Visual Attention and Motion Visibility Modulate Motor Resonance during Observation of Human Walking in Different Manners.

To advance our knowledge on the motor system during cyclic gait observation, we aimed to explore the effects of gaze fixation on corticospinal excitability evaluated by single-pulse transcranial magnetic stimulation (TMS). Fourteen healthy adult volunteers watched a video of a demonstrator walking on a treadmill under three different conditions: (1) observing the right lower limb, (2) observing the right ankle joint, and (3) observing the right lower limb on a video focused on the area below the knee. In each condition, motor-evoked potentials elicited by TMS in the tibialis anterior (TA) muscle were measured synchronously with the demonstrator's initial contact and toe-off points. Directing visual attention to the ankle joint and focusing on its movements caused corticospinal facilitation in the TA muscle compared with watching the video without any visual fixation. In addition, phase-dependent differences in corticospinal excitability between the initial contact and toe-off points were only detected when the visibility range was restricted to below the knee. Our findings indicated that motor resonance during cyclic gait observation is modulated by visual attention and motion visibility in different activation manners.