Effects of high-frequency repetitive transcranial magnetic stimulation on reach-to-grasp performance in individuals with Parkinson’s disease: a preliminary study

Individuals with Parkinson’s disease (PD) have deficits in reach-to-grasp (RTG) execution and visuospatial processing which may be a result of dopamine deficiency in two brain regions: primary motor cortex (M1) and dorsolateral prefrontal cortex (DLPFC). We hypothesized that improvement following M1 stimulation would be the result of a direct impact on motor execution; whereas, DLPFC stimulation would improve the role of DLPFC in visuospatial processing. The aim of pilot study was to investigate the effects of HF-rTMS on RTG performance by stimulating either M1 or DLPFC. Thirty individuals with PD participated (H&Y stages I–III). All of them were more affected on the right side. Participants were allocated into three groups. The DLPFC group received HF-rTMS over left DLPFC; while, the M1 group received HF-rTMS over left M1 of extensor digitorum communis representational area. The control group received HF-rTMS over the vertex. Before and immediately post HF-rTMS, right-hand RTG performance was measured under no barrier and barrier conditions. Additionally, TMS measures including motor-evoked-potential (MEP) amplitude and cortical silent period (CSP) were determined to verify the effects of HF-rTMS. For the results, there were no significant differences among the three groups. However, only the M1 group showed a significant decrease in movement time immediately after HF-rTMS for a barrier condition. Moreover, the M1 group showed a near-significant increase in hand opening and transport velocity. As for the DLPFC group, there was a near-significant increase in temporal transport-grasp coordination and a significant increase in velocity. Increased MEP amplitudes and a significantly longer CSP in the M1 and DLPFC groups confirmed the effects of HF-rTMS. Regarding non-significant results among the three groups, it is still inconclusive whether there were different effects of the rTMS on the two stimulation areas. This is a preliminary study demonstrating that HF-rTMS to M1 may improve RTG execution; whereas, HF-rTMS to DLPFC may improve visuospatial processing demands of RTG.