Psychonomic Bulletin & Review 2004, 11 (4), 642-666 Reconsidering evidence for the suppression model of the octave illusion CHRISTOPHER D. CHAMBERS and JASON B. MATTINGLEY University of Melbourne, Melbourne, Victoria, Australia and SIMON A. MOSS Monash University, Clayton, Victoria, Australia The octave illusion is elicited by a sequence of tones presented to each ear that continuously alter- nate in frequency by one octave, but with high and low frequencies always in different ears. The per- cept for most listeners is a high pitch in one ear, alternating with a low pitch in the other ear. The in- fluential suppression model of the illusion proposed by Deutsch and Roll (1976) carries three postulates: first, that listeners perceive only the pitch of the tones presented to their dominant ear; sec- ond, that this pitch is heard in whichever ear received the higher frequency tone; and third, that this ap- parent dissociation between what and where mechanisms arises from sequential interactions between the tones. In the present article, we reappraise evidence for the suppression model and demonstrate (1) the incompatibility of the theory with the existing literature on pitch perception, sound localization, and ear dominance and (2) methodological limitations in studies that have claimed to provide support for the suppression model. We conclude by proposing an alternative theory of the octave illusion that is based on established principles of fusion, rather than suppression, between ears. The octave illusion is a compelling perceptual phe- high-frequency dominance for sound localization, com- nomenon that arises when each ear is presented with an bined with ear dominance for pitch in which the domi- alternating sequence of tones separated by one octave, nant ear “exercises a steady suppression on the other but with the high- and the low-frequency tones in differ- [ear], so that only the frequencies arriving at one ear are ent ears (Figure 1). The majority of listeners are unable to heard” (p. 24). According to this theory, the interplay of accurately describe these stimuli and, instead, report a ear dominance for pitch and high-frequency dominance high pitch in one ear alternating with a low pitch in the for localization results in conflict when the high-frequency opposite ear. Deutsch (1974) noted that, when presented tone is presented to the nondominant ear. Under this con- with these stimuli via headphones, most listeners heard dition, Deutsch and Roll suggested that listeners perceive the higher pitch toward the right and the lower pitch to- the pitch presented to their dominant ear but localize this ward the left. This trend was particularly salient for right- percept in the opposite ear. The authors thus concluded handed listeners, but not for left-handed listeners. Deutsch that the octave illusion reveals separate neural mecha- (1974) suggested that this handedness effect indexed the nisms governing the what and where of auditory percep- lateralization of the higher pitch “toward the side produc- tion, which may be placed in conflict under appropriate ing the most effective input to the dominant hemisphere” conditions. Deutsch (1978, 1980a, 1988) later included a (p. 308). third tenet to the suppression model, suggesting that this The unique and influential suppression model of this unique division of object- and location-based auditory phenomenon was later proposed by Deutsch and Roll mechanisms was facilitated by sequential interactions (1976). From an analysis of subjective reports, the au- between the tones—specifically, the repeated alternation thors suggested that the octave illusion arises from a of the same frequencies between the ears. A recent study by Chambers, Mattingley, and Moss (2002) has challenged the validity of the suppression model. From a psychophysical investigation, their results Parts of this research were presented at the 28th Annual Australian Experimental Psychology Conference, April 2001. We thank Pierre Di- suggest instead that (1) the pitch variation during the oc- venyi and three anonymous reviewers for helpful comments on an ear- tave illusion may arise from established mechanisms of lier draft of this manuscript. We are also grateful to Dexter Irvine, Mel harmonic fusion, (2) the high-frequency dominance for Brown, Bruno Repp, Bob Carlyon, and Chris Darwin for valuable dis- localization proposed by the suppression model regularly cussions. Correspondence concerning this article should be addressed to C. D. Chambers, Cognitive Neuroscience Laboratory, Department of fails to emerge, and (3) both the pitch differences be- 1 Psychology, University of Melbourne, Melbourne, VIC 3010, Australia tween alternate dichotic octaves and the apparent later- (e-mail: [email protected]). alization of single dichotic octaves can occur indepen- Copyright 2004 Psychonomic Society, Inc. 642 OCTAVE ILLUSION REVIEW 643 of pitch. Section 2 shows that the suppression model is inconsistent with explanations of similar binaural phe- nomena that involve illusions of auditory localization. Sec- tion 3 demonstrates that the suppression model is incon- sistent with a related body of literature on ear dominance. Section 4 reviews three landmark investigations that ap- pear to provide evidence for suppression and sequential in- teractions but that relied on subjective report data and may, therefore, have been compromised by response bias. In section 5, we consider the contribution to this debate by two recent electrophysiological studies. Finally, in section 6, we propose an alternative theory of the octave illusion that reconciles the phenomenon with evidence for pitch perception based on harmonic fusion. 1. The Suppression Model Is Inconsistent With Theories of Pitch Perception The suppression model proposed by Deutsch and Roll (1976) suggests that during the octave illusion, listeners perceive only the frequencies presented to their dominant ear. Therefore, if the illusion is elicited with alternating se- quences of 400- and 800-Hz tones, the perceived pitch is predicted to alternate between 400 and 800 Hz. This tenet contrasts with theories of pitch perception, which predict that a dichotic complex of 400 and 800 Hz should either be perceptually segregated into two distinct pitches or be fused into a single percept with a pitch corresponding to the overall repetition rate of the complex (known as the fundamental frequency, F0). In the present section, this discrepancy will be explored through a brief review of pitch theories—specifically, those theories that deal with the pitch perception of complex tones. One of the oldest and most robust observations in psy- Figure 1. The octave illusion arises from an alternating se- choacoustics is that the pitch of harmonic complex tones quence of dichotic octaves presented via headphones (A). Most approximates F0, even when spectral energy at F0 is ab- listeners perceive these stimuli as a single tone shifting between a sent or masked, and even when harmonics are presented high pitch in one ear and a low pitch in the other ear (B). The suppression model (C) suggests that listeners perceive the pitch to different ears (Arehart & Burns, 1999; Demany & presented to their dominant ear (italicized) but localize this pitch Semal, 1988; Hermann, 1890; Houstma & Fleuren, 1991; in the ear receiving the higher frequency tone (white border). The Houstma & Goldstein, 1972; Licklider, 1954; Patterson, predicted percept according to suppression (D) thus comprises 1969; Seebeck, 1841; Small, 1955; Small & Campbell, an octave pitch alternation between 400 and 800 Hz. 1961; Thurlow & Small, 1955). This phenomenon is often referred to as virtual pitch, because the auditory system synthesizes a pitch sensation that is not physically pres- dently of sequential interactions. This interpretation con- ent in the stimulus. The perception of virtual pitch is trasts with the theoretical explanation of the illusion that thought to arise either from analysis of periodicity (e.g., has dominated opinion for the past quarter century. In Schouten, 1938, 1940a, 1940b, 1940c) or through a the present article, we consider this conflict from a the- pattern-matching mechanism that assigns pitch to the oretical standpoint and review evidence for the three most likely F0 (e.g., Terhardt, 1974). tenets of the suppression model. The purpose of this A distinction may also be drawn between virtual pitch analysis is to point out several inconsistencies between and spectral pitch. Spectral pitch arises when listeners per- the suppression model and previous research, in addition ceptually segregate a complex tone into distinct pitches, to identifying methodological limitations in past studies each relating closely to the frequency of the resolved sinu- that have been used to support a suppression hypothesis. soid (Terhardt, 1974). Thus, for a dichotic octave of 400 Section 1 of the present article considers the discrep- and 800 Hz, listeners should perceive either a single virtual ancy between the suppression model and evidence demon- pitch at F0 (400 Hz) or separate spectral pitches at 400 and strating fusion, rather than suppression, in the perception 800 Hz. Terhardt’s (1974) influential pattern-matching the- 644 CHAMBERS, MATTINGLEY, AND MOSS ory of pitch perception suggests that spectral and virtual The performance of listeners in a discrimination task pitches compete during pitch extraction, with the final has provided further evidence of harmonic fusion during percept determined by the listener’s