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Monika A. Formankiewicz, Sarah J. Waugh, Akash S. Chima; Contour interaction under monocular and dichoptic viewing conditions for luminance-modulated and contrast-modulated Cs. Journal of Vision 2011;11(11):1147. doi: 10.1167/11.11.1147.
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Resolution acuity is degraded by the presence of flanking bars. This contour interaction is observed under monocular and dichoptic viewing conditions, pointing to its cortical origin (Flom et al., 1963). The exact locus of the interaction remains unclear but areas beyond V1 have been suggested (review by Levi, 2008). The peak of contour interaction for contrast-modulated (CM) stimuli is higher than that observed for luminance-modulated (LM) stimuli under monocular viewing conditions (Hairol et al., VSS2010). CM stimuli are thought to be processed at a higher, more binocular cortical region than LM stimuli, and binocular interactions at the level of detection are different for the two types of stimuli (e.g. Waugh et al, VSS2009). We asked whether this is also true for suprathreshold stimuli and investigated contour interaction for LM and CM square-C stimuli, in monocular and dichoptic foveal viewing. In each condition, resolution thresholds were determined for a briefly presented isolated C and a C flanked by bars placed from abutting to one optotype width away from the target. The target was presented to the non-dominant eye. With monocular viewing, we again found that the peak contour interaction was greater for CM than for LM stimuli (by ∼0.1 logMAR). Under dichoptic viewing conditions, contour interaction for LM stimuli was on average greater (by ∼0.05–0.1 logMAR) and more extensive than in monocular viewing, but this change was not observed for CM stimuli. The lack of enhancement of contour interaction for CM stimuli under dichoptic conditions (where neural combination of target and flankers is required) suggests that these stimuli may be processed at the (binocular) locus of contour interaction. The differential effect of viewing conditions on the interaction for LM stimuli supports the notion that the processing of these stimuli takes place at a lower cortical level than that of CM stimuli.
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