An optical illusion (also  called a visual illusion) is  characterized by visually  perceived images that differ  from objective reality. The  information gathered by the  eye is processed in the brain  to give a percept that does  not tally with a physical  measurement of the  stimulus source. There are  three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, color, movement), and cognitive illusions where the eye and brain make unconscious inferences. They can also be known as “mind games”.

Physiological illusions, such as the afterimages following bright lights, or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type – brightness, tilt, color, movement, etc. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes aphysiological imbalance that alters perception.

The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround. Lateral inhibition has also been used to explain the Hermann grid illusion, but this has been disproved. “empirical” approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled.

Types Of Illusions:

Physiological illusions

Cognitive illusions

Explanation of cognitive illusions