Rethinking perceptual organization: The role of uniform connectedness

Abstract

A principle of perceptual organization, called uniform connectedness (DC), is described, and a theoretical approach to perceptual organization is proposed in which this principle plays a fundamental role. The principle of DC states that closed regions of homogeneous properties-such as lightness, chromatic color, texture, and so forth-tend to be perceived initially as single units. We demonstrate its effects and show that they occur even when opposed by powerful grouping principles such as proximity and similarity. We argue that DC cannot be reduced to such grouping principles, because it is not a form of grouping at all. We then propose a theoretical framework within which DC accounts for the initial (or entry level) organization of the visual field into primitive units. Classical principles of grouping operate after DC, creating superordinate units consisting of two or more basic-level units. Parsing processes also operate after DC, dividing basic-level units into subordinate parts. DC in the retinal image is proposed to be a necessary, but not a sufficient, condition for unit formation, since connected elements on the retina that are perceived to lie in separate depth planes fail to be perceived as units. This fact, together with other evidence that the Gestalt principles of grouping are based on perceived (rather than retinal) relations, suggests that the organization of visual stimulation into DC objects is ultimately achieved within a relatively late, postconstancy representation of environmental surfaces. The implications ofthis possibility are discussed in light of present theories of visual perception.In 1923, Max Wertheimer raised the fundamental question of perceptual organization: Why is it that people perceive an organized visual world consisting of discrete objects coherently arranged in space? Without some process of organization, he argued, perception should consist of a chaotic juxtaposition of different elementary sensations based on the intensities and colors registered by individual retinal receptors. Wertheimer recognized that perception cannot be explained by the mere presence of an image containing two-dimensional (2-D) regions that correspond to external objects. Rather, processesof organization within the visual nervous system must be hypothesized as appropriately unifying receptor stimulation that arises from a single object and as separating that which arises from two or more different objects.Wertheimer's major contribution to the solution of this problem was his proposal that perception organizes stimulation by grouping elements together according to certain properties present in the retinal image. He initially inves-

Segmentation, attention and phenomenal visual objects

“…In this way, attention researchers might exploit many of the advances which have been made by the vision research literature in understanding speci®c forms of image segmentation (e.g. Nakayama et al, 1989Nakayama et al, , 1995Palmer & Rock, 1994;Pomerantz & Kubovy, 1986).…”

“…Rock and Palmer have raised this general question for Gestalt grouping displays in a series of studies with their collaborators (e.g. Palmer, Neff, & Beck, 1997;Palmer & Nelson, in press;Palmer & Rock, 1994;Rock, Nijhawan et al, 1992). More speci®cally, they asked whether grouping might operate before or after various perceptual constancy mechanisms and various completion mechanisms.…”

Preemption effects in visual search: Evidence for low-level grouping.

“…In keeping with its emphasis on more integrated structures, Gestalt grouping also appears to operate at a relatively high level of visual processing (see e.g., Palmer & Rock, 1994). For example, Gestalt grouping acts on representations of three-dimensional surfaces rather than two-dimensional images (Rock & Brosgole, 1964) whereas low-level grouping operates on much simpler structures (although the involvement of three-dimensionality cannot be completely ruled out at this point).…”

“…In contrast, low-level grouping acts "preattentively", i.e., at a level where attention is not required (see, e.g., Treisman & Gormican, 1988). Palmer & Rock (1994) suggest that Gestalt grouping may act on "entry-level" structures that correspond to regions of relatively uniform image-based (or even scene-based) properties. If so, low-level grouping could play a role in the formation of these regions, e.g., via the alpha grouping of same-contrast segments into uniform assemblies.…”

Visual Endurance and Auditory Perdurance

“…According to psychological characterizations (see Palmer and Rock 1994;Scholl 2001), to perceive a fragment of the environment as an object, it has to be perceptually distinguished from the surrounding, usually in virtue of perceiving borders separating it from the rest of the environment. Actually, it often happens that two spatially connected regions ''compete'' for the status of an object.…”