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Apparent Speed in the Footstep Illusion is not Totally Contrast Dependent


Pretto,  P
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

Chatziastros,  A
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Pretto, P., & Chatziastros, A. (2005). Apparent Speed in the Footstep Illusion is not Totally Contrast Dependent. Poster presented at 8th Tübingen Perception Conference (TWK 2005), Tübingen, Germany.

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Recently, the role of luminance contrast on the perception of velocity has been emphasized by the so called “footstep illusion” [1]: when a grey bar drifts steadily across a background of stationary black and white stripes, it appears to stop and start as its contrast to the background changes. When two bars of unequal grey values drift in parallel, an impression of alternating footsteps of a moving person can be elicited. This change in apparent speed has been attributed to the different instantaneous contrasts of the grey bar against the white and black areas of the background (Contrast Ratio explanation, [1]). We report a series of experiments with further variants of the footstep illusion to test the validity of the Contrast Ratio (CR) explanation. In an adjustment task, we presented a standard footstep illusion together with second comparison stimulus. A total of five participants were required to adjust the contrast of the background of the comparison stimulus in order to match the strength of the standard illusion. The moving bars were presented in six luminance levels in the range between 6 and 103 cd/m2 . For each luminance level we computed the CR by the Weber fraction, following the original study. We compared the CR values of the standard and comparison stimulus and expected to obtain comparable CRs within matched illusion pairs. In the first experiment we found a discrepancy between predicted and obtained CR values. The strength of the illusion was matched at an increased CR, i.e. participants set the background stripes to higher contrast in order to obtain the same illusion. In a second experiment the moving bars were rendered transparent, with a transparency of 50, and we obtained basically the same results as in the first experiment. In the last experiment we tested whether the footstep illusion can be elicited with isoluminant red/green patterns. Individual isoluminance levels were identified with flickering stimuli. Subjects rated an apparent velocity change of the isoluminant moving bars again by adjusting the contrast of the standard luminance version. The results show clearly that the footstep illusion occurs also with isoluminant patterns. Furthermore, we found that the adjusted CR was always near the maximal possible value, suggesting that participants matched the isoluminant illusion to the strongest available luminance illusion. The present data suggest that foreground and background stimuli contribute differently to the footstep illusion, a finding which is not predicted by the current Contrast Ratio account. Most notably, the existence of the illusion with isoluminant stimuli shows the clearest limitation of the Contrast Ration explanation. That the strength of the isoluminant footstep illusion was comparable to the luminance version, suggests a crucial role of chromatic input in an adequate explanation of this phenomenon.