Objective: Resting tremor may compound the effects of bradykinesia to further prolong the initiation of voluntary movement in patients with Parkinson's disease (PD). However, the interaction between resting tremor and voluntary movements in these PD patients has not been well understood. Recently, we demonstrated that cutaneous afferents evoked by surface stimulation of superficial radial nerve can inhibit resting tremor effectively. The inhibition appears to take effect via spinal interneuronal pathways. This study evaluates how evoked cutaneous afferents would impact the performance of voluntary movements in PD subjects when tremor is inhibited.
Approach: Ten PD patients with tremor and eight age-matched control subjects were recruited to participate in this study. Both groups of subjects performed fast reaching movements, while cutaneous stimulation was delivered during reaching tasks on or off randomly. Kinematic performance, such as reaction time (RT), movement time (MT), and movement variability, as well as muscle synergy of tasks were evaluated and compared to assess the impact of evoked cutaneous afferents on movement performances.
Main results: Results indicated that the cutaneous stimulation significantly reduced RT in PD patients by 17.7%; but had an insignificant effect on RT in control subjects. Cutaneous stimulation, however, caused a significantly longer MT both in control subjects (8.6%) and in PD subjects (15.7%). Movement variability was not significantly altered in both groups of subjects by the cutaneous stimulation. Muscle synergy analysis revealed that cutaneous stimulation affected the power spectral densities (PSD) of time profiles of muscle synergies more significantly than the vector patterns of synergies in both control subjects and PD subjects.
Significance: These findings provide evidence that tremor increases the RT of voluntary motor control in PD patients, and demonstrate that cutaneous stimulation reduces the RT of voluntary movements significantly, in addition to suppressing tremor, yet without interrupting voluntary control of movements.