Transcranial direct current stimulation enhances motor learning in Parkinson's disease: a randomized controlled trial

Abstract

Writing training has shown clinical benefits in Parkinson's disease (PD), albeit with limited retention and insufficient transfer effects. It is still unknown whether anodal transcranial direct current stimulation (atDCS) can boost consolidation in PD and how this interacts with medication. To investigate the effects of training + atDCS versus training + sham stimulation on consolidation of writing skills when ON and OFF medication. Second, to examine the intervention effects on cortical excitability. In this randomized sham-controlled double-blind study, patients underwent writing training (one session) with atDCS (N = 20) or sham (N = 19) over the primary motor cortex. Training was aimed at optimizing amplitude and assessed during online practice, pre- and post-training, after 24-h retention and after continued learning (second session) when ON and OFF medication (interspersed by 2 months). The primary outcome was writing amplitude at retention. Cortical excitability and inhibition were assessed pre- and post-training. Training + atDCS but not training + sham improved writing amplitudes at retention in the ON state (p = 0.017, g = 0.75). Transfer to other writing tasks was enhanced by atDCS in both medication states (g between 0.72 and 0.87). Also, training + atDCS improved continued learning. However, no online effects were found during practice and when writing with a dual task. A post-training increase in cortical inhibition was found in the training + atDCS group (p = 0.039) but not in the sham group, irrespective of medication. We showed that applying atDCS during writing training boosted most but not all consolidation outcomes in PD. We speculate that atDCS together with medication modulates motor learning consolidation via inhibitory processes ( https://osf.io/gk5q8/ , 2018-07-17).

Keywords: Micrographia; Motor learning; Parkinson’s disease; Short interval intra-cortical inhibition; Transcranial direct current stimulation.

Fig. 1

CONSORT flowchart. After the first retention session, one participant dropped out due to unexpected side effects of the intervention. Due to the COVID-19 pandemic, two participants discontinued the TMS assessments (no 1.5 m distance possible)

Fig. 2

A Patients completed one baseline, two intervention and two retention sessions in ON and OFF (randomized). B Performance was assessed during training (online practice), offline (pre, post, retention) and continued learning. C Tablet tasks consisted of writing sequences with increasing size

Fig. 3

A Trained task writing amplitude (cm) (primary outcome), B amplitude (cm) of the untrained tablet task and C dual-task interference (%). *p value < 0.05, indicated for between group differences only