[Investigation of set-shifting ability in patients with Parkinson's disease: influence of task sequence type]

The aim of the present study was to investigate the origin of set-shifting deficits observed in Parkinson's disease (PD).

Seventeen patients diagnosed as having idiopathic PD were compared with 15 control subjects. We used a task-switching paradigm, including two tasks (task A and task B) so that subjects were required to switch either immediately after a switch-trial (i.e. alternating switch or ABA task sequence) or following one or two non-switch trials (ABBA or ABBBA task sequences).

In both groups, switch cost (SC) in ABA task sequence was larger than SC in ABBA task sequence (p<0.05) and SC was larger in ABBA than ABBBA task sequence (p<0.05). PD patients demonstrated an increased SC compared to controls for alternating switch trials (p<0.01). Alternatively, when required to switch to a task abandoned two or three trials earlier (i.e. ABBA and ABBBA tasks sequences), patients did not demonstrate increased SC compared to controls.

The fact that SC associated with alternating switch trials was exacerbated in PD patients may reflect difficulties for switching to a recently inhibited task-set. In conclusion, our results indicate that set-shifting deficits in PD patients may depend of the type of task sequence.