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Lynn A. Olzak, Patrick J. Hibbeler, Thomas D. Wickens; Perceptual segregation of center and surround does not yield independent processing. Journal of Vision 2009;9(14):48. doi: https://doi.org/10.1167/9.14.48.
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We have previously reported that center-surround stimuli produce asymmetric masking effects (surround masks center but not vice versa) when making fine spatial discriminations (spatial frequency or orientation) on sinusoidal grating patches when center and surround abut. Masking effects disappear upon introducing a tiny (4-minute) gap between center and surround, suggesting independent processing. We reported at VSS 2009 results of a concurrent response experiment with cues in both center and surround indicating that a standard bivariate Gaussian independence with separable criteria model was strongly rejected. Observers appeared to be using a two-stage decision process. Here, we ask whether introducing the tiny gap results in a better fit of the standard independence model. The stimuli were patches of 4 cpd vertical sinusoidal grating, arranged in a center-surround configuration (40 min center, 40 min width surround). A cue to discrimination was presented in both center and surround on each trial. Observers either made discriminations based on spatial frequency or on orientation, in different experiments. 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 (or higher frequency), 3) center titled left (lower frequency), surround titled right higher frequency), and 4) center tilted right (higher frequency), surround tilted left (lower frequency). 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. Despite apparent independent processing of center and surround observed when using the standard one-response masking paradigm with a gap, our concurrent response data again strongly reject the independence model. The data suggest that observers used a mixture of response strategies.
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