A within-trial measure of the stop signal reaction time in a head-unrestrained oculomotor countermanding task.

The countermanding (or stop-signal) task, which requires the cancellation of an impending response on the infrequent presentation of a stop signal, enables study of the contextual control of movement generation and suppression. Here we present a novel and empirical measure of the time needed to cancel an impending gaze shift by recording neck muscle activity during a head-unrestrained oculomotor countermanding paradigm. On a subset of stop signal trials, subjects generated small head movements toward a target even though gaze remained stable due to a compensatory vestibular-ocular reflex. On such trials, we observed a burst of antagonist neck muscle activity during the small head-only error. Such antagonist neck muscle activity served as an active braking pulse as its magnitude scaled with the kinematics of the head-only error. This activity was selective for trials in which the head was arrested in mid-flight and did not appear on trials without a stop signal, on noncancelled stop signal trials when the gaze shift was completed, or on stop signal trials without head motion. Importantly, the timing of this antagonist activity related best to the onset of the stop signal (lagging it by ∼180 ms), and strongly correlated with behavioral estimates of the time needed to cancel a movement (the stop signal reaction time). These results are consistent with the notion that such selective antagonist neck muscle activity arises as a peripheral expression of the oculomotor stop process that successfully cancelled the gaze shift. Studying movement cancellation within nested systems like the head-unrestrained gaze shifting system offers a unique opportunity for investigating underlying neural mechanisms as the overall goal (i.e., to cancel a gaze shift) can be achieved despite motion of other components; on such individual trials, the oculomotor stop process is expressed as an active braking pulse.