Here are two different melodies played one after the other, immediately followed by them both at once. What do you hear when they are played together?
On its own, neither melody gives the impression of an ascending or descending pattern. When the two melodies are played together however, most people hear this combination as being two scales that either ascend or descend in unison together.
Our brain groups sounds according to similar characteristics. When there are multiple ways to group something, a further hierarchy sets in. In this case, the close proximity of interleaved pitches between the parts takes precedence over individual melodies, creating the illusion of a third melodic trajectory. Interestingly, this illusion continues to occur over headphones when the two parts are simultaneously introduced, one to each ear.
This video is the second part in a new series of auditory illusion demonstrations adapted for musical instruments and voice. Some of this work appeared originally in a performance at Davies Symphony Hall for Pop-Up Magazine.
The Scale Illusion was discovered by Diana Deutsch.
Recorded by: Jeremy Harris and Meara O'Reilly
Performed by: Andrew Maguire
Animation: Isaiah Saxon