Transcranial Direct Current Stimulation (tDCS) Can Alter Cortical Excitability of the Lower Extremity in Healthy Participants: A Review and Methodological Study.

Objective: Transcranial direct current stimulation (tDCS) has been used to alter cortical excitability of the lower extremity (LE) and to influence performance on LE tasks like ankle tracking accuracy; but no study, to our knowledge, ever reported a significant change in cortical excitability relative to sham-tDCS. Additionally, because several different electrode montages were used in previous studies, it is difficult to know how stimulation should be applied to achieve this effect. Our objective was to determine whether active-tDCS alters cortical excitability of the LE and ankle tracking accuracy relative to sham-tDCS in healthy participants. The efficacy of two electrode montages and two conductance mediums were compared.

Methods: A triple-blind, fully randomized, within-subjects study was conducted with healthy participants (N=18, 24.2 (6.6) years). Cortical recruitment curves and measures of ankle tracking accuracy for the dominant lower extremity were obtained before and after participants received active-tDCS at 2 milliamps for 20 minutes using montage-medium combinations of M₁-SO:Saline, M₁-SO:Gel, C1-C2:Saline, and C1-C2:Gel and a sham-tDCS condition (M1-SO: Saline).

Results: The motor evoked potential maximum of the recruitment curve was significantly lower for active than sham-tDCS, but only for the M₁-SO:Saline combination. No other significant differences in the recruitment curve parameters or in ankle tracking were found.

Conclusions: This is the first study to our knowledge to demonstrate a significant difference in cortical excitability of the LE between active and sham-tDCS conditions. Given the order in which the experimental procedures occurred, the result is consistent with the concept of a homeostatic plasticity response.