Information processing in dorsal striatum subregions during instrumental learning

Previous research has suggested that the functions of the dorsomedial (DMS) and dorsolateral (DLS) striatum are dissociable, the former mediating goal-directed action-outcome (A-O) associations and the latter habitual stimulus-response (S-R) associations.Some of this evidence was obtained using different schedules of reinforcement in operant tasks, in combination with reinforcer devaluation.The present experiments used different versions of the win-stay task and single-solution T-maze tasks to assess the role of dorsal striatum subregions in instrumental learning.In contrast to previous studies, DMS but not DLS lesions impaired win-stay learning in both a conventional and in an automated radial maze, in which rats were allowed to obtain food multiple times from the same arms.This suggests that win-stay performance was due primarily to A-O associations.Consistent with this conclusion, devaluation of the reinforcer impaired the performance of control rats but not that of rats with either DMS or DLS lesions.This suggests that the rats also acquired S-R associations which were revealed when A-O associations were eliminated by the lesions.It was hypothesized that hippocampus learning to avoid revisiting baited arms suppresses DLS-based S-R associations, making win-stay performance dependent on DMS-based A-O associations.When the reinforcer was delivered on an interval schedule in the win-stay task both DMS and DLS lesions impaired performance, and reinforcer devaluation had no effect, suggesting that the interval schedule promoted S-R learning.Single-solution T-maze tasks were studied in an attempt to understand the roles of the striatal subregions in the components of win-stay learning.The results suggested that the DMS is required for the ability to acquire instrumental reinforcement contingencies with a proximal cue, a spatial location or a body turn.The availability of extramaze cues (EMCs) interfered with learning the cue task but not the turn or place tasks.DLS lesions also impaired cue learning (upon removal of EMCs), retarded but did not eliminate learning in the turn task, and enhanced learning in the place task when EMCs were available.Overall, these findings indicate that numerous different kinds of learning occur simultaneously and interact with each other in maze learning situations.Neurotoxically selective lesions of the striatal patch compartment had no significant effects on overall learning rates in the cue, turn or place tasks, but there were significant correlations between the residual patch area and several aspects of performance in the group of lesioned rats.The findings suggest that the patch compartment may mediate functions that can influence maze performance although its involvement may not be central to these forms of instrumental learning.I would like to thank my fellow graduate students and partners in crime, Maliha Naeem and Selma Hamdani, for their friendship and all the good times we have shared together inside and outside the lab.I would also like to acknowledge the technical assistance of Melissa Latourelle, Sintya Shortino and Karine Gamache.

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