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Abstract: Psychological, stcategic, and stimulus differentiation variables areusedto examine the nature of the functional relation between assimilation andcontrast distortions jn three experiments using the Delbouef (concentriccircle)illusion configurations. In Experiment Ⅰ, field dependence was foundto correlate positively with assimilation, but negatively with contrast, distor-tion magnitude in individuals. In Experiment Ⅱ, significant distortion reduc-tion was observed when observers were instructed to selectively attend tothe test element but only for the contrast configuration. In addition, observersgiven standard in.structions showed reductions in both distortions with loweredcontrast contour reflectance contrast, but those given selective attentioninstructions showed a similar reduction only in the assimilation distortion.InExperiment Ⅲ,suprafoveal (as opposed to subfoveal) viewing significantlyreduced the assimilation, but maximized the contrast, distortion. Moreover,significant distortion reduction with the "illusion decrement" procedure wasobserved only in suprafoveal conditions. The rate of dectement over trialsdiffered for the assimilation and contrast distortions, This pattern of findingsis best accounted for in terms of the composite congitive "Pool and Store"model, in which processes influencing Perceptual organization determine boththe type of distortion that is initially observed and the effectiveness of stra-tegic differentiation manipulations in reducing distortion magnitude.

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