The effect of temporal single alternation on learned increases in hippocampal unit activity in classical conditioning of the rabbit nictitating membrane response

Abstract

Rabbits were trained with a temporal single-alternation schedule in which reinforced (CS-US) trials were regularly alternated with nonreinforced (CS-alone) trials. Thus, CS-US trials were invariably preceded by CS-alone trials and CS-alone trials were invariably preceded by CS-US trials. In agreement with previous research, there was no evidence of behavioral discrimination; that is, conditioned nictitating membrane responses were no larger on CS-US trials than on CS-alone trials. In contrast, hippocampal multiple unit activity showed (1) greater conditioned increases on CS-US trials and (2) lowered pre-CS levels prior to CS-US trials. In a second experiment, CS-alone trials were replaced by “dummy” trials on which no stimuli were presented. There was no difference in pre-“CS” activity between CS-US and “dummy” trials, indicating that the effect shown in the first experiment was not produced by unlearned effects of US presentation. These results support the notion of an important role for the hippocampus in “working” memory.