Modulation of lower limb skill acquisition by cerebellar transcranial direct current stimulation

The involvement of the brain in controlling the lower limbs, for instance during walking, has clearly been demonstrated. Yet, the role of the brain in the different learning processes in the lower limbs is only starting to be unraveled. Recently, the cerebellum was shown to contribute to locomotor adaptation: cerebellar brain inhibition decreases with the amount of adaptation. Here, we investigated the role of the cerebellum in lower limb skill acquisition over multiple days by applying transcranial direct current stimulation (tDCS), to enhance the excitability of this area. We hypothesized that cerebellar tDCS would augment the online (within-session) learning and not influence offline (between-session) learning. Two groups of eight subjects trained on a challenging visually-guided sequential stepping task over three consecutive days. Subjects were instructed to perform the task as fast and accurate as possible. The groups received either anodal cerebellar or sham tDCS. Performance was measured using a skill measure, which was calculated from the movement speed and accuracy and reflected a true shift in a subject’s speed-accuracy tradeoff. We found that cerebellar tDCS did not significantly influence the within-session gains (Cerebellar: 0.60±0.07 [mean±SEM], Sham: 0.51±0.07, p-value=0.395). Strikingly, cerebellar tDCS resulted in a detrimental effect on the between-session gains (Cerebellar: -0.29±0.09, Sham: 0.04±0.05, p=0.008). This effect suppressed total learning, but there was no statistical difference (Cerebellar: 1.21±0.18, Sham: 1.60± 0.14, p=0.108). In contrast to our hypothesis, cerebellar tDCS did not affect the online learning, but negatively influenced the consolidation of what was learned, the offline effects. This shows the role of the cerebellum in consolidation.