Illusion
In order to answer this question we have to go back to the concepts of sensation and perception. Let us start with sensation. Our bodies, and for that matter most living things, have special cells called sensory receptors. They can detect light, sound, temperature, pressure, odor, taste, pain, pressure, balance, etc. Most of these receptors are on or near the surface of the body such as our eyes, ears, tongue, etc., but some are internal. Thus, we sometimes experience a sense of pain with a toothache, headache, stomachache, etc.
Each sensory receptor detects its own special form of energy and transmits a signal to the brain. The reception of the signal in the brain represents sensation. How the brain interprets these signals and makes them meaningful is called perception. Most of the time the interpretation of the received energy is consistent with it. Sometimes, however, our interpretation is incorrect. These misinterpretations are called illusions.
When we witness an illusion, we perceive something that does not correspond to what is actually out there—what exists in the real world. Illusions fool us. They convince us of things that are not true. Dictionary definitions of illusion usually state that an illusion is a sensory perception that causes a false or distorted impression, or a misrepresentation of a "real" sensory stimulus. While these definitions are correct, psychologists have also included a number of phenomena where there is no necessary incorrect interpretation of a stimulus but rather that there are two or more quite different interpretations from a single stimulus, but never more than one at a given moment. We will give examples of such illusions when we discuss visual illusions called figure-ground illusions and ambiguous figures. In these cases two or more different interpretations of a figure can be seen, but never at the same time.
