DISSOCIATING EXECUTIVE MECHANISMS OF TASK CONTROL FOLLOWING FRONTAL-LOBE DAMAGE AND PARKINSONS-DISEASE

Twelve patients with focal damage of the frontal cortex and 12 patient s with mild, medicated, early stage Parkinson's disease switched betwe en letter- and digit-naming tasks on every second trial of a task-swit ching paradigm. Compared with age- and IQ-matched control performance, patients with left-sided, but not right-sided, frontal damage exhibit ed markedly increased time costs associated with these predictable swi tches only when there was a general incidence of interference or 'cros stalk' between the tasks, and particularly so when the available task cues were relatively weak and arbitrary. The same patients also showed evidence of an increased sensitivity to the facilitatory and inhibito ry effects of previous processing, when required to switch between tas ks. Both groups of patients (with left-or right-sided frontal damage) exhibited slow, disorganized performance early in practice. In contras t to these frontal effects, the Parkinson's disease patients showed li ttle indication of larger time costs of task switches but they did sho w progressive increases in the error costs, while age-and IQ-matched c ontrol subjects showed reductions. We propose that while both left and right frontal cortical areas are involved in the organization of cogn itive and motor processes in situations involving novel task demands, only the left frontal cortex is involved in the dynamic reconfiguring between already-established task-sets, and specifically, that it is th e site of an executive mechanism responsible for the modulation of exo genous task-set activity. Finally, dopaminergic transmission, along th e nigrostriatal pathway, may be implicated in sustaining various cogni tive and motor processes over prolonged periods, including the operati on of those executive control mechanisms that accomplish reconfiguring between task-sets.