Horowitz T S, DiMase J S, Wolfe J M, 2003, "Visual search asymmetry for Brownian and ballistic motion trajectories" Perception 32 ECVP Abstract Supplement
Visual search asymmetry for Brownian and ballistic motion trajectories
T S Horowitz, J S DiMase, J M Wolfe
Some stimuli in the world change direction on their own, while others move in ballistic trajectories, changing direction (and speed) only when they encounter obstacles. Both ballistic and Brownian motions are known to support efficient visual search when distractors are stationary (Royden et al, 2001 Perception & Psychophysics 63 436 - 444). However, we do not know whether the visual system is preattentively sensitive to the difference between these types of trajectories. Ten observers were asked to detect Brownian targets among ballistic distractors and vice versa. Stimuli were light-gray disks (1.4 dva) on a dark-gray background. Brownian-motion stimuli changed direction every 27 ms, while ballistic-motion stimuli changed direction only to avoid the boundaries of the display. Set size was varied from 4 to 7 to 10 items. All stimuli moved at 8 deg s-1. However, the apparent speed of Brownian stimuli was slower than that of ballistic stimuli, since the net distance traveled in the Brownian condition over any time interval longer than 27 ms was smaller than in the ballistic condition. Observers made speeded present/absent responses. Median RTs were divided by accuracy to correct for errors. One might predict that apparently faster ballistic motion would pop out against a background of apparently slower Brownian distractors. In fact, search for a Brownian target among ballistic distractors was markedly more efficient than search for a ballistic target among Brownian distractors (10.7 ms item-1 versus 34.6 ms item-1). The ease of detecting Brownian targets among ballistic distractors suggests that changes in direction are visually salient.
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