Anterograde versus Retrograde Effects of Damage to Identified Learning and Memory Systems during Acquisition, Retention, and Re-Acquisition of an Instrumental Visual Discrimination Task: Dorsal Striatum, Perirhinal Cortex, and Hippocampus.

BACKGROUND

The goal of these experiments was to determine which learning and memory system(s) were necessary for the retention of visual discriminations and subsequent acquisition of a second problem. The dorsal striatum should be involved in the acquisition and expression of this task based on previous work implicating this region in instrumental learning and memory processes. The perirhinal cortex has been implicated in learning and memory processes associated with visual information like objects, and pictures and may also play a role in the acquisition and/or retention of visual discriminations. As there is no clear spatial/relational component to the task, the hippocampus should not be involved.

METHODS

Rats were trained on a two-choice visual discrimination task to criterion performance after which they received lesions to portions of the dorsal striatum (dorso-medial or dorso-lateral striatum) and medial temporal lobe (perirhinal cortex or hippocampus). After surgical recovery, the rats were tested for retention of the original discrimination, followed by training on a second problem on the same task.

RESULTS

The results showed that dorsal medial striatal lesions produced a retrograde deficit on picture discrimination, but dorsal lateral striatum lesions did not. Neither dorsal striatal lesion produced a deficit on acquisition of a second problem. Perirhinal cortex did not seem to make an essential contribution to the retention of the original discrimination or acquisition of the second problem. Surprisingly, subjects with hippocampal damage were severely impaired but eventually re-learned the discrimination. Damage to the hippocampus had no impact on acquisition of a second problem.

CONCLUSIONS

Taken together, the results of the present experiments show that the dorsomedial striatum and the hippocampus may support performance on this instrumental task if intact during acquisition but is not required for acquisition of a new problem. The implications of this pattern of results for our understanding of the organization of learning and memory in mammals is discussed.