The ever-decreasing glissando
Click on the ear to listen to a sliding scale.
(The sound file will play in a new window. When it has finished playing, close the window to return to this page.)
Is it getting higher or lower?
Most people will hear the glissando as continually descending in pitch, yet ultimately getting no lower. This illusion was initially created by the cognitive scientist Roger Shepard who put together a series of pure tones separated by octaves on the musical scale.
The ever-decreasing effect is achieved when the volume of the highest and lowest tones are adjusted to half that of the middle note. Because the high and low notes are equally likely to determine the pitch, the brain cannot resolve which one should dominate and the result is ambiguous.
Musician Jean-Claude Risset developed the version you hear here, known as the Shepard-Risset scale, in which the notes in the scale are continuous. The illusion is the auditory equivalent of Escher’s impossible staircase!
(The sound file was programmed by Gloumouth1 and uploaded to the Commons project: http://commons.wikimedia.org/wiki/Image:DescenteInfinie.ogg)