Damage to the prefrontal cortex disrupts the performance of self-ordered sequencing tasks, although the precise mechanisms by which this effect occurs is unclear. Active working memory, inhibitory control, and the ability to generate and perform a sequence of responses are all putative cognitive abilities that may be responsible for the impaired performance that results from disruption of prefrontal processing. In addition, the neurochemical substrates underlying prefrontal cognitive function are not well understood, although active working memory appears to depend upon an intact mesocortical dopamine system. The present experiments were therefore designed to evaluate explicitly the contribution of each of these abilities to successful performance of a novel spatial self-ordered sequencing task and to examine the contribution of the prefrontal cortex and its dopamine innervation to each ability in turn.
Excitotoxic lesions of the prefrontal cortex of the common marmoset profoundly impaired the performance of the self-ordered sequencing task and induced robust perseverative responding. Task manipulations that precluded perseveration ameliorated the effect of this lesion and revealed that the ability to generate and perform sequences of responses was unaffected by excitotoxic damage to prefrontal cortex. In contrast, large dopamine and noradrenaline depletions within the same areas of prefrontal cortex had no effect on any aspect of the self-ordered task but did impair the acquisition of an active working memory task, spatial delayed response, to the same degree as the excitotoxic lesion. These results demonstrate that a lesion of the ascending monoamine projections to the pre-frontal cortex is not always synonymous with a lesion of the prefrontal cortex itself and thereby challenge existing concepts concerning the neuromodulation of prefrontal cognitive function.