P 175. Modulation of motor cortex excitability and skilled task learning

Abstract

Question Primary motor cortex (M1) is the principal cortical motor output and thus should be involved in motor learning. Theta burst stimulation (TBS) provides a mean for a non-invasive transient modulation of cortical activity and excitability of the targeted cortical area. In this study we aimed to check whether changes in M1 excitability induced by facilitatory and inhibitory TBS protocols can affect learning of a skilled movement with non-dominant hand. Methods 30 right-handed healthy subjects (mean age 25 ± 7 y, 12 women) participated in the study. They were divided into three experimental groups (10 subject each) according to the intervention they had: (1) facilitatory, intermittent TBS (iTBS) protocol-the iTBS group; (2) inhibitory, continuous TBS (cTBS) protocol-the cTBS group; and (3) placebo, sham iTBS protocol-Placebo group. During iTBS protocol, short bursts of 50 Hz stimulation (3 pulses of 80% aMT) were applied at 5 Hz in 2 s trains every 10 s, for a total of 600 pulses. The cTBS protocol was the same except that bursts were applied continuously (600 pulses in total). The sham iTBS protocol was the same as the iTBS protocol, but a placebo coil was used. Motor cortex hand area for the non-dominant side was targeted. Motor cortex excitability was assessed by measuring motor evoked potentials (MEP) from the first dorsal interosseus muscle (single pulse TMS at 120% rMT). Motor performance was evaluated using Purdue peg-board (PPB) test and simple reaction time (RT) at 3 time-points: before (B), immediately after (T0), and at 30 (T30) min following TBS intervention. Results For MEP results (Fig. 1), ANOVA showed significant effects of factor Group-post hoc pair-wise analyses showed as significant differences between all groups. However, significant was Group × Time interaction as well: following placebo, MEPs remained unchanged in comparison to time B; following iTBS, MEPs increased significantly at T0, but then returned towards the baseline at T30; in contrast, following cTBS, MEPs were significantly reduced at both T0 and T30. For PPB results (Fig. 2), ANOVA showed significant effects of both factors, Group and Time. Post-hoc pair-wise analyses showed that cTBS differed significantly from both iTBS and Placebo groups, while the latter two did not differ. However, Group × Time interaction was significant as well: following placebo, number of pegs positioned on PBB test increased significantly at T0 in comparison to B, and continued with significant increase towards T30; following iTBS results on PBB test also increased significantly at T0, but subsequently remained at the same level up to T30 (although difference between Placebo and iTBS at T30 was not significant); following cTBS, the number of pegs did not change or decreased slightly at T0, only to show significant increase afterwards towards T30 (but still significantly below Placebo). RT did not change following either of the stimulation procedures. Conclusion Results suggest that changes in M1 excitability are of only partial functional significance for early phases of motor learning in healthy people. Decrease of excitability can considerably reduce efficiency of learning for up to 30 min. In contrast, increase of excitability seems not able to bring any additional improvement over one that is seen spontaneously and may even have mild disruptive effect. Alternatively, achieved increase of M1 excitability may have been below the threshold for inducing learning changes.