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...

Reaction times (RTs) to bimodal (visual and auditory) stimuli were examined using 3 different response systems: saccades, directed manual responses, and simple manual responses. The observed levels of intersensory facilitation exceeded race model predictions and therefore support summation (coactivation) models of bimodal processing. However, response-dependent differences suggest that the processing of bimodal targets also depends on the relevant sensorimotor pathways and requirements of the task. Coactivation of response mechanisms might account for the effects found using simple RTs. The results for saccades are consistent with known patterns of auditoryvisual convergence in the oculomotor system.

A great deal of evidence suggests that early in processing, retinal images are filtered by parallel, spatial frequency selective channels. We attempt to incorporate this view of early vision with the principle of global precedence, which holds that Gestalt-like processes sensitive to global image configurations tend to dominate local feature processing in human pattern perception. Global precedence is inferred from the pattern of reaction times observed when visual patterns contain multiple cues at different levels of spatial scale. Specifically, it is frequently observed that global processing times are largely unaffected by conflicting local cues, but local processing times are substantially lengthened by conflicting global cues. The asymmetry of these effects suggests the dominant role of global configurations. Since global spatial information is effectively represented by low spatial frequencies, global precedence potentially implies a low frequency dominance. The thesis is that low spatial frequencies tend to be available before information carried by higher frequency bands, producing a coarse-to-fine temporal order in visual spatial perception. It is suggested that a variety of factors contribute to the "prior entry" of low frequency information, including the high contrast gain of the magnocellular pathway, the amplitude spectra typical of natural images, and inhibitory interactions between the parallel frequency-tuned channels. Evidence suggesting a close relationship between global precedence and spatial frequency channels is provided by observations that the essential features of the global precedence effect are obtained using patterns consisting of low and high frequency sinusoids. The hypothesis that these asymmetric interference effects are due to interactions between parallel spatial channels is supported by an analysis of reaction times (RTs), which shows that RTs to redundant low and high frequency cues produce less facilitation than predictions that assume the channels are independent. In view of previous work showing that global precedence depends upon the low frequency content of the stimuli, we suggest that low spatial frequencies represent the sine qua non for the dominance of configurational cues in human pattern perception, and that this configurational dominance reflects the microgenesis of visual pattern perception. This general view of the temporal dynamics of visual pattern recognition is discussed, is considered from an evolutionary perspective, and is related to certain statistical regularities in natural scenes. Potential adaptive advantages of an interactive parallel architecture that confers an initial processing advantage to low resolution information are explored.

Simple reaction times (RTs) to a visual target are facilitated when the target occurs at a location expected by an observer, and are slowed when the target occurs at the mirror-symmetric location contralateral to the expectancy (e.g., Posner, 1978; Posner, Snyder, & Davidson, 1980). The spatial extent of this attention effect was examined by inducing subjects to expect the target at one location and introducing occasional probe flashes at other locations throughout the visual field. The results indicated that RTs to these probes were equivalent to those obtained at the expected location so long as the probe was in the same hemifield as the subject's expectancy. Conversely, RTs to probes in the hemifield opposite the expectancy generated uniformly slower response times. These results were obtained when the expected location varied in eccentricity from 2 degrees to 16 degrees along the horizontal meridian. In addition, when the expected and unexpected locations were within the same hemifield, no expectancy effects were observed. Under these conditions, the frequently used metaphor that directed visual attention operates like a spatially restricted "beam" appears inaccurate. The implications of these findings for current views of directed attention are considered.