The effects of centrally blocking hippocampal theta activity on learning and retention

Abstract

Listen

Translate article

Although hippocampal theta has been correlated with several possible functions of the hippocampus in mediating behavior, the necessity of this bioelectric response for the occurrence of these behaviors has not been substantiated. The present research investigated this issue by examining the effects of blocking hippocampal theta activity on learning and retention of a simultaneous brightness discrimination, a task whose performance is normally accompanied by theta on 90–95% of the approach trials. Our previous research suggested that the occurrence of theta in this situation is related to functions performed by the hippocampus in mediating specific processes of attention. Theta blocking was accomplished by injecting 2 μ liters of scopolamine hydrobromide (10 μg/μliter) into the medial septal nucleus of cats via a chronically implanted cannula. This procedure resulted in theta occurring on only 6–8% of the approach responses as compared to the normal 92–93% of the trials in the nondrugged state. Animals were administered the drug centrally either during original learning or during retention test sessions. It was found that neither the rate of original task learning nor the amount retained were affected by theta blocking. Thus, theta is not necessary for the occurrence, during learning, of behaviors with which it is normally correlated, and hippocampal theta is clearly not required for either the laying down of memory traces or for the subsequent recall of information from memory as some researchers have contended. The significance of the hippocampal theta pattern as an index of hippocampal functioning is also discussed. EEG and lesion literature are reviewed which suggest that when theta is present, the hippocampus is in a relatively inactive or inhibited state. It is proposed that differences in the frequency of theta in certain behavior situations reflect different degrees of inhibition of the hippocampus.