Probing the relevance of repeated cathodal transcranial direct current stimulation over the primary motor cortex for prolongation of after‐effects

Abstract

Key points To explore the capability of cathodal transcranial direct current stimulation (tDCS) to induce late‐phase long‐term depression (LTD) via repeated stimulation. Conventional (1 mA for 15 min) and intensified (3 mA for 20 min) protocols with short (20 min) and long (24 h) intervals were tested. Late‐phase plasticity was not induced by a single repetition of stimulation. Repetition reduced the efficacy of stimulation protocols with higher intensities. Abstract Transcranial direct current stimulation (tDCS) has shown promising results in pilot studies as a therapeutic intervention in disorders of the central nervous system, but more sustained effects are required for clinical application. To address this issue, one possible solution is the use of repeated stimulation protocols. Previous studies indicated the possibility of extending the after‐effects of single intervention cathodal tDCS by repeating the tDCS, with relatively short intervals between repetitions being most effective. In this study, we thus investigated the effects of repeated stimulation protocols at short and long intervals, for a conventional tDCS protocol (1 mA for 15 min) and a newly developed optimized protocol (3 mA for 20 min). In 16 healthy participants, we compared single interventions of conventional and optimized protocols, repeated application of these protocols at intervals of 20 min and 24 h, and a sham tDCS session. tDCS‐induced neuroplastic after‐effects were then monitored with transcranial magnetic stimulation (TMS)‐induced motor evoked potentials (MEPs) until the following evening after stimulation. The results revealed that the duration of the after‐effects of repeated conventional and optimized protocols with short intervals remained nearly unchanged compared to the respective single intervention protocols. For the long‐interval (24 h) protocol, stimulation with the conventional protocol did not significantly alter respective after‐effects, while it reduced the efficacy of the optimized protocol, compared with respective single interventions. Thus late‐phase plasticity could not be induced by a single repetition of stimulation in this study, but repetition reduced the efficacy of stimulation protocols with higher intensities. This study provides further insights into the dependency of tDCS‐induced neuroplasticity on stimulation parameters, and therefore delivers crucial information for future tDCS applications.

Complexity of cathodal tDCS: Relevance of stimulation repetition, interval, and intensity

“…In what follows, we discuss these parameters in the light of Mosayebi‐Samani et al . (2020) findings and also discuss the clinical significance of their findings.…”

“…Cathodal tDCS parameters that are investigated in recent years include stimulation intensity, duration, and repetition, with the latter parameter recently explored by Mosayebi‐Samani et al . (2020). The first systematic investigation of cathodal tDCS intensity was reported a few years ago.…”

“…The effect of repeated cathodal tDCS was further explored in the recent study by Mosayebi‐Samani et al . (2020) published in the Journal of Physiology providing further insight into this parameter. The authors specifically explored the capability of repeated cathodal tDCS over the primary motor area to induce late‐phase LTD. To this end, they first applied the conventional (1 mA, 15 min) and intensified/optimized (3 mA, 20 min) protocols and then repeated each protocol with short (20 min) and long (24 hr) intervals in 16 healthy, right‐handed volunteers.…”

“…Specifically, Mosayebi‐Samani et al . (2020) found that of all the active cathodal conditions (1 and 3 mA, 1 and 3 mA with 20 min interval, 1 and 3 mA with 24 h interval), only the intensified protocol with the 24 h interval failed to significantly reduce motor cortical excitability already in the early epoch (first 30‐min after‐effects). Interestingly, the neuroplastic after‐effects obtained from the single intervention of the intensified protocol were significantly stronger than those obtained from the repeated intensified protocol with 24 h interval at early (up to 30 min) and late (60–120 min) epochs (Mosayebi Samani et al .…”

“…Interestingly, the neuroplastic after‐effects obtained from the single intervention of the intensified protocol were significantly stronger than those obtained from the repeated intensified protocol with 24 h interval at early (up to 30 min) and late (60–120 min) epochs (Mosayebi Samani et al . 2020). These results indicate that the intensified protocol with single‐session repetition indeed reduced the efficacy of tDCS after‐effects.…”