GRE General Test: RC-644420 GRE Reading Comprehension

Recent findings suggest that visual signals are fed into at least three separate processing systems in the brain, each with its own distinct function. One system appears to process information about shape perception; a second, information about color; a third, information about movement, location, and spatial organization. An understanding of the functions and capabilities of these three systems can shed light on how artists manipulate materials to create surprising visual effects.

It is possible to summarize the functions of the three subsystems of the visual system as follows. The parvo system carries highly detailed information about stationary objects and about borders that are formed by contrasting colors. It does not, however, carry information about specific colors. Because much of the information about the shape of objects can be represented by their borders, we suspect that this system is important in shape perception. The blob system processes information about colors, but not about movement, shape discrimination, or depth. The magno system carries information about movement and depth. It is good at detecting motion but poor at scrutinizing stationary images. In addition it appears to be colorblind; it is unable to perceive borders that are visible only on the basis of color contrast.

Cells in the parvo system can distinguish between two colors at any relative brightness of the two. Cells in the color-blind magno system, on the other hand, are analogous to a black-and-white photograph in the way they function: they signal information about the brightness of surfaces but not about their colors. for any pair of colors there is a particular brightness ratio at which two colors, for example red and green, will appear as the same shade of gray in a black-and-white photograph, hence any border between them will vanish. similarly at some relative red-to-green brightness level, the red and green will. appear identical to the magno system. The red and green are then called equiluminant. A border between two equiluminant colors has color contrast but no luminance contrast.

Many artists have seemed to be empirically aware of these underlying principles and have used them to maximize particular effects. Some of the peculiar effects of Op Art, for example, probably arise from color combinations that are strong activators of the parvo system but are weak stimuli for the magno system. An object that is equiluminant with its background looks vibrant and unstable. The reason is that the parvo system can signal the object’s shape but the magno system cannot see its borders and therefore cannot signal either the movement or the position of the object. Hence it seems to jump around, drift, or vibrate on the canvas.
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