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Jingping Xu, Zijiang J. He, Teng Leng Ooi; The roles of boundary contour and stimulus onset asynchrony in triggering binocular rivalry alternation. Journal of Vision 2007;7(9):61. doi: 10.1167/7.9.61.
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The perceptual alternation of binocular rivalry is often experienced as a relatively abrupt change from one dominant percept to another. This switch in dominance could be caused by either a sudden change, or a gradual change, in the relative signal strength between the two eyes. In the latter, the signal strength changes gradually until it reaches a threshold that triggers the perceptual switch. To psychophysically test the sudden change hypothesis vs. gradual change hypothesis, we capitalized on a rivalry display (Ooi & He, 2006) where an image having the monocular boundary contour (MBC) has prolonged dominance duration. In the experiment, the MBC display consisted of a 1deg monocular vertical grating disk in the center of a 7.5×7.5deg binocular horizontal grating background (SF=5cpd; contrast=86%). At a predetermined stimulus onset asynchrony (SOA=0.15–4.5sec) following the onset of the MBC display, a horizontal grating disk whose spatial phase was shifted (22–72deg) relative to the larger horizontal grating background was introduced into the horizontal-grating-only half-image (suppressed eye), at a location corresponding to that of the vertical grating disk in the dominant eye. We found that when the spatial phase shift (i.e., the boundary contour) of the horizontal grating disk was sufficiently large, it disrupted the continued dominance of the vertical grating disk. To reveal the “latency-to-alternation”, we measured observers' reaction times to experiencing the change in percept from vertical to either horizontal or piecemeal/checkerboard. We found: (i) The larger the spatial phase shift of the horizontal grating disk (yielding a stronger boundary contour), the faster it was to suppress the vertical grating disk (shorter reaction time); (ii) For SOA durations between 0.15–1sec, the reaction times reduced significantly with increasing SOA. This finding supports the prediction of the gradual change hypothesis, and reveals the interplay between boundary contour strength and SOA in triggering binocular rivalry alternation.
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