Shimamura_Chp 3 - Chapter 3 Chapter 3. The Illusion of Form...

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Chapter 3 43 Chapter 3. The Illusion of Form Look around and describe what you see. Rather than noting the shadings, edges, and spa- tial frequencies that are registered by your brain, you name objects placed in a spatial environ- ment. Our corporate brain is exceedingly proficient in manufacturing forms . For example, in Sánchez Cotán’s still life, Quince, Cabbage, Melon and Cucumber (Figure 3.1), we easily recognize the scene’s layout and the objects in it. We see a sliced melon and cucumber on a window ledge. Hanging from above are a cabbage and a quince (or some kind of fruit). The realistic painting is enhanced by the rendering of shading and highlights, which de- fine a light source, presumably the sun, above us and to the left. With the presence of shadows we see that the small melon slice and cucumber ex- tend past the window’s ledge. Our ability to perceive this scenet occurs so rapidly that we fail to appreciate the complex brain processes necessary to perceive forms. As Gombrich noted in his seminal book, Art and Illusion , the realistic portrayal of objects on canvas is an illusion manufactured through artistic application of lines, colors, and shadings. As noted in Chapter 2, the brain must also create the illusion of a 3-D world by inter- preting edges, colors, and forms. How does the brain recognize forms given that erratic 2-D image that is constantly changing on our retina? First, the brain must isolate or segre- gate forms from each other. Once that is ac- complished, grouping processes put together edges and surfaces and creates blob-like (3- D) forms. Finally, meaning is attached to these forms so that knowledge is linked to sensory patterns. The goal is object recogni- tion , which gives us the ability to say, “that’s a melon” or “that’s my friend, Kathy.” Figure 3.2 illustrates some of the ways in which we create the illusion of 3-D objects from 2-D images. In Figure 3.2(a), shading and high- lighting define the shape of an abstract blob, and even though this form is unfamiliar, we easily perceive its 3-D structure. Figure 3.2(b) uses high-frequency contour lines to depict edges
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Chapter 3 44 and define features of a face. In Figures 3.2(c) and 3.2(d), what appears to be circles or parallel lines are distorted as if they were placed on curved surfaces. Of course, all of these impressions of 3-D forms are illusions, as each figure is of course only a 2-D illustration. Isolating Forms Form perception begins with the assumption that we’re looking at a spatial 3-D environ- ment with objects placed at various distances from us. We then must figure out what objects are in front of others, which can be tricky because objects will overlap and change in size depending on their distance from us. Psychologists call the process of isolating objects from each other as figure-ground segregation . That is, we must separate forms or figures from their background. This process takes place primarily in the occipital cortex where features such as color, edges,
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This note was uploaded on 11/04/2010 for the course PSYCH 39E SEM taught by Professor Shimamura during the Spring '10 term at University of California, Berkeley.

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Shimamura_Chp 3 - Chapter 3 Chapter 3. The Illusion of Form...

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