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Ryusuke Hayashi, Kenji Kawano; Paradoxical motion perception observed through contrast-alternating multiple-slit-viewing. Journal of Vision 2008;8(6):129. doi: 10.1167/8.6.129.
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It is known that the human visual system can extract motion from a moving pattern presented through an array of horizontal slits, each having one pixel width (Multiple-Slit-Viewing, MSV). Here we developed a new motion display in which the contrast of a pattern moving behind the slits keeps its original polarity or reverses to its negative alternatively (contrast-alternating MSV). In other words, this configuration is a variant of reversed-phi motion in alternating the contrast of a moving pattern not in the time domain (every frame), but in the space domain (every slit). To investigate the effect of contrast alternation in the space domain on motion perception, we asked participants (n=7) to report the perceived direction of the moving stimuli (upward or downward) under either standard or contrast-alternating MSV conditions by button-press (2AFC) while changing the width of interslit bands (from 0 to 8 pixels) and the monitor refresh rate (60, 75, or 100 Hz). Motion direction observed through standard MSV gradually degraded and eventually became indistinguishable as the width of the interslits increased. On the other hand, perceived motion under contrast-alternating MSV was intruded on by components moving in the direction opposite to the pattern shift behind the slits, depending on the width of the interslit bands and the monitor refresh rate. Counter-intuitive phenomena we found were that the participants reported the pattern-shift direction quite accurately (better than 75%) under contrast-alternating MSV in moderately wide interslit width conditions, while motion discrimination performance under standard MSV was chance level. We will further show that the power spectrum distribution of two MSV motion stimuli, weighted with the spatiotemporal sensitivity window (centered at 1.2 cpd and 3 Hz), can account for the motion discrimination limit under standard MSV, and the reversed motion perception and subsequent enhanced motion discrimination performance under contrast-alternating MSV.
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