While sensation has to do with detecting the presence of stimuli and changes in the stimulus field, perception has to do with forming mental representations of the objects that give rise to sensory experiences. Much perceptual research focuses on the process by which individuals determine the size, shape, distance, and motion of objects in the environment.

For most of its history, the study of perception has been dominated by the constructivist/phenomenalist tradition associated with Hermann von Helmholtz, Richard Gregory, Julian Hochberg, and Irvin Rock (among many others). The constructivist view assumes that the proximal stimulation impinging on sensory receptors is inherently ambiguous, and that there is an infinite array of distal configurations compatible with any momentary state of proximal stimulation. For example, consider the relationship between the size and distance of an object, on the one hand, and the size of the retinal image of that object, on the other hand. Holding distance constant, the size of the retinal image is directly proportional to the size of the stimulus; however, holding size constant, the size of the retinal image is inversely proportional to the distance between the object and the perceiver. Thus, given the size of a retinal image, the perceiver does not know whether he or she is viewing a large object far away or a small object close at hand.

According to the constructivist view, stimulation of this sort must be disambiguated by inference-like rules that compare the size of the object with the size of its background or make some comparison of the stimulus input with an a priori model of the world that tells us the size of various objects. In either case, perceiving entails thinking and problem solving. Sometimes, the solution can be wrong, such as when the perceptual system overcompensates for distance cues to generate the illusion that the moon on the horizon is larger than the moon at zenith. Helmholtz famously argued that the inferential rules that guide perception are part of our tacit knowledge: They can be discovered by the scientist but cannot be articulated by the perceiver. Because the thoughts that give rise to our percepts are unconscious, perception lacks the phenomenal quality of thought. However, it remains the case that the final product of perception is a mental representation of the stimulus world, constructed by cognitive operations such as computations and symbolic transformations. We are not aware of the world itself but only of our mental representation of it, which is projected onto the world so that the objects of perception and the objects of the world are coreferential. Even more than sensation, perception from the constructivist/phenom-enalist view has cognitive underpinnings that cannot be denied.

Nevertheless, a contrary, noncognitive view of direct realism was proposed by J. J. Gibson in his ecological theory of perception (interestingly, Neisser took a constructivist approach to perception in Cognitive Psychology, but has since embraced a version of direct realism). According to the ecological view, stimulation is ambiguous only at very elementary levels, but there is no ambiguity at higher levels. Therefore, for example, in determining an object's size, the perceptual system extracts information about the ratio of the size of an object to the size of its background; this ratio determines perceived size, not the size of the retinal image of the object alone. Thus, the perceived size of an object remains constant even as its distance from the viewer (and thus the size of its corresponding retinal image) varies. However, this requires no computations, inferences, or a priori models of the world on the part of the perceiver; size is perceived directly from information available in the environment about the ratio of the figure to ground, without need of any mediating cognitive operations. Because perceptual systems evolved in order to support adaptive behavior, Gibson further proposed that we perceive objects in terms of their affordances, or the actions that we can take with respect to them. Thus, in the same way that the ecological view of perception argues that all the information required for perception is "in the light,'' an ecological view of semantics argues that the meanings of words are ''in the world,'' available to be perceived directly.

The ecological theory of perception proposes that the perception of form, distance, motion, and other stimulus properties is no different from perceiving the hue of a light or the pitch of a sound. In each case, phenomenal experience occurs by virtue of the trans-duction of stimulation into perception, accomplished in a single step by specialized neuronal structures that have evolved to be selectively sensitive to higher order variables of stimulation available in an organism's environmental niche. The contrast between the con-structivist/phenomenalist and direct/realist views dominates much of contemporary perception research, with proponents of the ecological approach conducting clever experiments showing that percepts commonly attributed to computations, inferences, or world models are actually given directly by higherorder variables of stimulation. However, it is one thing to demonstrate that such information is available and quite another to demonstrate that such information actually contributes to perception. The occurrence of visual illusions strongly suggests that we do not always see the world as it really is, and that the perceiver must, in Bruner's famous phrase, go "beyond the information given'' by the environment in order to form mental representations of the world.

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