Previous studies have shown that cerebellar transcranial direct current stimulation (tDCS) leads to faster adaptation of arm reaching movements to visuomotor rotation and force field perturbations in healthy subjects. The first aim of the present study was to confirm a stimulation-dependent effect on motor adaptation. Second, we investigated whether tDCS effects differ depending on onset, that is, before or at the beginning of the adaptation phase. A total of 120 healthy and right-handed subjects (60 women, mean age 23.2 ± SD 2.7 yr, range 18–31 yr) were tested. Subjects moved a cursor with a manipulandum to one of eight targets presented on a vertically orientated screen. Three baseline blocks were followed by one adaptation block and three washout blocks. Sixty subjects did a force field adaptation task (FF), and 60 subjects did a visuomotor adaptation task (VM). Equal numbers of subjects received anodal, cathodal, or sham cerebellar tDCS beginning either in the third baseline block or at the start of the adaptation block. In FF and VM, tDCS and the onset of tDCS did not show a significant effect on motor adaptation (all P values >0.05). We were unable to support previous findings of modulatory cerebellar tDCS effects in reaching adaptation tasks in healthy subjects. Prior to possible application in patients with cerebellar disease, future experiments are needed to determine which tDCS and task parameters lead to robust tDCS effects. NEW & NOTEWORTHY Transcranial direct current stimulation (tDCS) is a promising tool to improve motor learning. We investigated whether cerebellar tDCS improves motor learning in force field and visuomotor tasks in healthy subjects and what influence the onset of stimulation has. We did not find stimulation effects of tDCS or an effect of onset of stimulation. A reevaluation of cerebellar tDCS in healthy subjects and at the end of the clinical potential in cerebellar patients is demanded.

AbstractPatients with cerebellar ataxia suffer from various motor learning deficits hampering their ability to adapt movements to perturbations. Motor adaptation is hypothesized to be the result of two subsystems: a fast learning mechanism and a slow learning mechanism. We tested whether training paradigms that emphasize slow learning could alleviate motor learning deficits of cerebellar patients. Twenty patients with cerebellar degeneration and twenty age-matched controls were trained on a visuomotor task under four different paradigms: a standard paradigm, gradual learning, overlearning and long intertrial interval learning. Expectedly, cerebellar participants performed worse compared to control participants. However, both groups demonstrated elevated levels of spontaneous recovery in the overlearning paradigm, which we saw as evidence for enhanced motor memory retention after extended training. Behavioral differences were only found between the overlearning paradigm and standard learning paradigm in both groups.Modelling suggested that, in control participants, additional spontaneous recovery was the result of higher retention rates of the slow system as well as reduced learning rates of the slow system. In cerebellar participants however, additional spontaneous recovery appeared only to be the result of higher retention rates of the slow system and not reduced learning rates of the slow system. Thus, memory resilience was reduced in cerebellar participants and elevated levels of slow learning were less resilient against washing out. Our results suggest that cerebellar patients might still benefit from extended training through use-dependent learning, which could be leveraged to develop more effective therapeutic strategies.

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