Express attentional shifts.

"Express" saccades, named for their extremely short latencies, occur more frequently in a paradigm with a "gap" in time between the disappearance of the fixation mark and the appearance of the target to be fixated. To explain this result, it has been hypothesized that movements of the eyes are preceded by movements of attention [Posner (1980) Quarterly Journal of Experimental Psychology, 32, 3-25], and that removing the fixation mark allows attention to disengage from the fovea and to be deployed more rapidly to the peripheral target, thus diminishing saccadic latency [Fisher (1987) Reviews of Physiology, Biochemistry and Pharmacology, 105, 1-35]. We measured attention using extra-foveal vernier acuity performance. Our results provide direct evidence supporting the above hypothesis. First, we found that the rise of performance for increasing cue lead times was much faster in the "gap" paradigm. Second, the time function relating gap duration to discrimination performance was remarkably similar to the one relating gap duration and rate of express saccades reported by Mayfrank, Mobashery, Kimmig & Fischer [(1987) European Archives of Psychiatry and Neurological Science, 235, 269-275]. Third, control experiments showed that it was the disappearance of the fixation mark rather than a non-specific warning that led to more rapid shifts of attention and, thus, to better performance. We therefore conclude that the short latencies of "express" saccades may be based on a mechanism involving unusually rapid shifts of attention.