Motor adaptation, or learning to adapt movement to changes in sensory feedback, is crucial to everyday functioning. For example, using an unfamiliar computer mouse with a large gain initially results in large movement errors, but with practice, we adapt the size of our wrist movements to reduce movement errors. Retention of motor adaptation is evident in savings, where initial learning improves subsequent learning, and anterograde interference, where initial learning impairs subsequent learning. Reinforcement and use-dependent movement biases induced by movement repetition have been proposed to contribute to retention of motor adaptation. However, the relative contributions of reinforcement and use-dependent mechanisms to savings and anterograde interference are unclear. Here, I describe a study using transcranial direct current stimulation which demonstrates that use-dependent movement biases induced by movement repetition contribute to anterograde interference, but not to savings.

Timing is crucial for effective motor control. Time intervals can be represented relative to a recurrent beat (beat-based timing), or as absolute durations (non-beat-based timing) (Grahn & Brett, 2007; Teki et al., 2011). Neuroimaging and neuropsychological work suggests involvement of the supplementary motor area (SMA) in beat-based timing, and the premotor cortices and the cerebellum in non-beat-based timing (Kung et al., 2013; Grahn & Rowe, 2013; Geiser et al., 2013), however, causal evidence is lacking. In this talk, I will describe experiments examining how performance on beat-based and non-beat-based timing is enhanced or impaired by altering excitability of the supplementary motor area (SMA), the right cerebellum, or the bilateral dorsal premotor cortices. Crucially, we found that increasing SMA excitability improved discrimination of beat rhythms, whereas decreasing SMA excitability impaired discrimination of beat rhythms. This polarity-dependent effect was absent for the premotor cortex and the cerebellum. These findings provide causal evidence that basal ganglia-SMA networks play a greater role in beat-based rhythm discrimination than premotor or cerebellar networks.

Transcranial alternating current stimulation (tACS) is a noninvasive method that allows the entrainment of brain oscillations at specific frequencies via alternating currents applied on the scalp surface. The goal of the current study was to investigate whether tACS can be used to entrain brain oscillations underlying the production of self-paced rhythmic movements. Twenty-four right-handed participants swung a wrist pendulum at their most comfortable tempo while stimulation was applied at the level of the left primary motor cortex (C3) at frequencies corresponding to participant’s movement (Experiment 1) or in the Alpha and Beta ranges (Experiment 2). Because entrainment is known to occur only if the difference between the frequencies of two rhythms is small enough, participants were stimulated at their preferred movement frequency and ± 12.5% in Experiment 1, and at 10Hz and 20Hz ±12.5% in Experiment 2. The analysis of the participants’ pendulum swinging frequency, amplitude, variability, and phase synchronization with the stimulation did not reveal evidence of entrainment or movement perturbations induced by tACS in any condition across the two experiments. Although it remains possible that other stimulation protocols may be influential, these results might suggest limited effects of tACS on movements that are low in goal-directedness and/or strongly driven by peripheral and mechanical constraints, as in pendulum swinging or locomotion.

The synchronization of movements between living beings is an important communication channel characterizing social behaviour in several species including humans. Correlational evidence suggests that successful behavioural synchronization among people is associated with brain-to-brain synchronization. Yet, because this evidence is correlational, it remains largely unknown whether brain-to-brain synchronization can cause interpersonal synchronization or simply emerges as an artefactual consequence of behaviour. To answer this question, we transcranially manipulated oscillatory synchronization between the motor cortices of pairs of individuals and measured their interpersonal synchronization in a joint tapping task. We show that 20Hz in-phase brain-to-brain synchronization improves pairs’ capacity to establish behavioural synchronization of their movements. This effect was specific for 20Hz brain stimulation (beta oscillations) and was not observed for other frequencies such as 2Hz (matching the tapping frequency) or 10Hz (alpha oscillations). Our results demonstrate that the synchronization of multiple individuals’ brains determines the success with which they coordinate their actions. Furthermore, the frequency-specificity of this finding support a cognitive account of the underlying mechanism for which, by synchronizing beta oscillations, we increased the likelihood of the participants initiating movements within overlapping temporal windows.