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Kazushi Maruya, Takao Sato; A contribution of early motion systems on stream-bounce perception. Journal of Vision 2005;5(8):138. doi: 10.1167/5.8.138.
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The stream-bounce stimulus induces a bi-stable motion perception (Metzger, 1934). In this phenomenon, two identical moving objects with trajectories crossing each other are perceived either as streaming through (stream) or colliding and bouncing off (bounce). Although this phenomenon is often related to a high-level motion processing, we explored a possibility of earlier contribution by examining the effect of inter-stimulus interval (ISI) which supposedly disrupt energy based detectors (Braddick, 1974).
Method: The stimulus comprised two moving white discs (1 deg diameter) on gray background. Each disc moved on one of two linear motion paths that inclined 30 deg from perpendicular i.e. 60 deg angle between the two trajectories. The motion was 9-frame apparent motion with 2 deg displacements and a fixed SOA of 120 ms. The two trajectories met at the center and the 5th disc shared a position, i.e. only one disc was presented there. ISI was manipulated in 6 steps from 0 to 100 ms and duration covaried accordingly to make SOA 120 ms. Subjects were asked to judge whether motion appearance was “stream” or “bounce” with a 2-AFC method.
Results and Discussion: The response rate that “stream” was perceived at 0 ms ISI was calculated for each subject. As a result, response rates were higher or lower than chance for 6 of 7 subjects. That is, although there was a strong individual difference in preferred percept, each subject had a dominant perception. Next, we calculated the response rate that “dominant perception” was reported (dominant rate) in each ISI condition. Regardless of the initial dominance type, dominant rate decreased as ISI increased and the dominance reversed at the largest ISI (100 ms). These results indicate that motion perception with the stream-bounce stimulus is sensitive to ISI, and thus suggest strong contributions of Fourier-type early motion detectors (Adelson & Bergen, 1985).
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