Effects of motor imagery on motor cortical output topography in Parkinson's disease

Background: The motor impairment in Parkinson's disease (PD) could partly r eflect a failure to activate processes of motor imagery. Objective: To veri fy any selective changes of motor output during motor imagery, lateralized to the hemisphere contralateral to the clinically affected side of hemipark insonian patients. Methods: Transcranial magnetic stimulation (TMS) was use d to map the cortical representations of the contralateral abductor digiti minimi muscle (ADM) during rest, contraction, and motor imagery in a group of patients with hemi-PD and in a group of healthy volunteers. Seven patien ts with hemi-PD and seven healthy subjects were examined. Focal TMS was app lied over a grid of 20 scalp positions on each hemiscalp. Maps were charact erized by area (number of excitable positions), volume (the sum of motor ev oked potential amplitudes at all scalp positions), and center of gravity (a map position representing an amplitude-weighted calculation of the excitab le area). Results: In healthy control subjects, the area of cortical repres entation of ADM was symmetrically increased in both hemispheres by mental s imulation of movement and real muscle contraction. In patients with hemi-PD , there was a hemispheric asymmetry in the area of cortical representation elicited by motor imagery. The area was reduced in the clinically affected hemisphere. The volume of cortical representation was increased under all c onditions and in both hemispheres in patients with PD. However, largely bec ause the volume was so high at rest in patients, the increment in volume as sociated with contraction was smaller than in control subjects. Conclusion: This study demonstrates the presence of a tonic hyperactivation of motor c ortical circuitry in PD in conjunction with an abnormality of either motor imagery or the process by which motor imagery engages the sensorimotor cort ices in the clinically affected hemisphere.