Are memories for stimulus-stimulus associations or stimulus-response associations responsible for serial-pattern learning in rats?

Previous research has provided convincing evidence that rats can learn to a nticipate the individual elements of a stimulus series consisting of differ ing amounts of food reinforcement. Rats prepared with lesions of the dorsal striatum or hippocampus were initially trained to acquire a three-element series consisting of 21 sucrose pellets, followed by 0- and 7-pellets (Noye s standard), respectively. During the initial 30 days of training, the anim als were run in two adjacent runways; the runways included either of a whit e, rough runway or a black, smooth runway as additional series cues. Thus, training included both floor (S-R) cues and the series (inter-item memory) cues. Anticipation was defined as faster running on the 21- than on the 7-p ellet element and 7- than on the 0-pellet element. While anticipation devel oped more slowly in the lesion groups than in the control group, all animal s eventually demonstrated the ability to track the elements of the series. Reversal of the floor cues disrupted tracking in the hippocampus-lesioned a nd control animals; dorsal striatum-lesioned rats were also affected but di d continue tracking. As a final test, shifting the order of the series prod uced a marked disruption in performance in the dorsal striatum-lesioned rat s but not in the hippocampus-lesioned or control rats. The results are cons istent with the proposal that integrated neural mediation is required for a nticipation, with a system that includes the dorsal striatum necessary for the promotion of a reinforced approach response and a system that includes the hippocampus necessary for associating and temporarily maintaining an in ternal record of the different elements of the stimulus series. (C) 2001 El sevier Science Inc. All rights reserved.