Abstract
Neuropsychological tasks such as a self-ordered task, which probe complicated behaviors that transcend simple stimulus-response reflexes, are particularly difficult for subjects with prefrontal lesions (Petrides and Milner 1982). Monkeys with lesions of the mid-dorsal prefrontal cortex cannot perform a three-step self ordered task in which they must choose three different objects, one by one, on successive steps of the task (Petrides 1995). The interpretation of this deficit is not clear. One would expect to find neurons in this area related to the mnemonic aspects of the task. Previous studies, however, suggest that this area contains few neurons that show evidence of object working memory. We hypothesized that the role of this area might be to regulate some general aspect of the performance of difficult tasks. To investigate this possibility we trained monkeys on an oculomotor version of the task, and studied the activity of neurons in this area while they performed it. We found few neurons selective for the object pattern. Instead, the largest number of classifiable neurons belonged to a new class of neurons whose activity increased as the step _ and hence task difficulty _ increased. In addition, we found that the monkey's performance and the background activity of prefrontal neurons fluctuated during the day, and that the fluctuation patterns were similar with some time gap. These results suggest that the mid-dorsal prefrontal cortex contributes to the self-ordered task, not by providing an object working memory signal, but by regulating some general aspect of the performance in the difficult task. [Jpn J Physiol 55 Suppl:S50 (2005)]
