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• Relative clarity can be seen when closer objects appear sharper than more distant, hazy objects.

• Texture gradient provides a cue to distance when closer objects have a coarser, more distinct texture than far away objects that appear more densely packed or smooth.

• Relative height or elevation can be seen when the objects closest to the horizon appear to be the farthest from you. The lowest objects in our field of vision generally seem the closest.

• Linear perspective provides a cue to distance when parallel lines, such as edges of sidewalks, seem to converge in the distance.

• Relative brightness can be seen when the closer of two identical objects reflects more light to your eyes.

• Optical illusions, such as the Muller-Lyer illusion, and the Ponzo illusion, in which two identical horizontal bars seems to differ in length, may occur because distance cues lead one line to be judged as farther away than the other. Similarly, the moon illusion may occur because the moon when near the horizon is judged to be farther away than when it is high in the sky, although in both positions it casts the same image on the retina.

At the San Francisco Exploratorium website, you can see examples of visual illusions and link to other great sites. Go to www.exploratorium.org.

Perceptual Constancy

As a car approaches, you know that it’s not growing in size, even though the image it casts on your retina gets larger, because you impose stability on the constantly changing sensations you experience. This phenomenon is called perceptual constancy. Three perceptual constancies are size constancy, by which an object appears to stay the same size despite changes in the size of the image it casts on the retina as it moves farther away or closer; shape constancy, by which an object appears to maintain its normal shape regardless of the angle from which it is viewed; and brightness constancy, by which an object maintains a particular level of brightness regardless of the amount of light reflected from it. The real shape, orientation, size, brightness, and color are perceived as remaining relatively constant even when there are significant variations in the image it projects. This enables you to identify objects no matter what your viewing angle is, how far away you are, or how dim the lights are.

Perceptual Adaptation and Perceptual Set

Have you ever looked through a periscope or displacement goggles and tried to reach for an object only to find it wasn’t where you thought it was? If you repeated your actions, after a short period of time you were probably able to reach the item easily. You adapted to the changed visual input. Newly sighted people who had been blind from birth are immediately able to distinguish colors and to separate figure from ground, but only gradually become able to visually recognize shapes. Visual perception can also be influenced by cultural factors, assumptions, and beliefs. To make use of the cue of relative size, you need to be familiar with the object and have been exposed to viewing objects in the distance.

Culture and Experience

Your perceptual set or mental predisposition can influence what you perceive when you look at ambiguous stimuli. Your perceptual set is determined by the schemas you form as a result of your experiences. Schemas are concepts or frameworks that organize and interpret information. This can account for people’s interpretations of unidentified flying objects (UFOs), the Loch Ness monster, or seeing a cloud of dust in a movie.

Extrasensory Perception

Parapsychologists study evidence for psychological phenomena that are currently inexplicable by science. They try to answer the question “Is there perception without sensation?” ESP (extrasensory perception) is the controversial claim that perception can occur apart from sensory input. Parapsychology, the study of paranormal events, investigates claims of ESP, including

• telepathy: mind-to-mind communication;

• clairvoyance: perception of remote events;