Perception is an organized process. The most common form of perceptual organization is called figure ground organization in which sensations are grouped into objects or figures that stand out on a plainer background.

Reversible goblet is a favourite demonstration of a figure-ground reversal. Note that either the light portion or the dark portion can be perceived as a figure against a background.

A. Laws of Perceptual Grouping

The factors which determine perceptual grouping are:

  1. Proximity: All other factors being equal, stimuli that are near one another tend to be grouped together. For example, if four stand near one another and a fifth 10 feet away, the adjacent four will be seen as a group and the distant fifth as an outsider. Events that are close in time and space are also perceived together.

  2. Similarity: Stimuli that are similar in size, shape or color tend to be grouped together.

  3. Continuity: Perception tends toward simplicity and continuity. Even if there are dots in a circular fashion, the person will see them as a complete circle.

  4. Closure: It is the tendency to complete a figure that is incomplete but has a consistent overall form.

  5. Common region: Stimuli that are found within a common area tend to be seen as a group.

B. Perceptual Constancy

Perception of an object’s shape, size or brightness remains the same even though its image on the retina has changed. This is called perceptual constancy and is found in all senses.

If the perceived size of an object remains the same, even though the size of its image on the retina changes it is called size constancy. In shape constancy the shape of an object remains stable even though the shape of its retina image changes.

Brightness constancy refers to the fact that the brightness of objects appears to stay the same as lighting conditions change.

C. Depth Perception

It is the ability to see three-dimensional space and to accurately judge distances. Without depth perception you can’t ride on a motorcycle, or drive a car, catch a ball, thread a needle or simply walk around a room. The world would look like a flat surface. The ability of depth perception is partly innate and partly learned.

Depth cues are features of environment and messages from the body that supply information about distance and space. The cues which work with just one eye are called monocular cues and those which require two eyes are called binocular cues. Binocular cues are the most basic source of depth perception that is caused due to retina disparity (a discrepancy in the images that reach the right and left eyes). A person with one eye will have very limited depth perception.

Pictorial cues for depth are features found in paintings, drawings and photographs that impart information about space, depth and distance. This influence causes apparent perception of things which are not there. For example, if you stand between two railway tracks, they appear to meet at the horizon, even though they actually remain parallel.

Illusions

Illusions are distorted perception of stimuli that exist, whereas hallucination is perception of objects or events that have no external reality. An example of illusion is the Muller-Lyer illusion in which horizontal line with outgoing arrow-heads appears shorter than the line with inward arrows.

The figure shows three powerful illusions. In drawing A, known as the Muller-Lyer illusion, the bottom line looks longer than the top line. Actually, they are equal in length. In drawing B, the Ponzo illusion, the top horizontal line looks longer; again, both lines are equal. In drawing C, known as the horizontal-vertical illusion, the vertical line looks longer, but both are equal.