Randomized vibrotactile fingertip stimulation modulates beta band in Parkinson's Disease

There is growing interest and need for non-invasive stimulation approaches for the treatment of Parkinson's disease (PD) and other neurological conditions. Pilot studies indicate that vibrotactile stimulation on the fingertips may reduce PD motor symptoms (Pfeifer et al., 2021; Syrkin-Nikolau et al., 2018). PD motor symptoms (e.g., rigidity, bradykinesia) are correlated with exaggerated beta power in the subthalamic nucleus (STN), where neurons are excessively synchronized (Brown 2003; Kuhn et al., 2006; Neumann et al., 2016; Yin et al., 2021), but the effect of vibrotactile stimulation on the STN has not been determined. Here, in 12 PD participants in the OFF deep brain stimulation (DBS) and OFF medication state, we investigated how unilateral vibrotactile stimulation applied to the fingertips affects local field potential (LFP) power in STN. We used a within-participants design to expose each participant to a treatment stimulation pattern, termed randomized vibrotactile stimulation (RVS), and a control stimulation pattern, with the order randomized and with intermittent acquisition of STN LFP. RVS yielded a modest but statistically significant 12% (SEM 4.6%) reduction in mean normalized STN beta power and a 48% (SEM 19%) reduction in peak beta power compared to the DBS-off baseline condition and was significantly different when compared to our control stimulus. Furthermore, we identified a biomarker in STN beta power that predicts which participants may benefit from RVS. We observed that participants that exhibited prominent beta peaks had stronger reductions in mean beta power (17% reduction, SEM 6.1%) and peak beta power (55% reduction, SEM 10%). Regressing against the magnitude of the peak in beta provides a moderate prediction of change in mean and peak beta power due to RVS (R2 = 0.58 for mean and 0.52 for peak). We then used our observations to construct a computational model where beta peaks in a simulated STN varied from prominent to diminished. We found that the efficacy of randomized treatments was dependent on the magnitude of beta peaking, mirroring our clinical findings, and showing that RVS may act by reducing intra-neuronal synaptic strengths in STN. Despite robust changes in STN LFP in our study population, we did not observe a significant change in motor symptoms. These results suggest that peripheral vibrotactile stimulation can reduce STN beta power and motivate additional studies to investigate its long-term effects on motor symptoms across a large population of participants.