Octave Illusion

Discovered by Diana Deutsch in 1973, the octave illusion is an auditory illusion produced by simultaneously playing two sequences of two notes that are spaced an octave apart, high to low, and low to high, in separate stereo channels over headphones. The tones are sine waves of constant amplitude, and switch between high and low four times a second, with no amplitude drops at the transitions. So the listener is presented with a single, continuous two-tone chord, with the ear of input of each component switching repeatedly. (Listen over headphones to the example linked below.)

This pattern is almost never heard correctly, and instead gives rise to a number of illusions. Most people hear a single tone that switches between left and right ears while its pitch simultaneously switches back and forth between high and low. Most right-handers hear the high tone as on the right and the low tone as on the left. When the earphones are reversed most right-handers hear the same thing – the tone that had appeared in the right ear still appears in the right ear and the tone that had appeared in the left ear still appears in the left ear.

Other people experience entirely different illusions; for example they may hear a single high tone in the left ear alternating with a single low tone in the right ear, or obtain complex perceptions that involve three or four tones of different pitch. Right-handers and left-handers vary statistically in how they perceive the octave illusion, with left-handers less likely to hear the high tone on the right, and more likely to obtain complex perceptions.