ where prior associations of a stimulus and an event prevent the subsequent association of a novel stimulus with the same event. Contrary to these findings, the cognitive map theory of spatial learning predicts that novel spatial cues should be attended to and rapidly incorporated into the representation of the environment. We therefore ask if blocking is seen for stimulus control over the firing fields of hippocampal place cells. Rats in two blocking groups foraged for food pellets in the presence of a white card for 40 mins. Rotations of the white card caused equal field rotations. Rats were then exposed to the white card and a novel black card for 10 mins (short compound group) or 60 mins (long compound group). Rotation of the black card alone elicited equal field rotations as well as a significant decrease in field coherence. Fields from 2 long compound rats remained in a similar position as the previous session suggesting that these rats were ‘blocked’. Rats in the control group were initially exposed to both black and white cue cards for 20 mins. Each card exhibited control over field location although significant decreases in firing rate and information content were found. The results suggest that rats tend to incorporate new spatial information in a map-like representation of the environment. However, increased compound exposure time may make spatial blocking possible.