Anodal Transcranial Direct Current Stimulation over the Vertex Enhances Leg Motor Cortex Excitability Bilaterally

In many studies, anodal transcranial Direct Current Stimulation (tDCS) is applied near the vertex to simultaneously facilitate leg motor cortex (M1) of both hemispheres and enhance recovery of gait and balance in neurological disorders. However, its effect on the excitability of leg M1 in either hemisphere is not well known. In this double-blind sham-controlled study, corticospinal excitability changes induced in leg M1 of both hemispheres by anodal (2 mA for 20 minutes) or sham tDCS (for 20 min) over the vertex were evaluated. Peak amplitudes of Transcranial Magnetic Stimulation (TMS) induced motor evoked potentials (MEPs) were measured over the contralateral Tibialis Anterior (TA) muscle before and up to 40 min after tDCS in 11 normal participants. Analysis of data from all participants found significant overall increase in the excitability of leg M1 after tDCS. However, in individual subjects there was variability in observed effects. In 4 participants, 20 min of tDCS increased mean MEPs of TAs on both sides; in 4 participants there was increased mean MEP only on one side and in 3 subjects there was no change. It's not known if the benefits of tDCS in improving gait and balance are dependent on excitability changes induced in one or both leg M1; such information may be useful to predict treatment outcomes.