Glass patterns: grouping by contrast similarity.

Recent work has shown that certain contrast phenomena associated with Glass patterns can be accounted for by filtering mechanisms applied within the luminance or energy domain. Hitherto, these phenomena were regarded as problematic for energy-processing models, and were taken as evidence in support of symbolic-processing accounts. An additional, and controversial, contrast effect is investigated. It is shown that in a Glass pattern consisting of simultaneous vertical and horizontal translations of different contrast strength, the most salient structure is determined by grouping (pairing) of the two low-contrast elements of the pattern. The finding that grouping in such patterns is by contrast similarity, as opposed to energy, is consistent with the symbolic-processing approach, but has yet to be accounted for by filtering mechanisms.