CONDITIONED RESPONSES OF HIPPOCAMPAL AND OTHER NEURONS

Changes in the firing rates of neurons in various subcortical structures induced by application of a 1 sec auditory stimulus were recorded in unrestrained animals during movement-free intervals. Pseudo-conditioning trials (500) were followed by conditioning trials (300). During the latter, a food pellet was presented to the animal by a dispenser after every stimulus, provided there was no detectable movement during the stimulus period. The difference between the response rates before conditioning and after conditioning was considered to be a measure of the effect induced by the conditioning procedure. 1. 1. Except for the early response in the dorsal reticular formation, responses consisting of incremental firing rate were increased in size and number and decremental response were attenuated or reversed. Even in the dorsal reticular formation the neuronal activity rate during the last half of the stimulus was augmented. 2. 2. Before conditioning an increment in the firing rate of neurons appeared in most areas during the first 300 msec of stimulation after which the firing rate returned almost to control levels or even systematically below control levels in some areas. In most cases, conditioning procedures caused the early increment to be augmented and the neuronal rate to stay elevated above control levels during the remainder of the 1 sec stimulus (thus reversing the decremental tendency where it had appeared). 3. 3. Incremental changes were largest in hippocampus. These changes were, on the basis of deductive arguments, considered to be independent of skeletal and attentional components of the conditioned response on the one hand, and unrelated to sleep and waking or inhibitory processes on the other. It seemed possible therefore that the changes might be related to a short term or temporary trace of a motivationally significant event. © 1969.