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Summary: Visual Information Processing

Visual scene analysis is a computer-based effort to simulate visual perception. It requires the computer to identify which parts of a scene "belong together" to form objects. The key process is called constraint satisfaction. The computer locates edges and corners where lines come together (called vertexes). The assignment of meaning to each line and vertex has the effect of limiting or constraining the interpretation put on other parts of the scene. The constraints "propagate." Each part of the scene, when interpreted, helps to limit possible interpretations of other parts of the scene.

Constraint-propagation eventually produces one interpretation of the whole scene that is consistent with all the evidence. Classic work on visual scene analysis in the mid-60s culminated in a successful program by the 1980s. Success, in this case, was defined by a computer's ability to start with a visual image from a camera and identify boundaries of all objects in the scene, while specifying which lines, edges, and corners belonged to particular objects.

Humans and computers both use a combination of bottom-up (data driven) and top-down (schema-driven) processing. Bottom-up processing is the flow of raw data into the system, which determines activity at higher levels. Top-down processing is the influence of organized knowledge upon the interpretation of data. A schema is a pattern or piece of knowledge based on past experience. The phenomena of set and expectancy, illustrated by the "rat man" example, show the effect of schemas on top-down processing. Cartoonists exploit top-down processing by using tiny cues to suggest emotions, movements, and mental states.

Mental imagery takes place in imagination. Cooper and Shepard showed that the speed of rotating an imagined shape varied with the amount of rotation required, suggesting mental images had characteristics similar to physical images. Kosslyn did research showing picture-like qualities of mental images. Brain scans show that the same areas involved in normal perception are involved in mental imagery. However, picture memory and spatial memory are distinct, involving different areas of the brain. Memory of images is not exact. We remember the meaning of an image, not the pictorial detail. The "haircut" experiment by Baggett showed that people lost memory for details of an image within about three days.

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