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K. Vallam, A.B. Metha; The Relationship Between Temporal Phase Discrimination and the Frequency Doubling Illusion With Equi–Luminant and Achromatic Gratings . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4578.
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The spatial frequency doubling illusion (FDI) occurs when a low spatial frequency (SF) sinusoidal grating is modulated at high temporal frequencies – its apparent SF increases. White et al (IOVS (43); 2002) suggested that loss of temporal phase discrimination (TPD) could be a possible cause of FDI. Purpose: We sought to explore the spatio–temporal characteristics of both the FDI and TPD thresholds with achromatic and equi–luminant (both red–green (RG) and blue–yellow (BY)) gratings. Methods: Equi–luminant points were determined by minimum motion technique and then crosschecked for luminance intrusion with a masking paradigm. The apparent size of 0.25–2.2cpd gratings counterphase flickering at 1–28Hz was determined at and above (2x, 4x and 8x) orientation identification contrast thresholds. Four subjects were asked to identify the orientation (45º or 135º) of flickering gratings and simultaneously match the apparent SF by one of two methods: 1) adjusting the SF of a stationary achromatic grating (contrast 0.3); and 2) by adjusting the distance between two parallel lines. To measure TPD thresholds, we created a novel stimulus consisting of three grating patches presented simultaneously for 2sec. A reference grating was presented superiorly, and two gratings (one a copy of the reference) were presented 6º below in one of two fixed positions. The odd grating had abutting regions of grating half cycles with alternate half cycles in temporal phase. The temporal phase difference between adjoining half–cycles was varied between 0 – 180º via a staircase, and subjects identified which stimulus appeared different to the reference grating at 2x, 4x and 8x contrast thresholds. Results: SF matches made using stationary grating revealed thinner matches in all conditions. SF matching at near threshold contrasts indicated subjects were biased to make thinner matches at low SF. For higher equi–luminant and achromatic contrasts, and with increasing flicker rate, the apparent size of local features shrink and TPD thresholds increase for low SF gratings; results were more variable for equi–luminant gratings (BY>RG>Ach). Conclusions: Overall, the data is consistent with the idea that frequency–dependent loss of TPD underpins the FDI in both equi–luminant and achromatic conditions.
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