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David F. Nichols, Howard S. Hock; Counterphase sine gratings flicker at the detection threshold but move above the detection threshold. Journal of Vision 2001;1(3):244. doi: https://doi.org/10.1167/1.3.244.
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Purpose. Basing their experiment on counterphase sine gratings being mathematically equivalent to the sum of two gratings drifting in opposing directions, Levinson & Sekuler (1975) showed that the threshold-level detection of counterphase gratings results from the activation of directionally selective motion detectors. This evidence for motion detector activation notwithstanding, the extent to which the actual perception of motion for counterphase gratings depends on luminance contrast has not been systematically investigated. In this study, therefore, we examine the effects of luminance contrast on both the detection of the grating and the perception of its motion. Method. Subjects viewed counterphase gratings that varied in luminance contrast. After each trial they indicated: 1) whether or not they detected the grating, and 2) whether it was moving or stationary. Results. Stationarity (with flicker) was perceived for luminance contrast levels that were near the threshold for detecting the sine wave pattern, but for the perception of motion to arise, larger contrast values were required. This was the case for the four combinations of spatial (2 and 4 cycle/deg) and temporal (1.6 and 3.1 Hz) frequency tested. Conclusion. The results indicate that pattern and motion detection are not equivalent for counterphase gratings. They also are inconsistent with claims (e.g., Adelson & Bergen, 1985) that motion is not perceived for counterphase gratings because motions in opposing directions cancel. Cancellation is equally likely at all levels of luminance contrast, but the perception of stationarity/flicker is replaced by motion perception as luminance contrast is increased.
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