Increase of the hippocampal theta activity in the Morris water maze reflects learning rather than motor activity.

The change in the percentage of rat hippocampal high-frequency theta activity from being immobile and awake to swimming behaviour was calculated for three groups of rats, trained in either place learning, cue learning or egocentric learning in the Morris water maze. The place-learning-trained rats showed an increase in the percentage of theta activity, along with a significant reduction in escape latency over the last 3 days of training. No changes were observed in the other two groups. Because the motor activity displayed by the three groups of rats was similar, we suggest that the increase in the percentage of theta activity concomitant with place-learning training could be related to the processing of information by the hippocampus, rather than to the displayed motor activity.