Fig. 2.

Example of a normal trial (left side) and a perturbed trial (right side) performed by a young adult. This figure shows the displacement, interference EMG, and rectified filtered EMG of the upper and lower lips. Beginning and end of each trial were selected manually by visual inspection. Open circles represent the beginning of the trial, and filled circles represent the end. Lip displacement was defined as the absolute movement amplitude (mm) of the upper or lower lips. The EMG interference (μV) was obtained at 5,000 Hz. The EMG signals were rectified and low-pass filtered at 40 Hz to facilitate the visual inspection. A direct perturbation effect can be seen as an abrupt decrease in the lower lip displacement prior to the trial