Further observations on the occurrence of express-saccades in the monkey

Express-saccades, i.e. goal directed eye movements with extremely short saccadic reaction times (SRT) have recently been observed in rhesus monkey (70-80 ms) and human subjects (around 100 ms). In the gap task which has been used so far, a central fixation point (Fp) was turned off a short time before a new target (Tg) in the near periphery was presented. Therefore, express-saccades occurred when the goal of fixation was no longer visible. To determine whether or not the absence of the Fp is a necessary condition for the execution of an express-saccade, we used an overlap task in which the monkeys had to change the direction of gaze in the presence of the Fp. The results for this overlap task were compared to those found in the gap task. Three major observations have emerged from the present study. Even though the Fp remained visible, a suddenly appearing peripheral target could be reached by an express-saccade. Express-saccades persisted if the location as well as the time of the appearance of the target was randomized. It appears that for an express-saccade to occur, the process of interruption of previous active fixation must be completed at the time when a new target becomes visible. The spectrum of the monkey's saccadic reaction times contains at least three different peaks: express-saccades with reaction times below 100 ms, fast regular saccades with reaction times around 130 ms, and slow regular saccades with reaction times around 180 ms.