The Magnitude of the Fixation Offset Effect with Endogenously and Exogenously Controlled Saccades

Two experiments examined saccadic reaction time (RT) in response to visual targets as a function of fixation offset condition (no offset; target simultaneous with offset and 200-msec offset-target SOA) in prosaccade and antisaccade tasks. The second experiment also included a condition in which saccades were made in response to verbal commands presented auditorally. To ensure that observers were equally prepared in each condition, auditory warning tones preceded target onset on every trial. The RT reduction associated with fixation offset (FOE, or gap effect) was identical with visual targets in the prosaccade task and in response to verbal signals, strongly implicating motor, rather than sensory, mechanisms in the FOE. The FOE in the antisaccade task was significant, but it was also significantly smaller than in the other tasks. We speculate that the reduced FOE in the antisaccade task may be due to the requirement to inhibit the superior colliculus when the target directed saccadic programs are, by instruction, erroneous.