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Jumping Frogs: Prior Knowledge Influences the Ternus Effect

26.405, Saturday, 17-May, 2:45 pm - 6:45 pm, Banyan Breezeway
Session: Motion Perception: Depth, higher order, illusions

Patty Hsu1, Wil Cunningham1, Jay Pratt1; 1Department of Psychology, University of Toronto

The Ternus effect is a robust bistable illusion of motion that produces element motion at short interstimulus intervals (ISIs; <50ms) and group motion at longer ISIs (> 50ms). Previous research has shown that the nature of the stimuli (e.g., similarity, grouping), not just ISI, can influence the likelihood of perceiving element or group motion. In the present study we examined if prior knowledge is also capable of influencing what illusory motion is perceived. To do so, our first experiment used a modified Ternus display with pictures of frogs in a jump ready pose that were facing forwards or backwards to the direction of illusory motion. The forward facing frogs afforded jumping over one another, which should promote element motion, while the backwards facing frogs should promote group motion. This was indeed found, as across ISIs ranging from 0 to 108 ms, participants perceived more element motion with the forward facing frogs and more group motion with the backward facing frogs. Our second experiment replicated the methods of the first experiment except that the frogs were shown in a sedentary "non-jumping" pose. As expected, we did not find a difference between forward facing and backward facing sedentary frogs. Presumably, the forward facing frog in this case did not produce more element motion because the sedentary pose did not afford a jumping motion. Thus, these two experiments provide more evidence that the Ternus effect cannot simply be attributed to the delay between the first and second display or to the physical characteristics of the stimuli. Rather, prior knowledge of the movement of certain animate objects, in this case frogs, also can modulate the perception of element or group motion.

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