Different effects of posttrial driving or blocking of the theta rhythm on avoidance learning in rats

Animals with and without septal electrodes were trained in two simple avoidance tasks, three weeks apart. Following the training trials, some implanted animals received 20 min of either low (LF) or high frequency (HF) septal stimulation, which drove or blocked, respectively, the hippocampal theta rhythm. Animals receiving the theta-driving stimulation subsequently exhibited improved retention performance in the 1-way active avoidance task in comparison to animals receiving HF stimulation or to implanted controls. In the 1-trial passive inhibitory task, LF animals subsequently performed better than did HF animals, but the LF animals' performance was not quite significantly higher than performance of implanted controls. Low frequency stimulation during testing also improved performance in the active avoidance task. Unoperated controls performed at a higher level than implanted controls and HF animals in both tasks, suggesting a lesion effect of the bilaterally-implanted septal electrodes. Nevertheless, the design of these experiments, in which posttrial treatment administration and two tasks requiring opposite responses were employed, appears to provide reasonably direct evidence that the activation of theta rhythms is not simply correlated with learning situations, but also apparently improves the retention of information that has recently been acquired. Thus, these rhythms could be associated with memory storage processes.