|Sensation and Perception: Visual Illusions|
The visual system is not as straightforward as it seems to the internal observer. We believe that we see things as they are, when in general we can be fooled as to the content and relative attributes of a picture fairly easily.
A lot of the classic illusions illustrate principles of gestalt perception, part of the gestalt school of psychology, especially popular in the first half of the twentieth century.
More recently, there has been work on trying to decipher the visual system itself, and such work leads to experiments like the one on blind-spots that attempts to figure out how much visual processing is inference by determining what fills a blank spot in the retina (the blind spot where the optic nerve passes out of the eye) -- there is no perceived blank spot, and lines seem to continue through the blind spot. The McCollough study investigates whether edge-detectors (cells or groups of cells that find edges in an image) adapt to color and whether there are separate edge-detectors for separate directions of line.