Effects of different transcranial direct current stimulation devices on motor cortical excitability

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique in which low‐amplitude electric currents are delivered to the brain via specific sites on the scalp. This electric current is purported to alter neuronal resting membrane potential and subsequently modulate cortical excitability. Over time, researchers have found numerous applications for tDCS including treatment of various neurological (eg, stroke, cerebral palsy, etc) and neuropsychiatric (eg, depression, anxiety, etc) disorders. However, a number of researchers have also shown that responses to tDCS are highly variable. It is possible that the differences in devices used in these studies could explain some of the observed variability. Hence, this study aimed at determining the effect of 2 different tDCS devices (Empi Dupel Dual Channel Iontophoresis; Isokentics Inc, AR, USA) and Soterix 1x1 (Soterix Medical, NY, USA) on motor cortical excitability of the quadriceps muscles. Twenty neurologically intact young adults participated in this crossover study. Subjects received anodal tDCS over their dominant leg (preferred kicking leg) motor cortex for 15 minutes on 2 different days that were separated by at least 48 hours. The tDCS devices were interchanged during these testing sessions and the device order was randomized. Motor cortical excitability of the quadriceps muscle was evaluated both before and after the tDCS by measuring the participant's recruitment curve (ie, the relationship between stimulus intensity and the evoked MEP torque) generated by stimulating the quadriceps hotspot at various intensities (70% to 140% of active motor threshold) using transcranial magnetic stimulation (TMS). The recruitment curve was fitted with a Boltzmann sigmoidal function (Levenberg‐Marquardt algorithm in LabVIEW) to determine the parameters relating to motor cortical excitability: (1) maximal MEP torque size (MEPmax), (2) curve slope (k), and (3) stimulus intensity leading to half‐maximal MEP torque size (S50). The results indicated that there were no changes in MEPmax, slope (k), and S50 before and after tDCS for both tDCS devices (P > .05). These results suggest that variability in tDCS‐induced motor cortical excitability changes are not explained by the differences in the tDCS units. The findings of this study have meaningful implications for interventions using tDCS as an adjuvant tool for neurorehabilitation.