Validation of a within-trial measure of the oculomotor stop process

The countermanding (or stop signal) task requires subjects try to withhold a planned movement upon the infrequent presentation of a stop signal. We have previously proposed a within-trial measure of movement cancellation based on neck muscle recruitment during the cancellation of eye-head gaze shifts. Here, we examined such activity after either a bright or dim stop signal, a manipulation known to prolong the stop signal reaction time (SSRT). Regardless of stop signal intensity, subjects generated an appreciable number of head-only errors during successfully cancelled gaze shifts (compensatory eye-in-head motion ensured gaze stability), wherein subtle head motion toward a peripheral target was ultimately stopped by a braking pulse of antagonist neck muscle activity. Both the SSRT and timing of antagonist muscle recruitment relative to the stop signal increased for dim stop signals and decreased for longer stop signal delays. Moreover, we observed substantial variation in the distribution of antagonist mus...