An auditory illusion can be thought of as a distortion of sound or a misinterpretation of sound. An auditory illusion is the sound equivalent of a visual or optical illusion. Auditory illusions seem to be less common than optical illusions, perhaps because only 20% of the population is estimated to have an auditory preference (sounds register in their brains faster and more intensely than other types of sensory data). In an auditory illusion, the listener hears either “impossible” sounds or sounds that are not actually present in the stimulus. The McGurk effect is one example. This illusion occurs when the auditory component of one sound is paired with the visual component of another sound, leading to the perception of a third sound. The Shepard tone is another example: an auditory illusion of a tone that continuously descends, while really not getting any lower. You can hear an example of the Shepard illusion at the following URL.