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Harald Ruda, Guillaume Riesen, Howard Hock; Reverse-Phi Experiments Support the Counterchange Model of Motion Detection. Journal of Vision 2016;16(12):668. doi: 10.1167/16.12.668.
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© ARVO (1962-2015); The Authors (2016-present)
Previous experiments and modeling inspired by the counterchange (CC) model of motion detection (Hock, Gilroy & Harnett, 2002; Norman, Hock, & Schöner, 2014) have suggested further experiments using reverse-phi stimuli to discriminate between different motion models. Because they mix the responses of contrast detectors with different polarities (e.g., edge detectors with excitatory lobes on the left or right), the Enhanced Reichardt Detector (ERD) and motion energy detector predict reverse-phi motion will be perceived equally well for stimuli whose luminance polarity is inverted and stimuli that maintain the same luminance polarity during successive frames. On the other hand, the counterchange detector, which segregates the responses of contrast detectors with different polarities, signals reverse-phi motion only from chance correlations. We used reverse-phi square wave gratings (Chubb & Sperling's, 1988, Gamma stimuli) to eliminate chance correlations, and determine whether motion is signaled only by same-polarity edge detectors. Method. A grating was displaced to the left or right during successive frames, for a total duration of 1.0 sec. There were three different displacements, each smaller than the width of a single bar of the grating. Observers matched the perceived speed of an adjustable sine-wave grating to the square wave's perceived speed. Results. For gratings maintaining the same luminance polarity, perceived speed increased with increasing displacement (same duration for longer displacements). This result also was predicted by the ERD and motion energy models for gratings whose polarity inverted during successive frames, but the opposite was obtained. That is, perceived speed was greater for for the smaller displacements. This was because the distance over which motion was perceived in the reverse-phi direction was greater for same-polarity edge detectors than edge detectors with different polarities. Conclusion. Motion detection for the Gamma square wave stimulus is more likely determined by counterchange than ERD or motion energy detectors.
Meeting abstract presented at VSS 2016
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