The role of fixation point and subjects' readiness in the occurrence of express saccades as revealed by the self-initiation paradigm.

The role of fixation and the subjects' response preparedness in producing express saccades were explored in seven human subjects. The occurrence frequencies of the express saccades were compared in the overlap (continuous presentation of fixation point), gap (fixation point offset 0-400 ms prior to target onset) and no-fixation tasks under the conventional and self-initiation paradigms. In the latter paradigm, the subjects, when ready, touched a sensor in order to ignite the target lamp with a delay time of 0-400 ms (target onset delay time). Therefore, the subjects' response preparedness might be expected to be higher than that in the normal paradigm and equated in each subject at the time when the subjects touched a sensor regardless of the paradigms. Although express saccades were produced neither in the normal overlap nor in the normal no-fixation tasks, they could be produced at the rate of 24 and 48% in the overlap and no-fixation tasks under the self-initiation paradigm, respectively. The highest occurrence frequency of express saccades was obtained when the gap paradigm was combined with the self-initiation paradigm with a delay time of 100 ms (62%). The value was higher by 20% than in the normal gap task. At a target onset delay time of 0 ms under the self-initiation paradigm, the occurrence frequency of express saccades was higher in the overlap task than in the gap task. These results suggest that the subjects' response preparedness has a potentiality to produce express saccades without fixation point offset and that fixation point offset at the same time of the target stimulus onset has an interference, rather than facilitatory, influence on the generation of express saccades.