National Geographic : 1992 Nov
4. Shape from shading Residents of a sunlit world, we assume light comes from above and use shading to determine shape. This pattern becomes an X of concave circles if you rotate it 90 degrees to the left. Turned to the right, it becomes an X of convex circles. KAISERPORCELAIN,JUDY COUSINS 7. Faces or vase? We see two faces or a vase. But we cannot see faces and vase at the same time, because our brain recognizes an object by separating it from its background. Here both perspectives are equally plausible, so our perception reverses again and again. 5. Is one line longer? Measure to convince yourself: The two dark lines in this corridor are actually the same length. The perspective tricks the brain into perceiving the near line as shorter. Even when we know this, our visual system is still confounded. 6. One or two grays? The color of the two small triangles is iden tical. It appears different to most people because the yellow and blue colors of the large triangles affect perception. When interpreting colors, the brain takes the larger scene into account. 8. Divergent or parallel? The short hash marks confuse the brain cells that gauge orientation, and we inter pret the longer lines as diverging. To negate this effect, tilt the diagram and look across it from the lower left corner. The divergent lines are in truth parallel. 9. Whence the gray spots? Where the white bars cross, dark spots pulse, induced by the way retinal cells react to light in this geometric scene. The contrast between black and white selec tively excites or inhibits the response of these cells, thereby creating the illusion.