Individualization of tDCS intensity according to corticospinal excitability does not improve stimulation efficacy over the primary motor cortex

Transcranial direct current stimulation (tDCS) applied at the same intensity for an entire group of people results in wide interindividual variability, limiting stimulation efficacy. Evidence suggests that tDCS efficacy might be linked to individual corticospinal excitability (CSE) levels measured by transcranial magnetic stimulation (TMS). However, no study has attempted to individualize tDCS parameters according to the CSE level. We aimed to investigate whether the tDCS effect could be improved by individualizing stimulation intensity based on CSE measured at baseline. Fourteen participants were included in a crossover single-blinded design study where anodal (1 mA), individualized anodal (between 0.9 and 1.6 mA) and sham tDCS were applied for 14 min over the primary motor cortex. The resting motor threshold (RMT), stimulus intensity for a 1 mV response (SI1mV) and the input-output curve (I-O curve) were measured before, immediately after, 15 after and 30 min after tDCS using single pulses of TMS. The tDCS intensity in the individualized anodal condition was determined according to the RMT value at baseline (i.e., CSE level). RMT, SI1mV and I-O curve MEPs did not change after any tDCS paradigm. Our results are consistent with previous investigations that did not show an effect of tDCS on CSE and supports that tDCS protocols suffer from large interindividual variability and a lack of efficiency. This calls for further investigations to find the optimal tDCS setting to reduce the inconsistency in the results and obtain reproducible effects.