Action observation facilitates practice-induced encoding of a motor memory

Introduction: Motor practice is associated with the formation of elementary motor memories. Here, we tested the hypothesis that observation of a motor training associated with physical training (concurrent imitation) will enhance the encoding process of a motor memory relative to physical training alone.

Methods: Ten healthy subjects (4 women, 31±8 years) participated in three different sessions that evaluated the ability to modify the direction of thumb movements elicited by transcranial magnetic stimulation (TMS) of the contralateral primary motor cortex (MI). In condition a, subjects practiced voluntary thumb movements in a direction opposite to the TMS-evoked baseline direction (physical training, PT). In conditions b and c, physical training of thumb movements was combined with observation of a video displaying synchronous thumb movements that were either directionally congruent (same direction, PT+AOc) or not congruent (opposite direction, PT+AOnc) to the practiced ones. The endpoint measure was the session-dependent percentage of TMS-evoked thumb movements falling in the direction of trained movements.

Results: PT and PT+AOc, but not PT+AOnc, led to a significant increase of the percentage of TMS-evoked movements falling in the trained direction (PT, from 2.2±2.9% to 20.8±20.7%, p=0.006; PT+AOc from 2.3±2.9% to 32.2±25.7%, p=0.001; PT+AOnc from 1.4±1.8% to 15.5±25.8%, n.s;) The increase was more prominent with PT+AOc compared to PT, or PT+AOnc (ANOVA on delta PTTZ, p=0.006). The acceleration of TMS-evoked movements along the principal movement axis changed toward the trained direction in PT (from 0.4±0.2 m/s2 to -0.3±0.7 m/s2, p=0.009) and PT+AOc (from 0.6±0.3 m/s2 to -0.3±0.6 m/s2, p=0.003), but not significantly for PT+AOnc (from 0.6±0.5 m/s2 to 0.0±0.7 m/s2, n.s.). Furthermore, the balance of excitability of muscles participating in the trained direction was altered in favor of the trained movement direction.

Conclusions: Observation of motor training can enhance the effects of a physical training when observed movements are similar to the practiced movements. This finding may provide evidence that matching observation and practice (concurrent imitation) could strengthen the process of motor memory encoding and motor learning.