Sceptics commonly dismiss supposed sightings of ghosts or other paranormal phenomena as optical illusions, which are false images caused by either quirks of human physiology or the way the brain processes information. Illusions that are physiological usually result from biochemical changes as the eyes send information to the brain. For example, when a person looks into a camera’s flash as it goes off, white spots seem to remain hovering in air. This is the aftereffect of the eyes’ retinas being unable to return to normal after sending the image of the flash to the brain. The other type of illusion is caused by the way the brain processes visual information. For example, when looking at a pattern consisting of alternating black and white squares, a person might at first perceive the black squares as being a bit farther away than the white squares; then, with a sudden switch of perception, the white squares will seem farther away than the black, creating the illusion of movement and depth in what is actually a two-dimensional image.
Because of such effects, sceptics note that the saying “seeing is believing” is not necessarily accurate. When people see moving lights in the sky, for example, the lights might not actually be moving, or they might not be there at all. This suggests that all reported sightings of UFOs, ball lightning, and other phenomena involving moving lights are suspect. Indeed, believers in these phenomena admit that some reports must be discounted as the products of optical illusions. However, they argue that not all reports could be caused by errors in visual perception.
Furthermore, they note that some reports include visual elements that cannot be explained away as an optical illusion. For example, some people have reported seeing UFOs that are not balls of light but are actual spaceships with windows. Sceptics dismiss such stories as being the result of cognitive illusions—that is, visual images created by the brain that do not involve the eye—created as part of elaborate hallucinations.
The Greenhaven Encyclopedia of Paranormal Phenomena – written by Patricia D. Netzley © 2006 Gale, a part of Cengage Learning