[ CogSci Summaries home | UP | email ]

Eysenck, M. W. & Keane, M. T. (1995) Chapter 4: Theories of perception, movement, and action. In Cognitive Psychology: A Student's Handbook, Lawrence Erlbaum, Hillsdale, USA.

Author of the summary: Debajyoti Pati, 2000, gte811q@prism.gatech.edu

There are two lines of thought in the realm of visual perception. One line of thought considers perception as indirect, where they assume the presence of several different kinds of information processing to transform the light intensities on the retina into perception of the visual environment. This approach is also known as the top-down (or concept driven) theory, signifying the importance of conceptually driven processes, and theorists in this category including Bruner (1957) and Gregory (1970) are sometimes referred to as the constructivists. In contrast, Gibson (1950, 1966, 1979) propounded the direct approach (otherwise known as the bottom-up approach) which emphasizes that the information provided by the visual environment is sufficient for moving around and interacting with the environment (without the role of any internal processes). Gibson"s theory is regarded as the ecological approach where the primary argument is that perception provides the information required for organization of action, and action along with movement facilitates accurate perception.

The constructivist theories differs from the ecological approach in many ways. It assumes that several other events intervene between stimulation and experience. According to it, perception is the end product of the interactive influences of internal hypotheses, expectation and knowledge (inferential processes) with the stimulus, as well as several motivational and emotional factors. Finally, owing to the above factors, it is assumed to be prone to error arising out of incorrect hypotheses and expectations. Gregory (1972) went further to propose that perceptions get constructed from "floating fragmentary scraps of data signalled by the senses and drawn from the brain memory bank". Some of the studies supporting this theory includes Parmer"s (1975) kitchen scene, and Ittelson"s (1952) Ames distorted room. Effects of motivation and emotion on perception were demonstrated by several studies including Myrphy (1943) and Smith and Hochberg (1954), and Bruner & Goodman (1947). Similarly, Gregory (1970,1980) demonstrated the cases of visual illusion arising out of his misapplied size-constancy theory (the Muller-Lyer illusion). The constrcutivist approach, however, has many shortcomings. The approach predicts that perception will be in error more often, where as it has been shown to be typically accurate. Further, many of the experiments and demonstrations carried out in support of this approach involved artificial or unnatural stimuli. The stimuli presented were so brief that it eliminated the possibility of any bottom-up process. In addition, it is difficult to ascertain the exact nature of hypotheses formed by observers. Finally, hypotheses of observers are very rigid, which goes against the "best guess" assumption of the constructivists.

Gibson"s ecological approach is not exactly the direct approach owing to his emphasis on the role of movement within the environment on perception. He assumed that the pattern of light reaching the retina includes all the visual information from the environment, and that this optical array provides unambiguous information about the layout of objects. The information, in his argument, comes in the form of texture gradients, optic floe patterns and affordances. Further, he argued that perception involves picking up information in a direct way, without the role of any information processing. According to Gibson, the potential use of objects (or affordances) could be perceived directly, which provides meaning to the environment. Further, most objects, according to him, are host to more than one affordances depending on the perceiver"s species and psychological state. This provides confirmation to the notion of close relationship between perception and action. Gibson also came up with the notion of resonance, where drawing analogy to the tuning of radios to electromagnetic radiation. He argues that picking up information from the environment is relatively automatic and effortless provided that the perceivers are attuned to the information. He draws parallel between the radio circuitry and the human nervous system. The ecological approach has been proved successful on many front. Philosophically, his view seem to have brought the environment back into focus. In addition, his contention regarding the richness of the visual stimuli has been shown to be correct, especially when coupled with motion. Similarly, he was correct in his argument that inaccurate perception depends, often, on artificial situations. Further, none of the constructivist experiments have actually disproved the ecological approach of Gibson. Gibson"s approach, however, has a few shortcomings. First of all, the process of discovering affordances and identifying invariants in the environment seem to be very complicated. Similarly, Gibson did not provide explanation for the distinction between "seeing" and "seeing as". Finally, the complete elimination of internal representations in the perception process seem to be wrong.

A synthesis of both the approaches appears to provide a better theory, where the bottom-up approach fits well in good viewing conditions while the top-down theory comes into play in bad viewing conditions or lack of stimulus clarity. Tulving, Mandler, and Baumal (1964) demonstrated this in their word identification study.

Some of the important study areas in perception include eye movement, reaching & grasping, time to contact, and optic flow pattern. Lee"s (1976) emphasis on tau (the inverse of the rate of expansion of the retinal image of an object) is one of the fundamental contributions to the field, which has been shown to be relevant to human perception, especially to athletes. While reaching and grasping seem to be dependent on information about spatial location, the time-to-contact decisions is more dependent on tau. Further, judgment about direction of heading could be based on optic flow pattern. Moreover, decisions about time to contact, detection of biological motion, apparent movement and perceived causality are all claimed to be direct, and not dependent on internal processes. Apparent motion, however, seem to involve certain low-level as well as higher level cognitive processes.

Back to the Cognitive Science Summaries homepage
Cognitive Science Summaries Webmaster:
JimDavies (jim@jimdavies.org)
Last modified: Wed Apr 26 11:29:24 EDT 2000