Abstract
When does the visual system decompose an image region to create a percept of transparency? According to Metelli's theory, the luminance range in the region of transparency must be lower than the luminance range in adjoining image regions. Previously, we demonstrated that Michelson contrast plays a critical role in assigning perceived opacity to transparent surfaces (Singh & Anderson, Psychonomics Meeting, 1999; ARVO 2000; Psychological Review, in press). In the current study, we examined the relative contributions of luminance range and Michelson contrast in initiating a percept of transparency. In two paradigms, we manipulated the mean luminance and luminance range of two adjoining regions with continuous textures. In one paradigm, the common border between the two regions was given near disparity relative to the texture elements. Depending on the relative contrasts in the two regions, either the left or the right side of the common border appeared to contain a transparent layer floating in front of a textured surface. This transparent layer perceptually owned the common border. In a second paradigm, we set the common borders between two alternating sets of textured regions in motion. Depending on their contrast relationships, motion was imparted to one or the other set of textures—which was seen as a series of transparent layers drifting over a textured surface. In both paradigms, observers judged which of the two sets of textured regions appeared to contain an overlying transparent layer. The results demonstrated a clear failure of Metelli's model, and a strong contribution of Michelson contrast in initiating a percept of transparency. In regimes where the two textured regions differed greatly in their mean luminances, however, the percept of transparency became multistable, sometimes leading to globally incoherent percepts. We discuss the implications of our results for junction classification based on contrast polarity, and its interaction with contrast magnitude.