Estimating the components of the gap effect.

Abstract

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 peripheral target. This reduction in saccadic reaction time (SacRT) is thought to be due to a general warning effect and an oculomotor specific fixation offset that occur when the fixation point is removed. In order to estimate the contribution of each of these effects to the overall gap effect, this paper 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 the fixation offset (i.e. similar to that in a gap trial) but does not provide the fixation offset effect (FOE). When the fixation point was abruptly reduced in size before the presentation of the target, the estimated decrease 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 auditory warning effect and the FOE interacted in reducing SacRTs. Additionally, when the fixation point was abruptly increased in size before the presentation of the target, SacRTs were slower than when the fixation point did not change in size and remained present for the entire trial (i.e. an overlap trial). We conclude that this new partial-gap paradigm is a useful method for researchers wishing to separately examine FOE and visual warning effects.