Stimulation-induced reset of hippocampal theta in the freely performing rat.

Previous research has suggested that visual and auditory stimuli in a working memory task have the ability to reset hippocampal theta, perhaps allowing an organism to encode the incoming information optimally. The present study examined two possible neural pathways involved in theta resetting. Rats were trained on a visual discrimination task in an operant chamber. At the beginning of a trial, a light appeared over a centrally located lever that the rat was required to press to receive a water reward. There was a 30-s intertrial interval before the next light stimulus appeared. After learning the task, all rats received surgical implantation of stimulating electrodes in both the fornix and the perforant path and recording electrodes, bilaterally in the hippocampus. After surgery, theta was recorded before and after the light stimulus to determine whether resetting to the visual stimulus occurred. During the intertrial interval, rats received single-pulse electrical stimulation of either the fornix or perforant path. Theta was recorded both before and after the electrical stimulation to determine whether resetting occurred. In this experiment, hippocampal theta was reset after all three stimulus conditions (light, perforant path, and fornix stimulation), with the greatest degree of reset occurring after the fornix stimulation. The results suggest that activation of the perforant path and fornix may underlie theta reset and provide a mechanism by which the hippocampus may enhance cognitive processing.