Rhythmic theta-like unit activity of the hippocampal formation during acquisition and performance of avoidance behavior in rabbits

Abstract

Neuronal activity was recorded from the hippocampal formation during instrumental conditioning of avoidance behavior in rabbits. The conditional stimuli (CSs) were pure tones, the unconditional stimulus (UCS) was a constant-current footshock, and the response was locomotion in a rotating wheel apparatus. Prior to conditioning the rabbits received pretraining, involving noncontingent presentations of the CSs and the UCS. The specific focus of this report is on neuronal records which displayed rhythmic bursts of action potentials similar to the theta rhythm of the EEG. Early in training, rhythmic theta-like neuronal bursts occurred throughout the CS-UCS interval on trials manifesting the initialmost behavioral conditioned responses (CRs). Late in training during asymptotic CR performance there was a build-up in the frequency and rhythmicity of neuronal bursts prior to the CR. The maximum frequency of bursting (9–10 Hz) always occurred just prior to the CR. Relatively little theta-like activity occurred in the early portions of the CS-UCS interval, in the late stage of training. During pretraining ample preresponse theta-like activity occurred in some subjects but not in others. However, no subject manifested a systematic build-up in bursting prior to behavioral responses in pretraining.