Mechanisms underlying attentional set-shifting in Parkinson's disease

Patients with Parkinson's disease (PD) show impairments on tasks that requi re them to switch attention between two perceptual dimensions (extradimensi onal (ED) shifting). It has been suggested that ED shifting deficits can be caused by two separate mechanisms, 'learned irrelevance' and 'perseveratio n'. This study set out to test the hypothesis that enhanced learned irrelev ance is present in medicated patients with PD. An enhancement of learned ir relevance in PD patients should result in increased errors on a 'deficit' s hift relative to controls and decreased errors on an 'improvement' shift. A similar pair of deficit and improvement shifts were used to detect possibl e enhanced perseveration in patients. Instead of showing the predicted patt erns of deficit and improvement, patients displayed a consistent deficit on those shifts that required that they switch their attention to a different dimension (ED shifts). In contrast, patients were not impaired on shifts t hat required no such shift of attention (intradimensional shifts). Although there was an increase in errors at the learned irrelevance deficit shift, a similar increase at the learned irrelevance improvement shift shows that enhanced learned irrelevance is not responsible for either of these results . Patients were no more distractible than controls, but displayed increased 'loss of set' as measured by errors generated after a rule was learned. Th ese results point to the existence of exaggerated, rigid selective attentio n in patients with PD rather than a breakdown in the ability to selectively attend. There was no evidence for the existence of enhanced learned irrele vance in the patients. (C) 1999 Elsevier Science Ltd. All rights reserved.