The disappearance of foveal and nonfoveal stimuli : Decomposing the gap effect

Abstract When a foveal stimulus disappears prior to the appearance of a peripheral target, saccadic reaction times (RTs) are reduced. We compared this gap ef%ect for foveal and nonfoveal stimuli when a highly predictive auditory warning signal was or was not presented. Without a tone, there was a gap effect for both foveal and nonfoveal stimuli; with a tone, there was a gap effect for foveal but not for nonfoveal stimuli. Highly predictive warning tones also modulated the gap effect in a manner that seemed to reflect top-down release of ocular inhibition. We argue that the gap effect therefore consists of three components: (a) warning effects; (b) release of ocular inhibition due to the disappearance of a foveal stimulus; and (c) release of ocular inhibition due to top-down processes. In a gap paradigm, participants are required to maintain gaze on a foveal stimulus until the onset of a peripheral saccade target. On some (overlap) trials, the foveal stimulus remains present; on other (gap) trials, the foveal stimulus is extinguished at some interval prior to the appearance of the target. Saccadic reaction times (RTs) are reduced on gap versus overlap trials (Saslow, 1967). Known as the gap effect, several explanations for this relative facilitation of RTs have been tested and rejected. The longer saccadic RTs on overlap versus gap trials do not result from refractoriness of the saccadic system following fixation-maintaining microsaccades (Kingstone, Fendrich, Wessinger, & Reuter-Lorenz, 1995). The facilitation of saccadic RTs on gap versus overlap trials is not the result of improved visual processing of the target (cf. Reulen, 1984a, 1984b; see Kingstone & Klein, 1993a; ReuterLorenz, Hughes, & Fendrich, 1991). In addition, the gap effect does not reflect the anticipatory execution of preprogrammed saccades on gap trials (cf. Juttner & Wolf, 1992; Kalesnykas & Hallett, 1987; see Kingstone & Klein, 1993a; Reuter-Lorenz et al., 1991). Fischer and colleagues proposed that the facilitation of saccadic RTs following the disappearance of a foveal stimulus may be related to the disengagement of visual attention: When attention is engaged, the saccadic system is inhibited; when attention is disengaged, the saccadic system is released from this inhibition (see Fischer & Weber, 1993 for a review). According to this account, RT facilitation in the typical gap paradigm occurs because the foveal stimulus is also the attended stimulus. It is possible to test this hypothesis by removing attention from the foveal stimulus. If participants are asked to maintain gaze on a foveal stimulus but to attend to a nonfoveal stimulus, the effects of extinguishing the unattended foveal stimulus can be distinguished from the effects of extinguishing the attended nonfoveal stimulus. Unfortunately, early studies that used this strategy to test the attentional disengagement hypothesis were compromised by a failure to assess whether attention was actually allocated to the nonfoveal stimulus (Braun & Breitmeyer,1987; Mayfrank, Mobashery, Kimmig, & Fischer,1986). When Kingstone and Klein (1993a) corrected this methodological weakness, they found that the facilitation of RTs depended on the location of the extinguished stimulus and not on the locus of covert attention. Kingstone and Klein (1993a) presented participants with three stimuli along the vertical meridian. Attention was directed on a trial-by-trial basis to one of the two nonfoveal stimuli on the vertical meridian. This was accomplished via a symbolic cue (Experiments 1 and 3) or via a brightening in the periphery (Experiment 2). On the majority of trials, participants were required to maintain central fixation and to make a speeded manual (button-press) response to report the detection of a target that appeared at one of the two locations on the vertical meridian. To the extent that attention was allocated to the cued location, manual detection RTs were expected to be faster at the cued nonfoveal location than at the uncued nonfoveal location (cf. …