The present results demonstrated for the first time that anodal tDCS over the leg motor cortex temporarily facilitated maximal leg PF contralateral to the stimulation. The enhancement lasted for 30 min after the end of stimulation and was not observed in the hand motor tasks. The effecter-specificity of the modulation indicated that the results were not caused by general effects such as increased arousal or changes in attention, motivation or mood. The effecter-specificity also indicates that tDCS could induce the change in the leg motor function without affecting the hand function, presumably because the hand motor cortex is about 3–4 cm apart from the leg motor cortex.
Hand motor performance enhanced by anodal tDCS has been observed in previous tDCS studies (Hummel et al. 2005
), and the degree of the improvement of leg PF in our study (20.5%) was comparable to the degree of tDCS-induced improvement observed in previous studies of hand motor function. In those studies, the current was 1 mA, whereas it was of 2 mA in our study because the leg motor areas are less excitable than hand motor areas (Jeffery et al. 2007
). Therefore, there might be the qualitatively similar effects on behavioral performance between 1-mA anodal tDCS over the hand motor cortex and 2-mA anodal tDCS over the leg motor cortex.
In the present study, the anodal tDCS over the leg motor cortex did not change the leg RT contralateral to the stimulation. The RT in hand motor tasks has been reported to be facilitated by anodal tDCS (Hummel and Cohen 2005
; Hummel et al. 2006
). The absence of an effect of tDCS on leg RT might be due to performance ceiling. In the present study, the subjects were healthy adult volunteers (mean age 23.8 years) and their RTs were already quite short before the intervention (mean RT 272 ms). The subjects in the previous studies, in contrast, were older patients (mean age 57.0 years) with chronic stroke and might therefore be expected to respond more slowly than younger healthy adults (Hummel et al. 2006
). Another possibility is that, because of low spatial focality of tDCS, anodal tDCS in the previous studies stimulated not only the hand motor cortex but also the premotor cortex. Then, anodal tDCS may facilitate externally triggered movement, a possible function of the premotor cortex (Goldberg 1985
; Wessel et al. 1997
; Crosson et al. 2001
), which is required for performing the RT task. It is also possible that the RT task might not be sensitive enough for detecting the effect of tDCS, or the stimulation strength and/or duration (10 min of 2 mA) might not be sufficient to induce meaningful behavioral gains in the task.
In accordance with tDCS effect on the excitability of leg motor cortex (Jeffery et al. 2007
), we did not observe significant effect of cathodal tDCS on the behavioral performances. This might be due to the leg motor cortex having fewer inhibitory circuits than the hand motor cortex, or cathodal current might be less effective in the leg motor cortex because of the different orientation and position of the leg motor cortex relative to the scalp (Jeffery et al. 2007
). One difference between the results of the electrophysiological study by Jeffery et al. (2007
) and our present results is the duration of the after-effect of tDCS. In their study, 10 min of anodal tDCS increased the excitability of the leg cotricospinal tract for over 60 min after the stimulation. The effect of anodal tDCS on the maximal PF of the leg that we observed, on the other hand, was relatively short-lasting, and the PF returned to its baseline value 60 min after the stimulation. It might be more difficult to induce long-lasting change of final behavioral output by tDCS. Alternatively, this discrepancy might be due to the differences in equipment, subject population and/or experimental conditions between Jeffery et al. (2007
) and ours.
Mechanisms underlying the enhanced leg PF are still speculative. One possibility is that an increase of corticospinal excitability by anodal tDCS in the leg motor cortex contributed to the behavioral gain. Another possibility is that tDCS increase intermuscular coupling. Anodal tDCS over the hand motor cortex has been reported to increase beta-band intermuscular coherence in the first dorsal interosseous and extensor digitorum communis muscles (Power et al. 2006
). Collaborative activation of lower limb muscles is needed for generation of the PF in toes. Thus, a more optimal coupling of these muscles by anodal tDCS may contribute to the enhanced maximal PF in the leg. Resolving this issue will require further experiments in which leg behavioral performance and brain and muscular activities are measured simultaneously and correlated.
In summary, we have shown that the leg motor performance of healthy adult subjects is transiently enhanced by anodal tDCS over the contralateral leg motor cortex. This is the direct evidence that tDCS can induce not only the change of local cortical excitability (physiological changes) but also behavioral gain of the motor function (behavioral/functional changes). Because tDCS can be applied while subjects are performing a leg motor task, it might be useful in the neuro-rehabilitation of patients with leg motor disabilities.