| One of the important processes involved in working memory is the monitoring of information held in memory. Monitoring refers to the tracking of different events that may have occurred at different times or places in order to make a decision or perform an action. It is required at times when executive tasks are involved, such as set-shifting tests. Indeed, in order to perform a set-shift, one often needs to keep track of the remaining available rules. Recently, fMRI studies have demonstrated that the dorsolateral prefrontal cortex (DLPFC) plays an important role in the monitoring of information held in working memory. However, imaging studies can only provide neuronal correlates of cognitive performance, without establishing their causal relations.;Second, we investigated the functional role of the left DLPFC during set-shifting, and its modulatory effect on the striatal dopaminergic system. We applied continuous theta burst stimulation, a newly proposed rTMS methodology, to the left and right DLPFC while subjects underwent positron emission tomography (PET) with [11C]raclopride. Stimulation of the left DLPFC, which transiently disrupted its function, impaired Montreal-Card-Sorting-Task (MCST) performance and affected dopamine release in the striatum. In contrast, right DLPFC stimulation had no significant effect on behaviour and striatal dopamine release.;Lastly, in order to investigate cortical dopamine transmission during executive function, we used PET with [11C]FLB 457. We observed significantly more dopamine release in the right anterior cingulate cortex (ACC) when subjects performed the MCST than during the control task. These findings are consistent with previous fMRI studies which demonstrate ACC activation in similar tasks involving conflict of monitoring.;In summary, these studies provide important insights on the mechanisms of executive functions of the human brain in vivo, and shed some light on the origin of executive deficits underlying certain neurological disorders associated with prefrontal and/or dopamine dysfunction, such as Parkinson's disease or schizophrenia.;Here we assessed the role of the right DLPFC in monitoring using repetitive transcranial magnetic stimulation (rTMS) to disrupt on-going processes within a given cortical region. When rTMS was applied to the right DLPFC during feedback reception on their previous response, the subjects' WCST performance deteriorated. This was not the case when rTMS was applied during other stages of executive processing. This result is consistent with the proposed role of the right DLPFC in the monitoring of events in working memory. |
