Gap duration and location of attention focus modulate the occurrence of left/right asymmetries in the saccadic reaction times of human subjects.

Five human subjects were trained in a single target gap saccade task (fixation point offset precedes target onset, target presentation random at 4 deg to the left or right of the fixation point). Four of them produced different distributions of saccadic reaction times (SRT) for left vs right directed saccades. These asymmetries consisted mostly in different numbers of express saccades, which the subjects produced to the left and to the right side. Using different gap durations (0, 100, 200, 300 and 400 msec) and an overlap task, we found a systematic modulation of the frequency of express saccades: for the shortest and longest gap durations, and in the overlap task, express saccades tended to decline. As a consequence of this effect asymmetric SRTs became rather symmetric for these gap durations. In a gap task where the central fixation point was replaced by a peripheral attention target the occurrence of express saccades was strongly modulated by the location of the attention target relative to the saccade target: for all subjects the frequency of express saccades decreased when the saccade target occurred in the close vicinity of the peripheral attention target. This effect resulted again in clear modifications of the observed asymmetries. We suggest that the occurrence of express saccades can be influenced in a dynamic way by the permanent allocation of the subject's visual attention. Moreover, the phenomenon of direction asymmetry in the SRT distributions can be modulated by such manipulations of the attentional focus.