The Octave Illusion Revisited Again

The octave illusion (D. Deutsch, 1974) occurs when 2 tones separated by an octave are alternated repeatedly, such that when the right ear receives the high tone, the left ear receives the low tone, and vice versa. Most subjects in the original study reported hearing a single tone that alternated from ear to ear, whose pitch also alternated from octave to octave, and D. Deutsch (1975a) proposed an explanation in terms of separate what and where auditory pathways. C. D. Chambers, J. B. Mattingley, and S. A. Moss (2002) argued that the perceived pitch difference generally corresponds more to a semitone and proposed an alternative explanation in terms of diplacusis. This article argues that Chambers et al. used problematic procedures and reports a new experiment on the octave illusion. The findings confirm that an octave difference is generally perceived, and they agree with the model of Deutsch (1975a) but are at variance with the diplacusis hypothesis. The octave illusion, which was originally described by Deutsch (1974), is a paradoxical auditory phenomenon that is characterized by large individual differences in perception. The pattern that was first used to create this illusion consisted of two tones that were spaced an octave apart and that were repeatedly presented in alternation. The identical sequence was presented via headphones to both ears simultaneously; however, when the right ear received the high tone, the left ear received the low tone, and vice versa. This pattern gave rise to a number of different illusory percepts, the most common one (termed octave) being of a single tone that alternated from ear to ear, whose pitch also alternated from one octave to the other in synchrony with the localization shift. Deutsch (1975a) proposed a model to account for the octave percept, based on a hypothesized separation between what and where decision mechanisms in the auditory system. The model, hereafter referred to as the two-channel model,assumes that (a) to produce the perceived pitches, the frequencies arriving at one ear are perceived, while those arriving at the other ear are suppressed from conscious perception and that (b) each perceived tone is localized at the ear receiving the higher frequency signal, regardless of whether a pitch corresponding to the higher or lower frequency is in fact perceived. The model therefore assumes that the octave illusion results from illusory conjunctions of pitch and location values. Chambers, Mattingley, and Moss (2002) argued from a series of experiments that the phenomenology of the octave illusion differs from that originally described by Deutsch (1974). More specifically, they asserted that, on listening to the octave illusion, the perceived difference between the alternating tones generally corresponds more to a semitone than to an octave. On this basis, the authors hypothesized that the tones at the two ears fuse harmonically to produce a pitch that corresponds to the low tone, 1 and that the slight pitch difference between the alternating tones that is perceived is the result of diplacusis. 2 They also reported that some of their subjects lateralized each tone to the ear receiving the higher frequency and that some lateralized each tone to the ear receiving the lower frequency, though they did not offer an explanation for the lateralization patterns they obtained. In this article, I first review early findings concerning the phenomenology of the octave illusion and describe the twochannel model that was proposed to explain the octave percept. There follows a critique of the study by Chambers et al. (2002), which questions the validity of their observations. Because their hypothesis could hold only if these observations were valid, this hypothesis is also challenged. Finally, a new experiment is reported in which the phenomenology of the octave illusion is documented more explicitly than in previous studies. The findings from this new experiment confirm that an octave difference between the alternating tones is generally perceived, and they are in accordance with the two-channel model but cannot be explained on the diplacusis hypothesis.