Estimating the components of the gap effect

The gap effect refers to the finding that saccadic latencies are typically reduced when a fixation point is removed prior to the appearance of a perip heral target. This reduction in saccadic reaction time (SacRT) is thought t o be due to a general warning effect and an oculomotor specific fixation of fset that occur when the fixation point is removed. In order to estimate th e contribution of each of these effects to the overall gap effect, this pap er introduces a new manipulation, the partial-gap trial, where the fixation point undergoes a change in size prior to the presentation of the target. The partial-gap trial is presumed to provide the visual warning effect of t he fixation offset (i.e. similar to that in a gap trial) but does not provi de the fixation offset effect (FOE). When the fixation point was abruptly r educed in size before the presentation of the target, the estimated decreas e in SacRT due to the visual warning effect was 5-7% and did not differ in the presence or absence of an auditory warning signal. It was found that au ditory warning effect and the FOE interacted in reducing SacRTs. Additional ly. when the fixation point was abruptly increased in size before the pres entation of the target, SacRTs were slower than when the fixation point did not change in size and remained present fur the entire trial (i.e. an over lap trial). We conclude that this new partial-gap paradigm is a useful meth od for researchers wishing to separately examine FOE and visual warning eff ects.