Expanding the parameter space of anodal transcranial direct current stimulation of the primary motor cortex

Abstract

Size and duration of the neuroplastic effects of tDCS depend on stimulation parameters, including stimulation duration and intensity of current. The impact of stimulation parameters on physiological effects is partially non-linear. To improve the utility of this intervention, it is critical to gather information about the impact of stimulation duration and intensity on neuroplasticity, while expanding the parameter space to improve efficacy. Anodal tDCS of 1-3 mA current intensity was applied for 15-30 minutes to study motor cortex plasticity. Sixteen healthy right-handed non-smoking volunteers participated in 10 sessions (intensity-duration pairs) of stimulation in a randomized cross-over design. Transcranial magnetic stimulation (TMS)-induced motor-evoked potentials (MEP) were recorded as outcome measures of tDCS effects until next evening after tDCS. All active stimulation conditions enhanced motor cortex excitability within the first 2 hours after stimulation. We observed no significant differences between the three stimulation intensities and durations on cortical excitability. A trend for larger cortical excitability enhancements was however observed for higher current intensities (1 vs 3 mA). These results add information about intensified tDCS protocols and suggest that the impact of anodal tDCS on neuroplasticity is relatively robust with respect to gradual alterations of stimulation intensity, and duration.

Figure 1

Diagrammatic representation of the experimental procedure. Each participant took part in 10 sessions of the experiment, with a minimum one-week inter-session interval to prevent carry-over effects. A condition as defined here refers to Intensity-Duration pairs (eg. 1 mA–30 min). All conditions were randomized. Each experimental session started with the identification of the ‘hotspot’ of the ADM, and the TMS intensity which resulted in a MEP amplitude of approximately 1 mV (baseline TMS intensity) was determined. MEPs were then recorded as a baseline excitability measure, after which tDCS was applied. Immediately after tDCS, MEPs were again recorded with the same baseline TMS stimulus intensity every 5 min until 30 min, every 30 min until 2 hr, SE, NM, NN, and NE (modified from Jamil, with permission of The Journal of Physiology, John Wiley and Sons).

Figure 2

Averaged MEPs post-stimulation for all intervention conditions and monitored time bins. MEPs were obtained before, immediately after tDCS at 5-minute intervals until 30 min after intervention and then every half an hour until 120 mins; same day evening (SE), next morning (NM), next noon (NN) and next evening (NE) for all three tDCS intensity-duration combinations. tDCS resulted in an enhancement of cortical excitability post-stimulation when compared to the respective baselines values, and sham. Sham tDCS did not alter cortical excitability with the exception of significant reduction of MEP amplitudes at NN. The three intensities and durations of stimulation show comparable effects. Comparison of all real tDCS conditions showed no significant differences between the respective induced cortical excitability alterations. Error bars represent standard error of the mean. Filled symbols represent a significant difference of MEP amplitudes compared to the respective baselines. Floating symbols represent significant differences between real and sham stimulation conditions (paired t test, two-tailed, p < 0.05).

Figure 3

Average MEPs post-stimulation for all intervention conditions and pooled time bins. Post hoc comparisons with sham tDCS indicate that all stimulation conditions resulted in significant cortical excitability enhancements within the first 30 minutes after stimulation (early epoch), except for the 2 mA–15 min condition, which did alter excitability only trend-wise. For the late epoch (60–120 min after stimulation), all conditions except 1 mA–30 mins, 2 mA–15 mins, and 2 mA–30 mins show a cortical excitability increase. Cortical excitability was also enhanced for all conditions in the very late epoch (same day evening to next day evening after stimulation), except for the 1 mA–30 min, 2 mA–30 min, and 3 mA–15 min conditions. Error bars represent standard error of the mean. Filled symbols in the graph represent a significant difference of MEP amplitudes compared to the respective baselines. The floating symbols indicate significant differences between real stimulation and sham conditions (paired t test, two-tailed, p < 0.05).