Functional Role of Cerebellar Gamma Frequency in Motor Sequences Learning: a tACS Study.

Although the role of the cerebellum in motor sequences learning is widely established, the specific function of its gamma oscillatory activity still remains unclear. In the present study, gamma (50 Hz)-or delta (1 Hz)-transcranial alternating current stimulation (tACS) was applied to the right cerebellar cortex while participants performed an implicit serial reaction time task (SRTT) with their right hand. The task required the execution of motor sequences simultaneously with the presentation of a series of visual stimuli. The same sequence was repeated across multiple task blocks (from blocks 2 to 5 and from blocks 7 to 8), whereas in other blocks, new/pseudorandom sequences were reproduced (blocks 1 and 6). Task performance was examined before and during tACS. To test possible after-effects of cerebellar tACS on the contralateral primary motor cortex (M1), corticospinal excitability was assessed by examining the amplitude of motor potentials (MEP) evoked by single-pulse transcranial magnetic stimulation (TMS). Compared with delta stimulation, gamma-tACS applied during the SRTT impaired participants' performance in blocks where the same motor sequence was repeated but not in blocks where the new pseudorandom sequences were presented. Noteworthy, the later assessed corticospinal excitability was not affected. These results suggest that cerebellar gamma oscillations mediate the implicit acquisition of motor sequences but do not affect task execution itself. Overall, this study provides evidence of a specific role of cerebellar gamma oscillatory activity in implicit motor learning.

Effects of low-gamma tACS on primary motor cortex in implicit motor learning.

In the primary motor cortex (M1), rhythmic activity in the gamma frequency band has been found during movement planning, onset and execution. Although the role of high-gamma oscillatory activity in M1 is well established, the contribution of low-gamma activity is still unexplored. In this study, transcranial alternating current stimulation (tACS) was used with the aim to specifically modulate low-gamma frequency band in M1, during an implicit motor learning task. A 40 Hz-tACS was applied over the left M1 while participants performed a serial reaction time task (SRTT) using their right hand. The task required the repetitive execution of sequential movements in response to sequences of visual stimuli. Sequential blocks were interleaved by a random block, which served as interference to sequence learning. Sham and 1 Hz tACS were used as control. Task performance was examined before, during and after tACS (pre-, online- and post-phase, respectively). Furthermore, cortical reactivity of M1 was assessed in the pre- and post-tACS phases, by measuring motor-evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation (TMS). Compared to sham and pre-tACS, the 40 Hz stimulation applied during SRTT slowed down response times in blocks that required retrieving previously learned sequences, after performing the random block. In addition, M1 cortical reactivity was selectively inhibited following 40 Hz-tACS, as quantified by reduced MEP amplitude. These results show that low-gamma tACS delivered over M1 during motor learning enhanced susceptibility to interference generated by the random sequence (i.e., proactive interference effect). Importantly, only low-gamma stimulation produced long-lasting effects on M1 cortical reactivity.