Both bias and disengagement related anomalies are implicated in attention-deficit/hyperactivity disorder (ADHD) as measured with various neuropsychological test and event related potentials (ERPs) (Kenemans, Bekker et al. 2005). A MEDLINE search was performed to identify articles published between January 1995 and July 2009 that assessed the visuo-spatial cuing (VSC) task, the stop task, and the continuous performance (CP) task, either alone or in combination with ERPs. Results show robust differences between ADHD patients and controls in both bias and disengagement as measured with the stop and the CP task. However, with respect to the VSC task the results were inconsistent. The dominant neurobiological theory suggests that bias is dependent upon acetylcholine (Ach) whereas disengagement is dependent upon norepinephrine (NE) (Corbetta and Shulman 2002). To investigate this theory, findings from various Ach and NE agonists and antagonists were summarized. ERP findings were only available for the NE agonist methylphenidate (MPH) in the stop and the CP task and the Ach agonist nicotine in the VSC task. MPH increased both bias and disengagement related ERP components in the stop as well as the CP task. However since MPH is a non-selective NE agonist it remains to be elucidated whether NE is responsible for the increase in bias or the increase in disengagement. Nicotine specifically increased the amplitude of a late positive deflection (LPD) wave, a disengagement related ERP component. These results suggest that nicotine increases disengagement and not bias, thus arguing against the dominant neurobiological theory. A new model is proposed in which bias is dependent upon NE and disengagement is dependent upon Ach
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