Relationships of hippocampal theta cycles with bar pressing during self-stimulation.

Previous reports have demonstrated a relationship between hippocampal rhythmical slow activity (theta) and movement, and have suggested that rhythmical movements tend to occur during certain phase of theta. Therefore, the relationships between theta cycles and voluntary motor activity were investigated. Bar pressing for electrical brain stimulation provided the reference to relate hippocampal activity. Ongoing theta increased in amplitude and frequency before and after pressing. Periodic waves preceding and following pressing. These waves only appear if the presses occur at particular phases of the theta cycles. They gradually disappeared during sessions lasting 4 to 8 hr. Light weights ( less than 90 g) added to the lever did not alter theta, but heavier weights (greater than 90 g) produced averages without periodic waves and with movement-related potentials. Introduction of a delay between pressing and electrical stimulation delayed evoked potentials, while averaging indicated that periodical waves persisted before and after bar pressing. Total lesions of the septum or superior fornix abolished theta and increased the frequency of self stimulation. In animals with partial lesions, theta reappeared during pressing. The above results indicating that the rats tended to lever press during particular phases of theta, suggest that phase-locked theta may be a corollary of motor mechanisms, and perhaps of the timing of motor responses.