Sunday, June 16, 2013

Vision and Inference


Charles D. Gilbert, "The Constructive Nature of Visual Processing," in Eric R. Kandel, James H. Schwartz, Thomas M. Jessel, Steven A. Siegelbaum & A. J. Hudspeth, eds., Principles of Neural Science 556, 556-557 (5th ed., 2013):

Vision is often incorrectly compared to the operations of a camera. Unlike a camera, however, the visual system is able to create a three-dimensional representation of the world from the two-dimensional images on the retina. In addition, an object is perceived as the same under strikingly different visual conditions.

A camera reproduces point-by-point the light intensities in one plane of the visual field. The brain, in contrast, parses scenes into distinct components, separating foreground from background, to determine which light stimuli belong to one object and which to others. In doing so it uses previously learned rules about the structure of the world. In analyzing the incoming scream of visual signals the brain guesses at the scene presented to the eyes based on past experience.

This constructive nature of visual perception has only recently been fully appreciated. Earlier thinking about sensory perception was greatly influenced by the British empiricism philosophers, notably John Locke, David Hume, and George Berkeley, who thought of perception as an atomistic process in which simple sensory elements, such as color, shape, and brightness, were assembled in an additive way, component by component. The modern view that perception is an active and creative process that involves more than just the information provided by the retina has its roots in the philosophy of Immanuel Kent and was developed in detail in the early 20th century by the German psychologists Max Wertheimer, Kurt Koffka, and Wolfgang Koehler, who founded the school of Gestalt psychology.

The German term Gestalt means configuration or form. The central idea of the Gestalt psychologists is that what we see about a stimulus--the perceptual interpretation we make of any visual object--depends not just on the properties of the stimulus but also on its context, on other features in the visual field. The Gestalt psychologists argued that the visual system processes sensory information about the shape, color, distance, and movement of objects according to computational rules inherent in the system. The brain has a way of looking at the world, a set of expectations that the rides in part from experience and in part from built-in neural wiring.

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