Associative and non-associative changes in unit activity of the rat hippocampus.

Two main types of neurons of the dorsal hippocampus were recorded in chronic rats during the classical conditioning of an arousal phenomenon (neocortical EEG desynchrony). The relative importance of the associative and non-associative factors was assessed by a differentiation procedure. In naive rats, during the "acquisition" session most type-I neurons rapidly acquired an "inhibitory" response closely parallel to the EEG response. Neither of these responses showed differentiation. Some type-II neurons acquired an "excitatory" response which was independent of the EEG response and which had a significant tendency to differentiation. These data were confirmed in rats submitted to several "retention" sessions. All the type-I neurons were undifferentiated while 11 out of 41 type-II neurons were differentiated. Type-I neurons were characterized by a bursting mode of discharge. Their activity was higher during slow wave sleep (SWS) than during wakefulness (W) or paradoxical mode of discharge. Most of the differentiated type-II neurons were more active during W and/or PS than during SWS.