(A) Time course of OO-EMG activity. The average OO-EMG across participants within each group (n = 20 participants per group) is shown at each contrast level (columns: 100%, 200%, and 400%). For each contrast level, responses from the 3 groups are superimposed and shown as a function of time over the 16-second recording interval. The 4-second stimulus pulse is indicated by a black bar and the middle 3,400 ms of this period is highlighted in yellow. The shaded area is the ±SEM across participants within a group. OO-EMG is expressed as percentage change relative to activity occurring prior to pulse onset. (B) The across-subject, average OO-EMG activity during the middle 3,400 ms of the stimulus pulse is shown for each group (n = 20 participants per group) at each contrast level. The stimuli were presented at 3 different contrast levels (100%, 200%, and 400%), and these (log-spaced) values define the x-axis. Error bars represents ±SEM across participants.