Time-course changes in motor cortical excitability following low level constant transcranial direct current stimulation

Introduction: Transcranial direct current stimulation (tDCS) is a non-invasive technique that has been found to modulate the excitability of neurons within the brain. The polarity of the current applied to the scalp determines the effect of tDCS on the underlying neural tissue. Anodal-tDCS increases excitability, whereas cathodal-tDCS decreases it. However, it remains unclear as to what amplitude of direct current results in the greatest increases in human primary motor cortex (M1) excitability. Therefore the aim of the present study was to establish the time-course effects of different amplitudes of anodal-tDCS onM1excitability and inhibition. Methods: In a cross-over design, with a one-week wash-out period, 8 participants (4 male & 4 female, age range 21–36 years) were exposed to 10 minutes of anodal-tDCS at 0.8, 1.0 and 1.2 mA. The applied current was induced by a saline-soaked pair of surface sponge electrodes (25cm2) delivered by a NeuroConn DC stimulator. Focal transcranial magnetic stimulation (TMS) was used to measure M1 excitability and short-interval-intracortical inhibition (SICI) of the contralateral extensor carpi radialis brevis muscle (ECR) at baseline, immediately post anodal-tDCS and every 5minutes up to 30 minutes following the removal of tDCS. Results:M1excitability was elevated in all conditions following tDCS with greatest increases at 15–25 minutes. Of the three conditions, 0.8mA demonstrated significant increase in M1 excitability with peak increases of 81%, 85%, 89% at 15, 20 and 25 minutes respectively (P < 0.05). SICI was observed in both 0.8mA and 1mA conditions with similar time-course patterns. Significantly reduced inhibition between 28–37% (P < 0.05) was found at all time points immediately following tDCS until 30 mins. Discussion: Two of the three tDCS conditions (0.8 and 1.0mA) significantly enhanced M1 excitability and reduced SICI. Further, low-level anodal-tDCS (0.8mA) provided the greatest effect on enhancing M1 excitability and reducing SICI. The effects of anodaltDCS were probably induced by modifications in synaptic plasticity between neurons, demonstrating short-term potentiation. This data demonstrates that transcranial electrical stimulation using weak currents may be a viable clinical tool to modulate cerebral excitability in a non-invasive, painless and focal way in the rehabilitation following brain injury.