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

Background: The motor impairment in Parkinson's disease (PD) could partly reflect a failure to activate processes of motor imagery. Objective: To verify any selective changes of motor output during motor imagery, lateralized to the hemisphere contralateral to the clinically affected side of hemiparkinsonian patients. Methods: Transcranial magnetic stimulation (TMS) was used 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 patients with hemi-PD and seven healthy subjects were examined. Focal TMS was applied over a grid of 20 scalp positions on each hemiscalp. Maps were characterized by area (number of excitable positions), volume (the sum of motor evoked potential amplitudes at all scalp positions), and center of gravity (a map position representing an amplitude-weighted calculation of the excitable area). Results: In healthy control subjects, the area of cortical representation of ADM was symmetrically increased in both hemispheres by mental simulation 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 conditions and in both hemispheres in patients with PD. However, largely because the volume was so high at rest in patients, the increment in volume associated with contraction was smaller than in control subjects. Conclusion: This study demonstrates the presence of a tonic hyperactivation of motor cortical circuitry in PD in conjunction with an abnormality of either motor imagery or the process by which motor imagery engages the sensorimotor cortices in the clinically affected hemisphere.