Octave illusion

The octave illusion is an auditory illusion discovered by Diana Deutsch in 1973. It is produced when two tones that are an octave apart are repeatedly played in alternation ("high-low-high-low") through stereo headphones. The same sequence is played to both ears simultaneously; however when the right ear receives the high tone, the left ear receives the low tone, and conversely. Instead of hearing two alternating pitches, most subjects instead hear a single tone that alternates between ears while at the same time its pitch alternates between high and low.

The two tones used were pitched at 400 Hz and 800 Hz, corresponding roughly to G4 and G5 in modern pitch notation. Each tone was played for 250 ms before switching ears. No gaps were allowed between tones. Both tones were therefore always present during the experiment; only the ears perceiving the high and the low tone at any one time changed repeatedly. After the initial test, the headphones were reversed, and the test was repeated.

86 subjects were tested, and none perceived the tonal pattern correctly. Most subjects heard a single tone that alternated in pitch by an octave as it alternated between ears. When the earphones were reversed, the ear that initially heard the high tone continued to hear the high tone, and the ear that initially heard the low tone continued to hear the low tone. Some subjects only heard a single tone that moved between ears but did not change in pitch, or changed very slightly. Several subjects heard various "complex" illusions, such as two alternating pitches in one ear and a third pitch that sporadically occurred in the other ear.

Handedness also played an important role in the results. 58% of right-handed subjects and 52% of left-handed subjects heard a single pitch that switched between octaves as it switched between ears. Of the remaining subjects, nearly three times as many right-handers than left-handers heard a tone that switched ears but not pitch. Left-handed subjects were varied in their localization of the high and low tones, while right-handed subjects were much more likely to hear the high tone localized to their right ear during both tests.

Deutsch proposed that when a single tone that alternates between octaves is heard, this illusion results from the combined operation of two decision mechanisms. First, to determine the tone's location, high pitches are given precedence; second, to determine the tone's pitch, tones in the dominant ear are given precedence over tones in the non-dominant ear. This is known as the two-channel model, since it is proposed that the operation of two separate "what" and "where" decision mechanisms combine to produce the illusion. The model is illustrated here.

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