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Iris K. Zemach, Davida Y. Teller, John Palmer; Substantial shifts of lightness matches can result from shifts of stimulus range. Journal of Vision 2005;5(12):99. doi: 10.1167/5.12.99.
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Using the method of constant stimuli, Teller et al. (2004) found that heterochromatic brightness matches between two test disks varied substantially with the range of test luminances used. Further experiments traced these bias effects to sensory adaptation as opposed to decision processes. We investigated whether achromatic lightness matches are similarly biasable with the choice of test stimuli. Stimuli were either two test disks presented against the same medium gray surround, or two disk-and-ring displays presented against a black surround. The ring luminances differed by 0.4 log units. The test disks were either luminance decrements or luminance increments. Stimuli were presented for 1 second with a 1 second inter-trial interval and a fixation point was employed. As in the heterochromatic matching experiments, the point of subjective equality (PSE) varied with the range of test luminances used. When plotted against the log of test range midpoints, the PSEs could be fit reasonably well by straight lines. For seven out of eight cases, the slopes of the best fitting lines ranged between 0.5 and 0.6 indicating PSE shifts of 50% to 60% of the shift of test disk luminance range. Further experiments investigating the sources of these lightness bias effects are in progress.
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