The octave illusion is an auditory illusion discovered by Diana Deutch in 1973 which is produced when two tones that are an octave apart are played simultaneously, in stereo, through headphones, switching between ears. Instead of hearing two alternating pitches, most subjects instead heard a single tone oscillating between ears and in pitch.
The two tones used were pitched at 400 Hz and 800 Hz, corresponding roughly to G4 and G5 in modern pitch notation. The tones were played for 250 ms before switching ears. No gaps were allowed between each set of pitches. Because the tones switched ears, both tones were always present during the experiment; only the ears perceiving them at any one time changed. Hand dominance of each subject was also noted. After the initial test, the headphones were reversed, and the test was repeated.
Out of the initial 86 subjects tested, none perceived the tonal pattern correctly. Most subjects heard a single tone that alternated in pitch as it oscillated between ears. Some only heard a single tone that moved between ears but did not change in pitch. Oddly, a few heard this single tone changing pitch very slightly. Several heard various ‘complex’ illusions, as Deutch categorized them, such as two alternating pitches in one ear and a third that sporadically occurred in the other.
Hand dominance also played an important role in the experiment. 58% of right-handed subjects and 52% of left-handed subjects heard a single pitch that switched between octaves and ears. However, nearly three times as many right-handed subjects heard a tone that switched ears but not pitch, when compared to left-handed subjects. Left-handed subjects were also slightly more likely to hear the high tone in their left ear and low tone in their right during both experiments, while right-handed subjects were much more likely to hear the high tone localized to their right ear during both tests. It is thought that two different factors are at work here: 1) high pitches are given precedence by the human brain, and 2) tones in the dominant ear are given precedence over tones in the non-dominant ear. Therefore, the brain becomes confused as to which tone to focus on and picks either a single tone or a single ear at a time to which it will listen. Deutch also believes that the degree of cerebral dominance may be reflected in the results. This conclusion is known as the suppression model.
Other Experiments by Deutch
Deutch conducted several more experiments to explore various implications of the octave illusion. In 1976 Deutch and Roll conducted an experiment in which 44 right-handed subjects were played a repeating pattern of tones pitched at 400 Hz and 800 Hz. This time the right ear was given three 800 Hz pitches alternating with two 400 Hz pitches, while the left ear heard three 400 Hz pitches alternating with two 800 Hz pitches simultaneously. A 250 ms pause was added between each pattern. Subjects were asked to report how many high tones and how many low tones they heard and in which ears they heard tones. The results were consistent with the initial experiment. Deutch conducted another group of experiments beginning with the same model and asked the subjects to report whether the pattern began with a high pitch or low, in order to determine which ear the subject was following for pitch. The amplitude of the unheard pitch was then manipulated in order to see how large it needed to be to counteract the effect. Deutch determined that a significant amplitude disparity was needed to counteract the effect. It was also determined that, when both tones were not present at the same time, the illusion was broken.
Alfredo Brancucci, Caterina Padulo, and Luca Tommasi argue that the octave illusion should be renamed the Deutch illusion since, according to their findings, the illusion is not limited to the octave. They performed an experiment similar to Deutch’s original, except the two tones used ranged in interval from a minor third to an eleventh. The tones were presented initially for 200 ms before switching ears, then again for 500 ms. While the illusion was present for several people at all intervals, the wider intervals more consistently made the illusion occur.
Christopher Chambers,Simon Moss, and Jason Mattingley believe that the illusion is caused by a combination of harmonic fusion and binaural diplacusis, a condition in which a pitch is perceived differently between ears. In the course of their experiments, not a single subject reported what they refer to as the ‘standard percept’ in which ‘a single pitch alternated between the ears, with the higher pitch towards the right, the lower pitch towards the left, the pitch variation equivalent to one octave, and the percept lateralized toward the higher frequency component within each dichotic octave.’ This does not necessarily conflict with other studies, as the other experiments do not clarify whether the high tone and low tone heard are perceived as being an octave apart. However, their results were not consistent with Deutch’s results in relation to hand dominance and ear dominance versus location of pitch.
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- Alfredo Brancucci, Caterina Padulo, and Luca Tommasi, “”Octave Illusion” or “Deutch’s Illusion”?,” Psychological Research 73 No. 3 (May 2009): 305, accessed 30 September 2013, doi:10.1007/s00426-008-0153-7.
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- Christopher D. Chambers, Jason B. Mattingley, and Simon A. Moss, “The Octave Illusion Revisited: Suppression or Fusion Between Ears?,” The Journal of Experimental Psychology. Human Perception & Performance 28 No. 6 (December, 2002): 1290, accessed 10 October 2013, doi: 10.1037/0096-15188.8.131.528.
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- Chambers, et al, "The Octave Illusion," 1293.