Hypometria in Parkinson's disease: automatic versus controlled processing.

OBJECTIVES The aim was to investigate whether patients with Parkinson's disease (PD) make movements that are of smaller amplitude when required to attend to a secondary task at the same time as performing a motor task. METHODS Thirteen patients with idiopathic Parkinson's disease (mean age, 67.1 yrs) and 14 healthy control subjects (mean age, 66.2 yrs) were tested. The motor task was repeated opposition of the thumb and forefinger and the secondary task was a lexical decision task. RESULTS The PD patients made hypometric movements, and the amplitude was further decreased when they performed the secondary task at the same time. There was no significant change for the control subjects. CONCLUSION The unpaced motor task was less automatic for the PD patients than for the control subjects, and hence more subject to interference from a secondary task. We relate this to the underactivation of the supplementary motor cortex (SMA).