Corticomotor facilitation associated with observation and imagery of hand actions is impaired in Parkinson's disease.

In the present report, we extend our previous observations on corticomotor facilitation associated with covert (action observed or imagined) and overt (action imitated) action execution in old adults (Leonard and Tremblay in Exp Brain Res 117:167-175, 2007) to investigate the impact of Parkinson's disease (PD). Participants consisted of 22 older adults (age range 58-76 years) of whom 11 were medicated patients diagnosed with PD (patient group) and 11 were age-matched healthy controls (healthy group). Corticomotor facilitation was assessed by monitoring the changes in the amplitude of motor evoked potentials (MEP) in muscles of the right hand (first dorsal interosseous: FDI; and abductor digiti minimi: ADM) in response to transcranial magnetic stimulation of the left motor cortex. In each group, corticomotor facilitation was assessed with participants seated in front of a computer screen under four testing conditions: (1) REST: eyes closed and instructions to relax for 10 s, (2) OBS: observe action, (3) IMAG: imagine action and (4) IMIT: imitate action. The action depicted in the video displayed the hand of a male subject cutting a piece of material with scissors. Comparison of variations in MEP amplitude revealed a significant interaction between groups and conditions. In the healthy group, the OBS and IMAG conditions were both associated with significant facilitation in the FDI and ADM, whereas the same conditions failed to produce facilitation in the PD group. In both groups, the IMIT condition produced the largest facilitation in hand muscles. Further planned comparisons revealed a significant difference between groups in the FDI for the OBS condition. From these findings, we conclude that, even when properly medicated, old adults with PD may experience major difficulties in engaging the motor system for covert actions, particularly when asked to observe another person's action. This failure of corticomotor facilitation for covert actions appears to be linked with the deficit in motor activation associated with basal ganglia dysfunction in PD and in line with the difficulty experienced in general by patients "to energize" the motor system in preparation for action.