Hippocampal theta activity, learning and behavioral tendencies

Abstract

Trace eyeblink conditioning is a method that models a declarative, awareness-dependent form of learning. Hippocampal theta activity, an approximately 3.5 - 8.5 Hz oscillatory pattern in rabbits, has been associated with learning-related phenomena: arousal, attentional processes and association forming. In some studies theta-contingent training, given during a brain state rich in hippocampal theta oscillation, has improved the initial learning in trace eyeblink conditioning. However, other laboratories have failed to replicate this effect. This study explored the effects of theta-contingent conditioning and extinction on learning, and the connection between the magnitude of spontaneous hippocampal theta activity and the behavioral tendencies or individual dispositions of a subject. In this study, Open Field test (OFT) was used to measure behavioral tendencies. Spontaneous hippocampal theta ratio was recorded, and theta-contingent conditioning was performed in three groups: high (T+) and low-theta training (T-) and yoked controls (Y). Unexpectedly, spontaneous hippocampal theta activity did not correlate significantly with the OFT variables. Moreover, while the amount of conditioned responses (CRs) increased as a function of training, the training group had no effect on the learning rate. However, the T- group had the highest proportion of subjects that reached the 60-% learning criterion, and the mean CR% in the best session was significantly higher in the T- group than in the T+ group. In extinction, the T+ group learned significantly faster than the Y group. When classifying the subjects as active versus inactive based on the OFT, T- training significantly improved the learning rate of only the active subjects. The results show that high-theta training did not improve learning in trace eyeblink conditioning, and suggest that theta-contingent training may affect differently subjects with distinct behavioral tendencies.