Dissociable effects of bilateral excitotoxic lesions of different regions of the rat neocortex, including medial prefrontal and anterior cingulate cortices, were investigated in a five-choice serial reaction time task that provides several indices of the accuracy and speed of attentional function. Whereas medial prefrontal cortical lesions impaired performance of the task as revealed by a reduction in choice accuracy, an increase in the latency to respond correctly to the visual target and enhanced perseverative responding, lesions of the anterior cingulate cortex specifically increased premature responding. By contrast, lateral frontal cortical lesions did not significantly disrupt baseline performance of the task, but rather increased the latency to respond correctly to the visual target during various behavioral manipulations, for example, when the length of the intertrial interval was varied unpredictably and during interpolation of distracting bursts of white noise. Lesions of the parietal cortex failed to disrupt any aspect of task performance investigated.
These behavioral effects in the five-choice task were compared with the effect of these same lesions on acquisition and retention of a one-trial passive avoidance task. The main finding from this paradigm was that lesions of the lateral frontal cortex produced a significant disruption to the retention of passive avoidance, which stands in marked contrast to the successful retention observed by animals of the other lesion groups. In addition, this pattern of results reveals that the “disinhibitory” effect of cingulate cortex lesions are relatively specific to the five-choice attentional task.
Finally, the results of the present study are compared with the findings of previous experiments using the five-choice task, which have examined the effect of selective manipulations of the ascending noradrenergic, cholinergic, dopaminergic. and serotonergic projections. In particular, the deficits in attentional function observed following cholinergic lesions of the nucleus basalis magnocellularis appear to be attributable to cholinergic denervation of the medial frontal cortex. These results are discussed in terms of the role of parallel distributed neural systems within the neocortex that mediate continuous attentional performance in the rat.