K. Forbes et Rm. Klein, THE MAGNITUDE OF THE FIXATION OFFSET EFFECT WITH ENDOGENOUSLY AND EXOGENOUSLY CONTROLLED SACCADES, Journal of cognitive neuroscience, 8(4), 1996, pp. 344-352
Two experiments examined saccadic reaction time (RT) in response to vi
sual targets as a function of fixation offset condition (no offset; ta
rget simultaneous with offset and 200-msec offset-target SOA) in prosa
ccade and antisaccade tasks. The second experiment also included a con
dition in which saccades were made in response to verbal commands pres
ented auditorally. To ensure that observers were equally prepared in e
ach condition, auditory warning tones preceded target onset on every t
rial. The RT reduction associated with fixation offset (FOE, or gap ef
fect) was identical with visual targets in the prosaccade task and in
response to verbal signals, strongly implicating motor, rather than se
nsory mechanisms in the FOE. The FOE in the antisaccade task was signi
ficant, 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.