Elementary processes of response selection mediated by distinct regions of the striatum.

The relative contributions of the medial and lateral caudate putamen to performance of a visual reaction time task were compared by examining the effects of unilateral ibotenate-induced lesions to these regions in the rat. Different groups of rats were trained to respond either towards (Discrimination 1) or away from (Discrimination 2) unpredictable, brief visual stimuli presented to either side of the head. Lateral striatal lesions produced a strong spatial bias towards the side of the lesion but left the latency for the initiation of responses to the visual cues unchanged. By contrast, the medial striatal lesions resulted in a smaller degree of spatial response bias but a significant slowing of initiation latency to the side contralateral to the lesion. These effects were irrespective of the side of the stimulus presentation and thus represent response-related impairments. The lesions were further dissociated in their effects on responding inappropriately to task requirements. These results demonstrate a double dissociation of behavioral effects of lateral and medial striatal damage that can be used to infer the operation of distinct elementary processes of response output within a single task. They are important not only in demonstrating functional effects of hypothetical segregated parallel corticostriatal loops but also in showing that these loops must interact to produce integrated performance.