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Evelina Tapia, Bruno Breitmeyer, Jane Jacob; How the unseen informs us about the seen: Metacontrast masking with texture-defined second-order stimuli. Journal of Vision 2011;11(11):1058. doi: 10.1167/11.11.1058.
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What is known about contour and surface feature processing in the primate visual system applies mainly to first-order stimuli, such as objects defined by wavelength- or luminance-contrast differences. It is unclear whether the same processes and their properties apply to perception of second-order stimuli, such as shapes created in random-dot stereograms or contours defined purely by surface texture differences. Presently, evidence indicates that processing and perception of first- and second-order features at least partially relies on separate neural mechanisms. However, the similarities and differences between first- and second-order feature and object processing have not yet been fully established. We employed a visual masking methodology to examine how the human visual system handles contours and surfaces of texture-defined second-order stimuli. The experiments revealed that (1) both the monotonic type A as well as the standard nonmonotonic (U-shaped) type B metacontrast effect, which has been extensively examined with first-order stimuli, can also be obtained with texture-defined second-order stimuli; and (2) that neither the size nor orientation contrast between texture elements defining the target, mask and background stimuli had a significant impact on the magnitude or shape of metacontrast, unlike analogous contrasts with first-order stimuli. In light of these and previous findings, evidence suggests that the metacontrast suppression mechanism can be activated not only by first- but also by second-order (cyclopean and texture-defined) features and objects. This additionally indicates that first- and second-order contours are processed in a similar manner; hence, possibly, by a similar neural mechanism. However, since neither size nor orientation contrast between texture elements defining the target, mask and background stimuli had a significant impact on the magnitude of metacontrast masking in the present experiments, this suggests that there also may be processing of second-order contour and surface features that is distinct from processing of first-order contours and surfaces.
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