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Lynn A. Olzak, Patrick J. Hibbeler, Thomas D. Wickens; Processing cues to discrimination in center-surround stimuli. Journal of Vision 2009;9(8):1018. doi: 10.1167/9.8.1018.
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Fine spatial discriminations made on center-surround stimuli show lateral masking effects. Unlike overlaid (spatially coincident) masking patterns, these effects are asymmetric; the surround affects the center but not vice versa. Also unlike effects found with overlaid patterns, there seem to be no higher level circuits that sum (or difference) responses from cues in two different components, demonstrated in a two-cue, single-response configural (Olzak & Thomas, 1991) effect test. In the current study, we tested the hypothesis that cues to discrimination in center and surround portions of a stimulus are in fact processed independently, with no excitatory or inhibitory interactions, and no correlation in noise. We used a concurrent response paradigm and associated multivariate SDT analyses to identify the underlying response space and to test whether an independence model adequately describes how two cues to discrimination, one in the center and one in the surround, are processed. The stimuli were patches of 4 cpd vertical sinusoidal grating, arranged in a center-surround configuration (40 min center, 20 min width surround). Observers either made discriminations based on spatial frequency or on orientation, in different experiments. A cue to discrimination was presented in both center and surround on each trial. Four stimulus types were created and intermingled in a single session of 160 trials (40 of each stimulus): 1) both center and surround tilted left slightly (or were of slightly lower frequency), 2) both tilted right, 3) center titled left, surround titled right, and 4) center tilted right, surround tilted left. Observers made separate decisions on center and surround patches following each trial, rating their certainty that each component was tilted left or right on a 6-point rating scale. The results supported the notion of no higher-level summing circuits, but strongly rejected a bivariate-Gaussian independence model.
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