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7. Visual Perception

DescriptionComprehensive text book which provides a useful overview of recent psychophysical research and theoretical models. It gives a background to the development of psychological and physiological theory. Gibson's 'ecological' approach to visual perception is examined and used selectively in support of Marr's work to develop the authors' own theoretical position.
MethodologyCritical review of theoretical and empirical studies on visual perception.
ResultsThe authors conclude that both retinal image and dynamic patterns of light play an important part in visual processing. They accept that human perception operates in a cultural as well as physical environment but advocate further investigation of the psysiological bases of visual perception and the formulation and testing of algorithmic theories pioneered by Marr (p.379).
PublishedVisual Perception
AuthorsBruce, V., Green, P.R., and Georgeson, M.A.
Date1996
PublisherPsychology Press, Hove,
Price35.95 (hardback) 14.95 (paperback)
Keywordsphysiology, psychology, ecological theory, visual perception
Comments

Useful text book which contrasts traditional psychological theories of perception, which rely on the concept of processing of one or more retinal image, with 'direct' theories of perception. The review of research and theoretical debate is given dynamic force by the authors' own, clearly stated theoretical standpoint. Several chapters of the book involve the interpretation of the work of Gibson and Marr. Further reading of the text and source material is recommended.

Ecological Approach to Visual Perception

Direct theory believes that there are two levels at which perception can be explained: the 'ecological' level and the 'physiological' level. The ecological level considers that light intensity is directly detected. The physiological level is concerned with the way in which nerve cells are organised. Direct theories consider that there is no need to study the link between the ecological and physiological levels. The authors consider that whilst direct theories are a useful basis for study, further research of these links is necessary.

Chapter 11 outlines the ecological approach to visual space perception developed by Gibson, (Gibson 1966, 1979; Reed and Jones, 1982).

Gibson holds that perception is direct and unmediated by inference and problem-solving. He contends that movement is essential for seeing and that it is the flow and disturbances in the structure of the total optical array rather than bars and blobs or forms in an image which provides the information for perception, (p255).

The successes of the ecological approach have been in understanding seeing - how we use information to walk upright or catch a ball - but its attempts to explain seeing as have not progressed beyond general assertions (p377).

Perceptual Organisation

Chapter 6 provides a brief review of the Gestaltists' phenomenological way of seeing. It comments on Marr's use of the Gestalt principles of organisation. Gestalt laws are suggested as useful descriptive tools for discussing perceptual organisation in the real world but, it is suggested, they do not provide an adequate theory of why the principles work or how perceptual organisation is achieved, (p118).

Computational Model of Visual Perception

Marr argues that there must be a set of computational procedures that enable the detection of structures in light and that these procedures are implemented by the nervous system. He considers that a theory of algorithms is needed to explore perception at a psychological level.

Marr's theories outlined in Ch. 4, suggest that early visual processing involves a 'primal sketch' made from the light reflected by the physical structures being viewed and focused by the observer's eye. The 'raw primal sketch' locates edges and blobs and their orientations, etc. From this complex set of statements larger structures, i.e. boundaries and regions, are differentiated through grouping procedures to form a 'full primal sketch'. Depth, motion and shading yield a '2 1/2D sketch': a second level of representation which is viewer centred. This is followed by a third level, termed a '3D model representation' which is centred on the object(s) being viewed. Object recognition is achieved when the image viewed matches a representation of a known object stored in the brain.

Conclusions: contrasting Theories of Visual Perception

The author's reach their conclusions from contrasting 'traditional' and 'ecological' approaches:

  • They adopt aspects of 'direct' theories which suggest that properties of the world can be detected without "cognitive" processes of inference, interpretation, and judgement, but argue with 'indirect' theorists such as Marr and Ullman, that such processes of detection do rely on computation (p370.)
  • They suggest that perception involves representations and computations. Different kinds of computation require different sorts of representations. Computations may be different, for example, if a dynamic pattern of light and not a retinal image is considered as the input to visual processing.

We cannot always assume that different perceptual tasks are actually tapping the same underlying visual processes, however similar they appear. (p378)

  • They accept that people may see objects and events in terms of a culturally given conceptual representation of the world.
References

Gibson, J.J. (1966). The senses considered as perceptual systems. Boston, MA: Houghton Mifflin.

Gibson, J.J. (1979). The ecological approach to visual perception. Boston, MA: Houghton Mifflin.

Marr, D. (1982). Vision: A computational investigation into the human representation and processing of visual information. San Francisco, CA: Freeman.

Marr, D., and Poggio, T. (1979). A computational theory of human stereo vision. Proceedings of the Royal Society of London, B, 204, 301-328.

Marr, D, and Ullman, S. (1981) Directional selectivity and its use in early visual processing. Proceedings of the Royal Society of London, B, 211, 151-180.

Reed, E., and Jones, R. (Eds.) (1982). Reasons for Realism: selected essays of J.J. Gibson. Hillsdale, NJ: Lawrence Erlbaum Associates Inc.