Eye movements are primed toward the center of multiple stimuli even when the interstimulus distances are too large to generate saccade averaging.

Abstract

Prior oculomotor research has established that saccades tend to land near the center of multiple saccade targets when they are near each other. This saccade averaging phenomenon (or global effect) has been ascribed to short-distance lateral excitation between neurons in the superior colliculus. Further, at greater inter-stimulus distances, eye movements tend toward the individual elements. This transition to control by local elements (individuation) with inter-stimulus distance has been attributed to long-range lateral inhibition between neurons in winner-take-all models of oculomotor behavior. We hypothesized that the traditional method of requiring a saccade to an array of multiple, simultaneous targets may entail response ambiguity that intensifies with distance. We resolved the ambiguity by focussing on reaction time of our human participants to a single saccade target after one or more simultaneous priming stimuli. At a 50-ms prime-target interval, saccadic reaction time was shortest for targets closer to the center of the prime stimuli independent of the distance between the primes. This effect was gone at 400 ms. These findings challenge the typical inferences about the neural control of oculomotor behavior that have been derived from the boundary between saccade averaging and individuation and provide a new method to explore eye movements with lessened impact from decision processes.