The page you are reading now looks flat and feels flat, but your brain isn't so certain. That's because when the human brain evolved into its current form about 30,000 years ago, it was programmed for interpreting 3-D objects, especially predators, not images on flat surfaces. As a result, your brain plays tricks on you. For example, the dimensions of the two tabletops above look different, but they are actually identical. The front edge of the table on the left is the same length as the left edge of the table on the right, but it looks shorter because perspective makes it appear closer and therefore smaller. Similarly, the front and rear edges of the right-hand table are actually longer than the apparently receding long edges of the left-hand table. Although many optical illusions depend upon erroneous transformations of two dimensions into three, your brain's tendency to project depth onto flat surfaces can occasionally help it correctly interpret what it sees.

Experiment 1   Examine the figure on the top, then rotate the page 180 degrees and study the figure below it. The letter fragments in both figures are identical, but those covered by the gray bars are easier to read. This is because your brain interprets the bars as being in front of the letters, and so they "fill in" the blanks to create the perception of a single word, as opposed to isolated, fragmented letters.



Experiment 2   Your brain's ability to fill in gaps behind obstructions is not restricted to letters and words. Notice how much more "together" the cube on the right seems than its neighbor, even though it contains no more information.Such perceptual anomalies, known as occlusion effects, provide neuroscientists with clues about how the brain processes information. For example, by studying electrical responses of visual neurons to fragmented visual stimuli, a research team at the Massachusetts Institute of Technology led by Mriganka Sur discovered that the filling in of missing information occurs in the tiniest building blocks of our nervous systems: individual nerve cells.Given that our brains happen to be wired this way, it appears that we are doomed forever to read not just between the lines but behind them as well.