Early motor memory consolidation: Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) over the left primary motor cortex (M1)

Question: Synchronized oscillatory activity predominantly at alpha (8 – 12 Hz) and beta (13 – 30 Hz) frequencies represents a key mechanism for motor control. But, the exact significance of these frequencies for motor learning and motor memory consolidation remains to be solved. Transcranial alternating current stimulation (tACS) allows the frequency specific modulation of cortical brain areas. The present study investigates the significance of 10 Hz and 20 Hz motor cortical oscillations for early consolidation of a newly learned motor sequence by means of tACS. Methods: 36 right-handed subjects participated in the study and were assigned to three groups. Subjects learned a sequential 8-digit serial reaction time task (SRTT) interleaved by a random pattern with the right hand. tACS was applied with 10 Hz or 20 Hz for 10 min at 1 mA over the left primary motor cortex (M1) immediately after motor learning during rest. Sham stimulation served as control condition. After stimulation cessation, retrieval as a proof of early motor memory consolidation was tested. Motor learning and retrieval were measured by reaction times. Results: All subjects initially showed motor learning as determined by faster reaction times during the sequential but not the random SRTT. During retrieval, 20 Hz tACS yielded significantly faster reaction times for the sequential but not the random pattern, whereas 10 Hz tACS as well as sham stimulation showed no significant modulation. Conclusion: The present findings underline the relevance of synchronized oscillatory activity at 20 Hz for motor retrieval by showing fascilitatory effects of 20 Hz tACS.