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Evelina Tapia, Bruno G. Breitmeyer, Jane Jacob; Temporal dynamics of contour and surface processing of texture-defined second-order stimuli. Journal of Vision 2010;10(7):1169. doi: 10.1167/10.7.1169.
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Psychophysical and neurophysiological experiments have demonstrated that at the implicit (i.e. nonconscious) level first-order luminance-defined contours are processed on average 30-60 ms before surface information. Here, figure-ground segmentation processes establish contours that are later filled in with surface (e.g. wavelength and brightness) information. The present work, using metacontrast masking paradigm, examines whether the same sequence of processing also characterizes extraction of second-order features. On the one hand, first- and second-order contour and surface features might be processed in an analogous succession – form followed by surface details. On the other hand, a preliminary segmentation based on differences between foreground and background surface elements might be required before texture-defined second-order contours can be fully established. In this study texture-defined second-order target stimuli were followed, at varying stimulus onset asynchronies (SOAs), by a surrounding but spatially non-overlapping texture-defined second-order mask. First, we established that a typical U-shaped metacontrast function, obtained with first-order stimuli, can be also obtained with these stimuli. Secondly, in contour-discrimination tasks the subjects identified the shapes of the target stimulus; while in surface-discrimination tasks the subjects identified the surface texture elements of the targets. The results indicate that the suppression of texture-defined second-order contour visibility occurs at an earlier SOA than the suppression of texture-defined second-order surface information. These findings are similar to those reported with first-order stimuli and indicate that texture-defined second-order contours are processed before texture-defined second-order surfaces. These findings bear on theories of metacontrast and place constraints on models proposing that first- and second-order features are processed by separate neural circuits.
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