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Alex O. Holcombe; Rapid image alternation: a new route to transparency and rivalry. Journal of Vision 2001;1(3):391. doi: 10.1167/1.3.391.
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Bartley (1939) reported that alternating two superposed color patches at high rates produced a transparent percept. We show: first, that alternating between two *patterns* can also result in transparency. Second, such image alternation constitutes a new route to transparency, mediated by a mechanism distinct from those documented previously. Two sinusoidal gratings differing in orientation and brightness were alternated. At low rates, (1-∼5 Hz), observers experienced first one pattern, then the other. At higher rates (∼6–20 Hz) observers experienced transparency: the two patterns seemed to be available at the same time, at different depths with an accompanying percept of flicker. At still higher frequencies the temporal resolution of the early visual system was exceeded and the sum was experienced with no transparency. When three patterns were alternated at ∼6–20 Hz, observers often experienced slow rivalry. The transparent and rivalrous percepts cannot be explained by the mechanisms which yield transparency from static monocular cues (Metelli). An account based on this monocular mechanism would posit that, when alternated very rapidly, the two patterns sum internally and static cues would then allow decomposition into separate surfaces. To the contrary, observers did not experience transparency when the two sinusoids were summed instead of alternated. Also, the results of a contrast-matching experiment make it unlikely that an early nonlinearity results in a distorted sum that is then decomposed by the monocular mechanism. Bartley's original observations provide further support for this view: two color patches alternated rapidly can appear transparent whereas their sum does not. We suggest that early stages of the visual system separate two alternating images into separate channels (such as ON- and OFF- pathways). Later stages before awareness then integrate these channels over ∼100 msec, resulting in the experience of both images at the same time.
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