Deutsch's Octave Illusion

The octave illusion was discovered by Deutsch in 1973. It was first reported by Deutsch at a meeting of the Acoustical Society of America (Deutsch 1974a) and first published by Deutsch, Nature (1974b). 

This illustration shows the pattern that produces the illusion. Two tones that are spaced an octave apart are alternated repeatedly at a rate of four per second. The identical sequence is played over headphones to both ears simultaneously, except that when the right ear receives the high tone the left ear receives the low tone, and vice versa. The tones are sine waves of constant amplitude, and follow each other without amplitude drops at the transitions. So in fact the listener is presented with a single, continuous two-tone chord, with  the ear of input for each component switching repeatedly.

            Despite its simplicity, this pattern is almost never heard correctly, and instead produces a number of illusions.  Many people hear a single tone which switches from ear to ear, while its pitch simultaneously shifts back and forth between high and low.  So it seems as though one ear is receiving the pattern 'high tone - silence - high tone - silence' while at the same time the other ear is receiving the  pattern 'silence - low tone - silence - low tone'.  Even more strangely, when the earphone positions are reversed many people 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.

            The octave illusion has another surprising property - righthanders and lefthanders tend to hear this pattern in different ways. Righthanders tend to hear the high tone on the right and the low tone on the left, regardless of how the earphones are positioned. Yet lefthanders vary considerably in terms of where the high and low tones appear to be coming from.  Moreover, the tendency to hear the high tone on the right and the low tone on the left is stronger among people with only righthanders in their family than among those with a lefthanded parent or sibling. 

            Other people experience quite different illusions.  Some hear a single tone which switches from ear to ear,  whose pitch remains the same or changes only slightly as the tone appears to change location. Yet others describe quite complex perceptions. For example, some  hear a low tone which switches from ear to ear, whose pitch shifts back and forth by a semitone, together with an intermittent high tone in one ear. Other people hear a high tone which switches from ear to ear, together with an intermittent low tone in one ear. Some listeners report that the pitches appear to change gradually as the sequence continues, while for yet others, the pattern appears to speed up and slow down in unpredictable ways. Some people say that the high and low tones sound as though they were produced by different instruments - for example, the high tone may sound like a flute and the low tone like a gong. Some listeners experience different illusions at different times, so that the pattern appears to be constantly changing. A few  complex illusions that people have described are illustrated below.

Play Deutsch's Octave Illusion 

References:

Deutsch, D. An auditory illusion. Journal of the Acoustical Society of America 1974a, 55, s18-s19.

Deutsch, D. An auditory illusion.  Nature, 1974b, 251, 307-309. [PDF Document]

Deutsch, D. Musical Illusions. Scientific American , 1975, 233, 92-104.

Deutsch, D. and Roll, P. L. Separate "what" and "where" decision mechanisms in processing a dichotic tonal sequence.  Journal of Experimental Psychology: Human Perception and Performance, 1976, 2, 23-29. [PDF Document]

Deutsch, D. Lateralization by frequency for repeating sequences of dichotic 400 Hz and 800 Hz. tones. Journal of the Acoustical Society of America, 1978, 63, 184-186.

Deutsch, D. Ear dominance and sequential interactions.  Journal of the Acoustical Society of America, 1980, 67, 220-228.

Deutsch, D. The octave illusion and the what-where connection.  In R. S. Nickerson (Ed.), Attention and Performance VIII, 1980, 575-594.

Deutsch, D. The octave illusion and auditory perceptual integration.  In Tobias, J.V., and Schubert, E.D., Hearing Research and Theory, Vol. I, New York: Academic Press, 1981, 99-142. [PDF Document]

Detusch, D. The octave illusion in relation to handedness and familial handedness background.  Neuropsychologia, 1983, 21, 289-293. [PDF Document]

Deutsch, D. Auditory illusions, handedness, and the spatial environment. Journal of the Audio Engineering Society, 1983, 31, 607-618.

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. Lateralization and sequential relationships in the octave illusion, Journal of the Acoustical Society of America,  1988, 83, 365-368.

Deutsch, D. Grouping mechanisms in music. In D. Deutsch (Ed.) The psychology of music, 2nd Edition, Academic Press, 1999, 299-348. [PDF Document]

Deutsch, D. The octave illusion revisited again. Journal of Experimental Psychology: Human Perception and Performance, 2004, 30, 355-364. [PDF Document]