When does grouping happen?
Recent research on perceptual grouping is described with particular emphasis on identifying the level(s) at which grouping factors operate. Contrary to the classical view of grouping as an early, two-dimensional, image-based process, recent experimental results show that it is strongly influenced by phenomena related to perceptual constancy, such as binocular depth perception, lightness constancy, amodal completion, and illusory contours. These findings imply that at least some grouping processes operate at the level of phenomenal perception rather than at the level of the retinal image. Preliminary evidence is reported showing that grouping can affect perceptual constancy, suggesting that grouping processes must also operate at an early, preconstancy level. If so, grouping may be a ubiquitous, ongoing aspect of visual organization that occurs for each level of representation rather than as a single stage that can be definitively localized relative to other perceptual processes.