The reduction of saccadic latency by prior offset of the fixation point: An analysis of the gap effect

Abstract

The latency to initiate a saccade (saccadic reaction time) to an eccentric target is reduced by extinguishing the fixation stimulus prior to the target onset. Various accounts have attributed this latency reduction (referred to as the gap effect) to facilitated sensory processing, oculomotor readiness, or attentional processes. Two experiments were performed to explore the relative contributions of these factors to the gap effect. Experiment 1 demonstrates that the reduction in saccadic reaction time (RT) produced by fixation point offset is additive with the effect of target luminance. Experiment 2 indicates that the gap effect is specific for saccades directed toward a peripheral target and does not influence saccades directed away from the target (i.e., antisaccades) or choice-manual RT. The results are consistent with an interpretation of the gap effect in terms of facilitated premotor processing in the superior colliculus.The latency to initiate a saccade in response to an eccentric target is typically on the order of 180-250 msec (Carpenter, 1977;Wheeless, Boynton, & Cohen, 1966). Saccadic reaction times (RTs) can be substantially reduced, however, simply by extinguishing the fixation stimulus 200-300 msec prior to target onset (see, e.g., Fischer & Ramsperger, 1984;Saslow, 1967). In addition to reducing the average latency of saccades, a temporal gap between fixation point offset and target onset (henceforth referred to as the gap condition) may produce a subpopulation of saccades with a modal latency of 120 msec (Fischer & Ramsperger, 1984. These have been called "express saccades" (e.g., by Fischer, 1987;Fischer & Boch, 1983;Fischer & Breitmeyer, 1987). The latency reduction produced by fixation stimulus offsets have been variously attributed to facilitated sensory processing (see, e.g., Reulen, 1984a), oculomotor readiness (Kalesnykas & Hallett, 1987;Saslow, 1967), or attentional factors (see, e.g., Fischer, 1987. In the present experiments, we attempted to clarify the basis of latency facilitation in the gap condition by examining the effects of target luminance and response requirements.Fixation point offsets could conceivably exert their effect by altering visual sensitivity. It might be easier, for instance, to detect eccentric flashes in a blank field than in the presence of a fixation stimulus. This possibility is considered in Reulen's (1984aReulen's ( , 1984b model, which attributes latency reduction in the gap condition to enhanced processing of the visual target. In this model, it is assumed that saccadic RT represents the linear sum of several seri- 167ally organized processing stages. Following Grice's random threshold theory of response latency (Grice, 1968), a "sensory stage" is posited, in which neural responses to target onset accumulate until a threshold is reached. Subsequent events represent oculomotor programming and efferent processes. In this model, the accumulation rate is a direct function of signal intensity and is constant over time. Reulen's (1984aReulen's ( , 1984b model a...

Gap effect and reaction time distribution: simple vs choice manual responses

“…Many investigators have confirmed the presence of the gap effect for SRT (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Some have proposed that the gap paradigm reduces saccade latency and also facilitates the appearance of a very short latency population in the SRT distribution, the so-called express saccades (ES) (for a review, see Ref.…”

Looking for the GAP effect in manual responses and the role of contextual influences in reaction time experiments

“…For example, in the studies by Bekkering, Pratt and co-workers, when pointing movements were used (to move a handle to the left or to the right) a reliable and significant GAP effect was obtained -24 to 50 ms (7,8). However, when the choice keypress was introduced, the magnitude of the GAP effect was considerably smaller -6.3 to 11 ms (7), or did not reach significance, as reported by Reuter-Lorenz et al (5). Nevertheless, to move a handle is a mutually exclusive movement, i.e., you cannot move it to both sides at the same time, while it is perfectly possible to press both left and right keys simultaneously.…”

“…On the basis of their data, both ReuterLorenz et al (5,6) and Kingstone and Klein (9) proposed the so-called "two-component model" to explain the GAP effect. According to these investigators, the reduction in motor responses observed in the GAP condition should have two components: i) a warning component which follows any warning event and is common to both saccadic and manual responses, and ii) a fixation offset component which is specific for saccades and is observed when the FP is turned off.…”

“…Competing theories have been proposed to explain it (4-7), but there is as yet no agreement about the cause of such effect. Moreover, some groups claim that there is a GAP effect for manual reaction times (MRTs) (7,8), while others do not agree (5,9).…”

Manual responses to visual stimuli: early and late facilitatory effects due to the offset of a peripheral cue

“…Moreover, after training, some subjects were able to reduce or even eliminate the later peaks, thus resulting in an unimodal distribution with a single peak around 100 ms (7) -absolute latency criterion. Other researchers (10)(11)(12)(13)(14) , however, though confirming the reduction on SRTs with the gap paradigm, do not support the idea of ESs. According to their interpretation, neither the bimodality nor the latency criterion are convincing enough to propose the existence of ESs.…”