Control of saccade initiation in a countermanding task using visual and auditory stop signals

We examined inhibitory control in an oculomotor countermanding task, where the primary task required a saccadic eye movement be made to a target and a less-frequent secondary task required that the movement be halted. Previou s studies have used a visual stimulus presented centrally on the fovea as t he signal to stop or countermand a saccade. In these previous studies, ther e are at least two possible sources of saccadic inhibition: (1) sensory sti mulation at the fovea can elicit a bottom-up mechanism, where a visual tran sient signal can delay or inhibit the developing saccade command; and (2) i nformation based on the task instruction can be used to initiate a top-down mechanism to halt the movement. In the present study, we used both visual and auditory stop signals to test the hypothesis that the bottom-up mechani sm is activated only after presentation of a foveal visual stop signal. Sub jects were instructed first to look at a central spot and then to look to a n eccentric visual target that appeared randomly to the left or right of ce nter. On about one-third of the trials, a stop signal was presented. Three types of stop signals were used with equal probability: a broad-band noise burst (auditory), a central fixation spot (visual), and a combination of th e auditory and visual stimuli (combined). Saccadic reaction time and stop-s ignal accuracy were used to calculate stop signal reaction time (SSRT), an estimate of the time required to inhibit the eye movement. Mean SSRT was lo nger for the auditory stop signals (201 ms) than for the signals with a fov eal visual component (visual 113 ms; combined 91 ms). We conclude that a fo veal visual stop signal in an oculomotor countermanding task changes the me asure of inhibitory control to reflect not only inhibitory processes but al so the sensory information afforded by stimulation at the fovea.