gre考试阅读真题解析(B)_GRE阅读

gre考试阅读真题解析(B)

　　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 objects 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.