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Hall of Illusions

Illusion of Circular Pitch


Click on the Photograph of the ever rising staircase to hear the ever rising scale.

     An auditory illusion of an endlessly rising or descending scale can be obtained by playing on a keyboard an ascending or descending chromatic or diatonic scale using four parallel octaves simultaneously. If we had three hands, we could use six parallel octaves, and that would be even better! The larger number of tones the better the effect. You perceive the tones as continuously increasing or decreasing in pitch; however, after traveling over an octave, they are the same in pitch as when you first started.

     In 1964 psychologist Roger Shepard developed this illusion using the chromatic scale, and computer produced tones ranging over ten octaves. Shepard's tonal illusion is a musical equivalent of the Penrose staircase (seen above) which seemingly rises forever, but nevertheless always comes back to the same starting point. The version heard here uses the C major diatonic scale and a four octave range. The scale was generated on a synthesizer and was harmonized by American composer Ruth Schonthal to make it musically more accessible.

     The illusion, in principle, can be played on any keyboard instrument, but would take considerable practice and skill. The illusion is much more startling when you actually see it played, because what you expect to hear and see are in direct conflict with what you actually hear and see. To get this illusion to work have the tones in the middle of your range the loudest and those tones at either end the softest. If you are using a computer or synthesizer to shape the loudness of the tones, use a fixed bell-shaped amplitude envelope.


     In perceiving this illusion your brain uses proximity between tones that follow each other in time. For example, when you go from C to C sharp your brain chooses to hear an ascending pattern since this is a small distance in pitch rather than a descending pattern which would be a large distance in pitch. The effect is analogous to apparent motion (like a neon light display) where you perceive apparent motion between lights that come on and off and are close to each other in space.

     At present we do not know where in the brain this illusion is occurring.

     To find out more about auditory and musical illusions, please visit the web site of Diana Deutche at the University of San Diego. She also has a musical illusion CD for sale!

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