TMS over M1 late in learning impairs retention of motor memory


The primary motor cortex (M1) is important for the long-term retention of motor memory (Hadipour-Niktarash et al., 2007). However it is still unclear whether M1 contributes to memory storage as a result of the processing of movement errors early during learning (when motor commands are constantly changing; Tan et al., 2014), or when performance is stabilizing late in learning (Classen et al., 2008). To address this issue, we used a visuomotor adaptation paradigm in which participants performed 500 reaching movements toward 10 visual targets. During the first 250 trials, we introduced a gradual rotation of the visual representation of the hand (1° every 10 trials for a total rotation of 25°). The last 250 trials were performed with a constant 25° rotation. We interfered with M1 processing using single-pulse transcranial magnetic stimulation (TMS) delivered at movement end, either when motor behavior was gradually changing (trials 0-250; Early TMS group) or when performance was stabilizing (trials 250-500; Late TMS group). A control group received no TMS during adaptation. Retention was assessed 24h later by measuring the angular error at peak velocity. Participants in all groups showed identical performance during adaptation and none consciously perceived the induced visuomotor rotation. Participants of the Late TMS group demonstrated significantly lower retention compared to the Control group (p<0.01). This suggests that post-movement activity in M1 late in learning, when motor performance is stabilizing and motor commands tend to repeat, is crucial for the long-term retention of motor memory during visuomotor adaptation.

Acknowledgments: This project has been funded by a Natural Sciences and Engineering Research Council of Canada (NSERC) award, a Fond de Recherche du Québec - Santé (FRQ-S) subvention and a Senate Research Committee Grant from Bishop's University.