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NR A Parry, DJ McKeefry, IJ Murray; Dependence of Reaction Time on Isoluminance, Contrast and Colour Vector . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3988.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To generate simple reaction times (RTs) at different saturations (contrasts) along 8 vectors in isoluminant colour space, including specific cone-isolating axes which represent the activity of geniculate opponent (L-M) or S-(L+M) cell types. Methods: RTs were measured to circular, coloured, gaussian patches (s.d. = 0.2) presented with a raised cosine temporal profile which reached a maximum 190ms after onset. These chromatic shifts were presented on a 12.5 cd.m2 illuminant C background (CIE chromaticity coordinates x = 0.310, y = 0.316). Isoluminance was obtained by heterochromatic flicker photometry. The directions (ϕ) in colour space tested were 0, 53, 90, 130 180, 233, 270 and 310. Four of these axes isolate LM (0 and 180) and S (90and 270) cone mechanisms. Depth of modulation (RMS cone contrast) was expressed as the Weber contrast between the background and the centre of the stimulus. Results: RT was highly sensitive to departures from isoluminance. For example, for the 90 stimulus, a 5% change in luminance ratio resulted in a decrease in RT of about 100ms. Note that 5% is the approximate level of between-subject variation in isoluminance, emphasising the importance of individual isoluminance settings. RTs varied systematically as a function of ϕ at all levels of contrast, whether expressed in terms of equal RMS cone contrast or equal suprathreshold increment. The longest RTs were obtained at 90 and 270 and the shortest at 0 and 180. From the family of RT versus contrast functions generated at different vectors we have drawn a series of iso-RT functions. These show that, for a given RT, up to 10x more contrast is required at 90 than at either 0 or 180. Conclusion: We conclude that, in the colour domain, variations in RT are dependent on opponent mechanisms and are thus dominated by early visual processing mechanisms.
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