Simon effect and attention in Parkinson's disease: a comparison with Huntington's disease and Tourette's syndrome.

Patients with hyperkinetic basal-ganglia disorders (Huntington's disease, HD; and Tourette's syndrome, TS) have difficulty with spatially incongruent stimulus-response configurations, the Simon effect, and with inhibiting inappropriate responses in a conditionality paradigm. However Parkinson's disease (PD) patients with hypokinetic basal-ganglia disorder show normal (for their age) conditionality and congruency effects, probably because the task is extremely sensitive to aging and PD patients are typically older than HD and TS individuals. Overall, HD patients were by far the most affected, reflecting the likely greater involvement of the caudate (with its predominantly cognitive role) than the putamen; the latter structure, with a predominantly motor involvement, is the more affected in PD.