The difference in the mean reaction time between gap and overlap conditions is called the gap effect. In case of saccadic responses the gap task favours the occurrence of express saccades. Express saccades do not occur in all naive subjects, yet they show a gap effect. Express saccades can also be observed in the overlap task. Therefore, the gap effect and the occurrence of express saccades must be considered two different phenomena. The express phenomen seems to be confined to the saccade system.
Disengagement of fixation and of sustained attention modulate the size of the gap-effect in saccadic reactions.