The scale illusion was discovered by Diana Deutsch in 1973. It was first reported by Deutsch at a meeting of the Acoustical Society of America (Deutsch 1974) and first published by Deutsch, Journal of the Acoustical Society of America, 1975. The pattern that produces the scale illusion is shown in Figure A. This consists of a major scale with successive tones alternating from ear to ear. The scale is played simultaneously in both ascending and descending form; however when a tone from the ascending scale is in the right ear a tone from the descending scale is in the left ear, and vice versa (Figure B shows these ascending and descending scales separately, and you can see that the pattern shown in Figure A is produced by the superposition of the patterns shown in Figure B). The tones are equal-amplitude sine waves, and the sequence is played repeatedly without pause at a rate of four tones per second.
An illusion that people frequently experience when listening to the scale illusion through earphones is shown in Figure C. A melody corresponding to the higher tones appears to be coming from one earphone, and a melody corresponding to the lower tones appears to be coming from the other one. When the earphone positions are reversed, the ear that had heard the higher tones continues to hear the higher tones, and the ear that had heard the lower tones continues to hear the lower tones.
As with the octave illusion, righthanders and lefthanders tend to differ in how they hear this pattern. Righthanders tend to hear the higher tones on the right and the lower tones on the left, whereas lefthanders are less likely to localize the tones in this way.
People have also described a number of different perceptions on listening to the scale illusion. Some of these are illustrated below.
Follows is sound example (make sure you listen in stereo):
The video below features Deutsch's Scale illusion being performed and experienced by the fifth graders at Atwater School, Shorewood, Wisconsin. The video was created by their music teacher Walt Boyer, and is posted with permission.
Deutsch, D. An illusion with musical scales. Journal of the Acoustical Society of America, 1974, 56, s25.
Deutsch, D. Two-channel listening to musical scales. Journal of the Acoustical Society of America, 1975, 57, 1156-1160. [PDF Document]
Deutsch, D. Musical Illusions. Scientific American , 1975, 233, 92-104.
Deutsch, D. Binaural integration of melodic patterns. Perception and Psychophysics, 1979, 25, 399-405. [PDF Document]
Deutsch, D. Auditory illusions, handedness, and the spatial environment. Journal of the Audio Engineering Society, 1983, 31, 607-618.
Deutsch, D. Dichotic listening to melodic patterns, and its relationship to hemispheric specialization of function. Music Perception, 1985, 3, 1-28. [PDF Document]
Deutsch, D. Auditory pattern recognition. In K. Boff, L. Kaufman and J. Thomas (Eds.), Handbook of Perception and Human Performance, Wiley, 1986. 32, 1-44.
Deutsch, D. Illusions for stereo headphones. Audio Magazine, March, 1987, 36-48. [PDF Document]
Deutsch, D. Grouping mechanisms in music. In D. Deutsch (Ed.) The psychology of music, 2nd Edition, Academic Press, 1999, 299-348. [PDF Document]