Rewarding intracranial stimulation,movement and the hippocampal theta rhythm

Some workers have suggested that hippocampal theta rhythm reflects activity of the neural reward system, while others have thought it to be primarily a correlate of voluntary movement. In order to examine these views, rats were trained to remain immobile while receiving rewarding intracranial stimulation; in this way the effects of the rewarding stimulation on hippocampal electrical activity were separated from the effects of movement (exploratory locomotion) that usually accompanies such stimulation. Rewarding intracranial stimulation was found to be neither necessary nor sufficient for the appearance of theta rhythm. Theta rhythm was present when rats performed large continous movements and absent when they remained immobile or performed small interrupted movements. Thus, theta rhythm is not an intrinsic correlate of rewarding stimulation but is closely associated with mechanisms that produce movement.