National Geographic : 1992 Nov
Practicing to deceive "People often think visual illusions are only curios ities. That's a misconcep tion," says Vilayanur S. Ramachandran, professor of neuroscience at the University of California at San Diego. "It's important to study illusions because they teach us how the visual sys tem works, what its ground rules are." Normally we do not expe rience illusions in our every day, three-dimensional world, which offers multiple cues to help our brains interpret images. When these cues are selec tively reduced ina two dimensional world, the brain can be fooled. In illusion 4, Ramachan dran says, we are forced to respond exclusively to the cue of shading. Our brains, accustomed to light from the sun overhead, assume objects are lit from above. Rotate the diagram. The brain keeps to its assump tion and creates a new per ception of the scene. Illusion 2 shows how the brain is primed to construct outlines from incomplete data. A useful capability, says Ramachandran, if you ever need to recognize a tiger lurking in tall grass. 1. Filling in the blind spot With your right eye closed, stare at the cross and slowly move the diagram closer to your eyes. The red dot will disappear when its focused image on your left retina covers the optic nerve head, called the blind spot because it has no photoreceptors. How ever, using data from adjacent receptors, the brain improvises, filling the void with slanted lines from the surrounding area. A 2. Illusory triangles The white triangle (above left) seems superimposed on the blue sided triangle. But upon study, the sides of the white triangle disappear: Its brightness is the same as that of the surrounding white. The illusion doesn't work for most people when blue and magenta are shown at equal luminosity, perhaps because the brain's contour-detecting cells are partly color-blind. 3. Afterimages Stare at the cross in the color pie for a minute or so, and then shift your gaze to the blank square. Patches will appear in colors complementary to the original hues. Clockwise from upper right, red, blue, yellow, and green replace the original colors. Possible explanation: When neurons sensitive to one color become fatigued, antagonistic neurons produce its complementary color.