Set-shifting aptitude in Parkinson's disease: external versus internal cues.

A modified version of the odd-man-out test was used to investigate set-shifting aptitude in 12 patients with Parkinson's disease. We asked subjects to execute in alternation two different sorting rules over successive items. External and internal cueing conditions were employed. Patients with Parkinson's disease were impaired on the tasks with internal cues but were normal on the tasks with external cues. Moreover, the shift costs were consistently larger for the shift to the easier task than the shift to the more difficult task. These findings indicated that the model of 'Supervisory Attentional System' may not be sufficient to explain the data as Brown and Marsden (1988) originally suggested.