Reaction times of vertical prosaccades and antisaccades in gap and overlap tasks

Horizontal saccadic reaction times (SRTs) have been extensively studied over the past 3 decades, concentrating on such topics as the gap effect, express saccades, training effects, and the role of fixation and attention. This study investigates some of these topics with regard to vertical saccades. The reaction times of vertical saccades of 13 subjects were measured using the gap and the overlap paradigms in the prosaccade task (saccade to the stimulus) and the antisaccade task (saccade in the direction opposite to the stimulus). In the gap paradigm, the initial fixation point (FP) was extinguished 200 ms before stimulus onset, while, in the overlap paradigm, the FP remained on during stimulus presentation. With the prosaccade overlap task, it was found that most subjects (10/13) — whether they were previously trained making horizontal saccades or naive — had significantly faster upward saccades compared with their downward saccades. One subject was faster in the downward direction and two were symmetrical. The introduction of the gap reduced the reaction times of the prosaccades, and express saccades were obtained in some naive and most trained subjects. This gap effect was larger for saccades made to the downward target. The strength of the updown asymmetry was more pronounced in the overlap as compared to the gap paradigm. With the antisaccade task, up-down asymmetries were much reduced. Express antisaccades were absent even with the gap paradigm, but reaction times were reduced as compared to the antisaccade overlap paradigm. There was a slight tendency for a larger gap effect of downward saccades. All subjects produced a certain number of erratic prosaccades in the antitaks, more with the gap than with the overlap paradigm. There was a significantly larger gap effect for the erratic prosaccades made to the downward, as compared to the upward, target, due to increased downward SRTs in the overlap paradigm. Three subjects trained in both the horizontal and the vertical direction showed faster SRTs and more express saccades in the horizontal directions as compared to the vertical. It is concluded that different parts of the visual field are differently organized with both directional and nondirectional components in saccade preparation.