The amplitude and angle of saccades to double-step target displacements.

Two experiments examined the magnitude and direction of the initial saccade to a target that underwent two displacements within 200 msec. When the amplitude of the two target displacements was held constant at 10 deg but the angle of the displacements differed by 45 deg, a small but significant number of intermediate-angle saccades occurred. These intermediate-angle saccades were directed to locations between the two targets, thereby generating an angle transition function, and their amplitude was 10-20% less than the amplitude on single-step displacements. These intermediate-angle saccades were not simply the result of programming an oblique saccade because amplitude transition functions virtually identical to those reported by Becker and Jürgens [Vision Res. 19, 967-983 (1979)] for horizontal saccades were obtained for double-step target displacements limited to oblique saccades. Finally, when both target amplitude and target angle were varied in double-step displacements, it became clear that the timing of the amplitude transition function and the angle transition function was not coincident. Across conditions, the angle transition function occurred at a consistent time prior to the initial saccade, whereas the amplitude transition function occurred at a variable time prior to the initial saccade. Because these amplitude and angle transition functions appeared to be dissociated, a modified model of the saccadic programming system for double-step displacements was proposed.