Perceptual Constancy Research Paper

1. Stability And Constancy In Perception

The ability of our perceptual system to overcome the effects of changes in stimulation and maintain a veridical and stable percept is called perceptual constancy (Epstein 1977, Walsh and Kulikowski 1998). In discussing the constancies it is common to make a distinction between the distal stimulus and proximal stimulus. The distal stimulus is the actual physical stimulus, the physically objective dimensions of the viewed object. The proximal stimulus, in the case of vision, is the very image that falls on the retina. It changes with changes in position or lighting of the physical stimulus. Constancy can be defined as the ability to correctly perceive the distal stimulus; that is, the stable properties of objects and scenes, in spite of changes in the proximal stimulus.

One example of constancy is shape constancy, which refers to the ability of the visual system to ascertain the true shape of an object even when it is not viewed face on but slanted with respect to the observer. In an example of shape constancy the distal stimulus might be a door half open, while the proximal stimulus will be the trapezoid shape it projects on the retinas. Shape constancy in this case would be the perception of the distal rectangular shape of the door in spite of the fact that the proximal shape is far from rectangular. Another example of constancy is lightness constancy. Lightness refers to the perceived reflectance of an object; high reflectance is perceived as white and very low reflectance as black, with intermediate reflectances as various shades of gray. We are capable of perceiving the distal lightness of a surface in spite of the fact that the proximal amount of light reaching our eyes changes with changes in the amount of light illuminating that surface. For example, if we hold a piece of chalk in one hand and a piece of charcoal in the other, the chalk will be perceived as white and the charcoal as black. This will be true if we observe the two in a dimly lit room or in bright sunshine, in spite of the fact that the amount of light reaching the eyes from the charcoal in the sunshine might be greater than that from the chalk in the dim room lighting.

The perceptual constancy that has received the greatest amount of research attention is size constancy (Ross and Plug 1998). If the distance is not very great we usually perceive the distal size of objects in spite of the changes in their proximal size as their distance from us changes. The two theoretical approaches mentioned above, the constructivist and the direct, deal with size constancy differently. According to the constructivists, size constancy is achieved through some process whereby the perceptual system perceives the object’s distance and then takes the distance into account and ‘corrects’ the proximal image to yield a true distal percept. The direct approach asserts that there is no need to involve the perception of distance in the perception of size. Instead, it claims that enough information exists in the visual environment to allow direct perception of size without the need for aid from additional ‘higher’ mental processes.

2. Two Theoretical Approaches To Perception

The two approaches, the constructivist and the direct, differ in many respects, but the main differences between them revolve around two interrelated issues, the richness of the stimulation reaching our sensory apparatus, and the involvement of ‘higher’ mental processes in the apprehension of our environment. The constructivists see the stimulation reaching our senses as inherently insufficient for veridical perception, necessitating an ‘intelligent’ mechanism that utilizes inferential processes to supplement the information available to the senses and resolve its inherent ambiguity. The direct theorists, in contrast, argue that the information in the environment is sufficiently rich to afford veridical perception with no need for sophisticated algorithms. The constructivists see perception as multistage with mediational processes intervening between stimulation and percept; i.e., perception is indirect. The direct theorists see perception as a single-stage process; i.e., it is direct and immediate. For the constructivists, memory and schemata based on past experience play an important role in perception. The direct approach sees no role for memory in perception. Finally, the two approaches differ in the aspects of perception they emphasize: the constructivists excel at analyzing the processes and mechanisms underlying perception, whereas the direct approach excels at the analysis of the stimulation reaching the observer.

2.1 The Constructivist Approach

This approach has also been called Helmholtzian, indirect, cognitive, algorithmic, and mediational, among other labels. It is the older of the two, ascribing some of its roots to Helmholtz’s (1867) notion of ‘unconscious inference,’ which maintains that veridical perception is achieved by an inference-like process which transpires unconsciously. A somewhat similar approach was adopted and elaborated more recently by Rock (1983)in his The Logic of Perception. This book contains the most detailed account of the constructivist view and the evidence in its favor. The first sentence in the book is, ‘The thesis of this book is that perception is intelligent in that it is based on operations similar to those that characterize thought.’ (Rock 1983, p.1). Rock makes it clear that this thought like process occurs unconsciously. He also writes, ‘… a summary statement of the kind of theory I propose to advance in the remainder of the book. My view follows Helmholtz’s (1867) that perceptual processing is guided by the effort or search to interpret the proximal stimulus, i.e., the stimulus impinging on the sense organ, in terms of what object or event in the world it represents, what others have referred to as the ‘‘effort after meaning’’.’ (Rock 1983, p. 16). In other words, Rock is conceiving of perception as an effortful, but unconscious, attempt at identifying an object or event.

In his discussion of size perception, Rock specifically invokes a syllogistic inferential mechanism: ‘I will argue that the process of achieving constancy is one of deductive inference where the relevant ‘‘premises’’ are immediately known. That is to say, in the case of a specific constancy such as that of size, two aspects of the proximal stimulus are most relevant, one being the visual angle subtended by the object and the other being information about the object’s distance.’ (Rock 1983, p. 240). Rock sees size perception as depending on two perceptions, that of proximal size and that of distance, together leading through a syllogism to the veridical distal percept. Like other constructivists, this approach posits some sort of combination of proximal size information and distance information in the achievement of size constancy. In a similar manner, the same combinatorial process holds for all the constancies, according to the constructivist view.

The equation of perceptual and thought processes is difficult to verify empirically, but there are implications from this approach that are verifiable. As can be seen above, the constructivist approach implies that at least two perceptions are involved in perceiving size, that of the proximal size of the object and that of its distance. In a later book, Indirect Perception, Rock (1997) collected a large group of studies showing ‘that perception is based on prior perception, implying a perception–perception chain of causality.’ This interdependence of perceptual processes is something that can be examined empirically and, indeed, the studies reprinted in that book clearly evidence that chain of causality. However, it should also be noted that all of these studies utilize somewhat artificial situations where a manipulation is used in order to create some sort of illusory perception or misperception. A simple example is studies where subjects are presented with monocular views of the stimuli causing misperception of the three-dimensional spatial arrangement. Here, for example, misperceived distance is accompanied by the misperception of other perceptual qualities.

2.2 The Direct Approach

This approach is also referred to as ecological, Gibsonian, sensory, and immediate, among other labels. It is in the main an approach developed by James J. Gibson who expounded it in his The Ecological Approach to Visual Perception (Gibson 1979). In that book, Gibson proffered an exciting new approach to the study of visual perception that included many new concepts and new ways of looking at perception. The entire first half of the book is devoted to a novel analysis of the ambient, or surrounding, environment and the information it provides the observer. Gibson finds the classical approach of describing the stimuli for perception in terms of stimulus energies impinging upon the receptors completely unsatisfactory. He points to the differences between these energies and the optical information available in the ambient optic array. That information is picked up by a stationary or moving observer. Perception consists of perceiving events, i.e., perceiving changes over time and space in the optic array.

What sort of information is picked up in direct perception? Gibson suggests that there exist invariants in the optic array that serve to supply the observer with unequivocal information. These invariant aspects of the information remain constant in spite of changes in the observer’s viewpoint. He musters a great deal of evidence to prove this point. Among the items of evidence he presents is a study of size perception of his where observers were asked to match the height of stakes planted at various distances in a very large plowed field with a set of stakes of varying size nearby. His finding was that size perception remained veridical no matter how far away the stake was planted. Unlike the constructivists, Gibson does not ascribe this size constancy to the taking into account of distance, but rather to the pickup of invariant ratios in the ambient optical array surrounding the observer. In the case of perception of size, he proposes two such invariant ratios. One is the amount of surface texture intercepted by the object’s base as it sits on that surface. The second is the horizon ratio, the ratio between the object’s height and its distance from the horizon, both measured in the two-dimensional projection of the optic array. There is no need, according to his view, for perceived distance to be involved here, or for the inferential mental processes that the constructivists purport to underlie size perception.

One of Gibson’s most important contributions is the concept of affordances. He writes, ‘The affordances of the environment are what it offers the animal, what it pro ides or furnishes, either for good or for ill’ (Gibson 1979, p. 127). In other words, affordances refer to the possible actions that can be taken by the perceiver vis-a-vis objects in the environment. Thus, for example, some surfaces in the environment are ‘stand-on-able,’ or ‘climb-on-able,’ or ‘sit-on-able.’ ‘The psychologists assume that objects are composed of their qualities. But I now suggest that what we perceive when we look at objects are their affordances, not their qualities.’ (Gibson 1979, p. 134), and, ‘… the basic affordances of the environment are perceivable and usually perceivable directly, without an excessive amount of learning.’ (Gibson 1979, p. 143). Gibson is suggesting that the direct perception of, say, a chair does not constitute recognizing it as a type of furniture labeled ‘chair,’ i.e., categorizing it, but rather the perception of the chair’s affordance, i.e., that it contains a surface that is sit-on-able.

As may be clear from the foregoing description, Gibson’s conception is one of an active perceiver exploring his environment. Eye, head, and body movements are part and parcel of the perceptual process. Perception transpires continuously over both time and space. ‘Space’ here refers not to an empty space but to the many surfaces that make up the environment, the most important being the terrain that at times reaches the horizon. The horizon is of importance as it serves as a useful reference standard, and when it is occluded Gibson speaks in terms of an implicit horizon, presumably similar to what architects and others have called the eye-level plane. With such a conception, Gibson is totally averse to the reductionist experimental paradigms used by the constructivists. Brief exposures or looks through monocular ‘peepholes’ do not represent true perception, in his view.

In his book, Gibson almost totally refrains from discussing the processes underlying perception. Perception is simply the pickup of information from invariants in the ambient environment. His only allusions to underlying processes are in terms of resonance: ‘In the case of the persisting thing, I suggest, the perceptual system simply extracts the invariants from the flowing array; it resonates to the invariant structure or is attuned to it. In the case of substantially distinct things, I venture, the perceptual system must abstract the invariants. The former process seems to be simpler than the latter, more nearly automatic’ (Gibson 1979, p. 249).

3. Summing Up And A Possible Reconciliation

The two theoretical approaches, the constructivist and the direct, obviously differ in many respects. The former endows the perceiver with ‘higher’ mental capacities that allow him her to compensate for the purported insufficiency of the stimulation reaching the sensory apparatus. In contrast, the direct approach attempts to delineate the richness of information available in the ambient environment as picked up by an active perceiver, allowing direct perception without the need for additional mental processes. What is more, the proponents of each of these two approaches utilize research methods commensurate with the tenets of the respective approach. Thus, for example, constructivist researchers utilize impoverished or illusory stimulus conditions (e.g., brief and/or monocular presentations), calling for the intervention of ‘higher’ processes to compensate for the lack of adequate stimulus information. The adherents of the direct theory, on the other hand, utilize rich stimulus conditions without time limitations.

In spite of the seemingly irreconcilable differences between the two theoretical approaches, a means of bridging them does exist. They can co-exist within a broader theory of perception (Norman 2001). This broader theory is based on the accumulating neurophysiological, neuropsychological, and psychophysical research findings that point to the existence of two distinct visual systems, the dorsal and the ventral (see, e.g., Milner and Goodale 1995). The central idea is that the direct approach broadly parallels the functions of the dorsal system, and the constructivist approach broadly parallels those of the ventral system. These two visual systems are found in different parts of the brain, the dorsal mainly in the parietal lobe, the ventral mainly in the temporal lobe. The two deal with different aspects of perception. The dorsal system deals mainly with the utilization of visual information for the guidance of motor behavior in one’s environment. The ventral system deals mainly with the utilization of visual information for ‘knowing’ one’s environment, i.e., identifying and recognizing items previously encountered and storing new visual information for later encounters. However, it should be stressed that both systems process similar information for somewhat different purposes. Thus, for example, both systems are involved in the perception of size, albeit with somewhat different specializations. The dorsal system picks up size information for motor tasks, such as grasping an object. The ventral system utilizes size information for distinguishing between objects, say, between a real car and an accurate toy model.

Emerging from this attempt at integrating the two approaches into a single theory is a dual-process approach to visual perception. According to this approach, much of our day-to-day pickup of visual information is carried out by the dorsal-direct system without involving much awareness. In the main the information picked up is that which allows the organism to function within its environment, i.e., Gibson’s affordances. The ventral-constructivist system, on the other hand, deals with the interface between the visual input and cognition, and we are normally aware of its output. Only it possesses a longterm memory and therefore any type of identification or recognition must transpire within it. As the dorsal system is mainly concerned with directing motor behavior in one’s environment it must rely on bodycentered information, in absolute units, about the environment and the objects in it. In contrast, the ventral system in its attempt to recognize objects can suffice with relative, object-centered, information. It is probably for this reason that the dorsal system yields more accurate information about the visual environment than does the ventral system. Thus, it is suggested that perceptual constancy is usually attained by the dorsal system. However, there are cases where the ventral system is also involved, as when the visual information is sparse or ambiguous, or when the observer is required not only to act upon some aspect of the visual environment, but also to make a judgment about some object in that environment.

Bibliography:

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