PART THREE PERCEPTUAL PROCESSES - Numerons 2012-04-03¢  PERCEPTUAL ORGANIZATION The term perceptual

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  • PA RT T H R E E

    PERCEPTUAL PROCESSES

  • CHAPTER 7

    Visual Perception of Objects

    STEPHEN E. PALMER

    179

    PERCEPTUAL ORGANIZATION 179 Perceptual Grouping 180 Region Segmentation 184 Figure-Ground Organization 185 Parsing 187 Visual Interpolation 187 Parts and Wholes 191 Frames of Reference 192

    OBJECT IDENTIFICATION 194 Typicality and Basic-Level Categories 195 Perspective Effects 195

    Orientation Effects 197 Part Structural Effects 197 Contextual Effects 198 Visual Agnosia 199

    THEORIES OF OBJECT IDENTIFICATION 200 Representing Objects and Categories 200 Comparison and Decision Processes 203 Part-Based Theories 203 View-Specific Theories 205

    REFERENCES 207

    Visual perception begins when light entering the eye acti- vates millions of retinal receptors. The initial sensory state of the organism at a given moment can therefore be completely described by the neural activity of each receptor. Perhaps the most astonishing thing about this description of sensory information, aside from its sheer complexity, is how enor- mously it differs from the nature of the visual experiences that arise from it. Instead of millions of independent points of color, we perceive a visual world structured into complex, meaningful objects and events, consisting of people, houses, trees, and cars. This transformation from receptor activity to highly structured perceptions of meaningful objects, rela- tions, and events is the subject matter of this chapter. It is divided into two related subtopics: how people organize visual input into perceptual objects and how people identify these objects as instances of known, meaningful categories such as people, houses, trees, and cars.

    This chapter describes perceptual organization and object identification in the visual modality only. This is not because either organization or identification is absent in other sensory modes—quite the contrary. But the specific stimulus infor- mation and processing mechanisms are different enough across modalities that it makes more sense to discuss them separately. Some of the issues covered in this chapter for vision are therefore also discussed in the chapter by Yost for audition, in the chapter by Fowler for speech perception, and

    in the chapter by Klatzky and Lederman for touch (all in this volume). Indeed, the present chapter concentrates mainly on organization and identification in static scenes because dynamic issues are considered in the chapter by Proffitt and Caudek in this volume for visual perception of depth and events.

    PERCEPTUAL ORGANIZATION

    The term perceptual organization refers somewhat ambigu- ously both to the structure of experiences based on sensory activity and to the underlying processes that produce that per- ceived structure. The importance and difficulty of achieving useful organization in the visual modality can perhaps be most easily appreciated by considering the output of the reti- nal mosaic simply as a numerical array, in which each num- ber represents the neural response of a single receptor. The main organizational problem faced by the visual nervous sys- tem is to determine object structure: what parts of this array go together, so to speak, in the sense of corresponding to the same objects, parts, or groups of objects in the environment. This way of stating the problem implies that much of percep- tual organization can be understood as the process by which a part-whole hierarchy is constructed for an image (Palmer, in press-b). There is more to perceptual organization than

  • 180 Visual Perception of Objects

    just part-whole structure, but it seems to be the single most central issue.

    The first problem, therefore, is to understand what part- whole structure people perceive in a given scene and how it might be characterized. Logically, there are limitless possible organizations for any particular image, only one (or a few) of which people actually perceive. A possible part-whole struc- ture for the leopard image in Figure 7.1 (A) is given in Fig- ure 7.1 (C). It is represented as a hierarchical graph in which

    each node stands for a perceptual unit or element, and the various labels refer to the image regions distinguished in Figure 7.1 (B). The top (or root) node represents the entire image. The scene is then divided into the leopard, the branch, and the background sky. The leopard is itself a complex per- ceptual object consisting of its own hierarchy of parts: head, body, tail, legs, and so forth. The branch also has parts con- sisting of its various segments. The sky is articulated into different regions in the image, but it is perceptually uniform because it is completed behind the leopard and branches. The bottom (or terminal) nodes of the graph represent the millions of individual receptors whose outputs define this particular optical image.

    The second problem is how such a part-whole hierarchy might be determined by the visual system. This problem, in turn, has at least three conceptual parts. One is to understand the nature of the stimulus information that the visual system uses to organize images. This includes not only specifying the crucial stimulus variables, but also determining their eco- logical significance: why they are relevant to perceiving part- whole structure. It corresponds to what Marr (1982) called a “computational” analysis. The second problem is to specify the processing operations involved in extracting this infor- mation: how a particular organization is computed from an image via representations and processes. It corresponds to what Marr called an “algorithmic” analysis. The third is to determine what physiological mechanisms perform these op- erations in the visual nervous system. It corresponds to what Marr called an “implementational” analysis. As we shall see, we currently know more about the computational level of perceptual organization than about the algorithmic level, and almost nothing yet about the neural implementation.

    Perceptual Grouping

    The visual phenomenon most closely associated historically with the concept of perceptual organization is grouping: the fact that observers perceive some elements of the visual field as “going together” more strongly than others. Indeed, per- ceptual grouping and perceptual organization are sometimes presented as though they were synonymous. They are not. Grouping is one particular kind of organizational phenom- enon, albeit a very important one.

    Principles of Grouping

    The Gestalt psychologist Max Wertheimer first posed the problem of perceptual organization in his groundbreaking 1923 paper. He then attempted a solution at what would now be called the computational level by asking what stimulus

    A

    B

    C

    Figure 7.1 A natural image (A), its decomposition into uniform connected regions (B), and a hierarchical graph of its part-whole structure (C). Source: From Palmer, 2002.

  • Perceptual Organization 181

    Figure 7.2 Classical principles of grouping: no grouping (A) versus group- ing by proximity (B), similarity of color (C), similarity of size (D), similar- ity of orientation (E), common fate (F), symmetry (G), parallelism (H), continuity (I), closure (J), and common region (K).

    time 1

    time 2

    time 3

    (etc.)

    change change

    change change

    Figure 7.3 Grouping by synchrony of changes.

    factors influence perceived grouping of discrete elements. He first demonstrated that equally spaced dots do not group together into larger perceptual units, except the entire line (Figure 7.2; A). He then noted that when he altered the spac- ing between adjacent dots so that some dots were closer than others, the closer ones grouped together strongly into pairs (Figure 7.2; B). This factor of relative distance, which Wertheimer called proximity, was the first of his famous laws or (more accurately) principles of grouping.

    Wertheimer went on to illustrate other grouping princi- ples, several of which are portrayed in Figure 7.2. Parts C, D, and E demonstrate different versions of the general principle of similarity: All else being equal, the most similar elements (in color, size, and orientation for these examples) tend to be grouped together. Another powerful grouping factor is com- mon fate: All else being equal, elements that move in the same way tend to be grouped together. Notice that both com- mon fate and proximity can actually be considered special cases of similarity grouping in which the relevant properties are similarity of velocity and position, respectively. Further factors that influence perceptual grouping of more complex elements, such as lines and curves, include symmetry

    (Figure 7.2; G), parallelism (Figure 7.2; H), and continuity or good continuation (Figure 7.2; I). Continuity is important in Figure 7.2 (I) because observers usually perceive it as con- taining two continuous intersecting lines rather than as two angles whose vertices meet at a point. Figure 7.2 (J) illus- trates the further factor of closure: All else being equal, ele- ments that form a closed figure tend to be grouped together. Note that this display shows that closure can overcome conti- nuity because the very same elements that were organized as two intersecting lines in part I are organized as two angles meeting at a point in part J.

    Recently, two new grouping factors have been suggested: common region (Palmer, 1992) and synchrony (Palmer & Levitin, 2002). Common region refers to the fact that, all else being equal, elements that are located within the same closed region of space tend to be grouped together. Figure 7.2 (K) shows an example analogous to Wertheimer’s classic demonstrations (Figures 7.2;

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