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Maria Pereverzeva, Davida Y. Teller; Centering biases in heterochromatic brightness matching. Journal of Vision 2004;4(8):347. doi: https://doi.org/10.1167/4.8.347.
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When the method of constant stimuli is used, heterochromatic brightness matches are biased toward the center of the stimulus range (Teller et al, 2003). In the present experiment we investigate whether such biases are specific to physical location or to retinotopic location. To separate these two sources, we designed a paradigm in which chromatic stimuli from two different luminance ranges are presented in two different physical locations, A and B, in an achromatic surround. An observer is asked to judge whether the stimuli are brighter or darker than the surround. The stimuli are viewed in either the same (Expt 1) or different (Expt 2) retinal locations (by shifting fixation between the two physical locations, or by maintaining central fixation). The data are scored separately for physical locations A and B. In Expt 1, a difference in PSEs would indicate location specific biases. In Expt 2, a difference in PSEs would indicate retinotopically specific biases. The stimuli were 2 red squares embedded in a 68 by 42 achromatic surround. The stimuli belonged to one of two ranges described below. The stimuli from the two ranges were presented in separate physical locations, 7 to the left (A) or right (B) of the center of the screen. The stimulus ranges spanned .3lu in .05lu steps. The center of the higher range was isoluminant to the surround. The center of the lower range was shifted downwards by .15lu. The stimuli were viewed either in the same retinal location (Expt 1), or in two separate retinal locations (Expt 2). Three observers were tested to date. The results showed no significant difference between the PSEs in Expt 1. In Expt 2, however, PSEs differed by an average of 0.05 log units. The results indicate an absence of a physical location specific centering bias and the presence of a bias specific to retinotopic location. These results implicate a sensory rather than a cognitive site for at least some of the centering bias seen in heterochromatic brightness matching.
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