Glass pattern studies of local and global processing of contrast variations

Using Glass patterns [Nature 223 (1969) 578; Nature 246 (1973) 360; Perception 5 (1976) 67], we have studied the role of contrast differences in local and global processes of form perception. The virtue of these patterns (composed of a set of randomly distributed elements combined with a geometrically transformed copy) for studying object formation is that they allow ready isolation of local processes, the combination of dots to form a perceptual pair, from global processes, the combination of dipoles into the percept of an overall rotational or translational pattern. We find that a contrast difference within dot-pairs reduces the ability to resolve local features; large differences totally abolish the perception of the pattern. Contrast differences between dot-pairs lessen, but do not abolish, the global integration among local features. In both cases the effect is proportional to the ratio of the two contrast levels employed. Effects which differ for rotations and translations, are consistent with the greater areal integration required to resolve rotational patterns.