What Auditory Illusions Reveal About the Nature of Sound

Tchaikovsky's handwritten score for the opening of his 6th Symphony's final movement gives no single violin part the full theme, yet audiences hear a continuous theme and accompaniment when it is performed. Nikisch rewrote the score to literally assign the theme to one violin section, producing a result that sounds identical to audiences. The dispute between the two raises the question of which one was experiencing an illusion.

Diana Deutsch's scale illusion sends alternating high and low tones separately into each ear, but listeners hear all the high tones as coming from one side and all the low tones from the other, based on handedness rather than actual input. This is top-down processing, where the mind imposes a probable pattern on the world rather than registering the real physical arrangement of sound.

Casey O'Callaghan draws on Wittgenstein's duck-rabbit distinction between seeing and seeing as to explain cases like sine wave speech and the mysterious melody, where identical sound is first heard as meaningless noise and then, once primed with knowledge, is heard as speech or as a recognizable tune. This suggests much of auditory experience is a construction built from raw sound plus prior knowledge.

Some philosophers argue from the existence of illusions that there are no sounds in the external world at all, only mental constructions. O'Callaghan disagrees, holding that sounds are real things in the environment with genuine properties like frequency, timbre, and loudness, even though features like location and melody may be partly constructed by the mind.

Deutsch's tritone paradox presents an ambiguous pair of tones that different listeners consistently and confidently hear as moving in opposite directions, some hearing it ascend and others descend. This shows that even relative pitch direction is not simply given by the sound itself but is resolved differently by different minds, unlike the scale illusion where the explanation is well understood.

Christine Howlett argues that in the 19th-century seating arrangement, with first violins on the left facing the audience and second violins on the right facing away, there would actually have been a real balance difference in the sound reaching listeners. She suggests Tchaikovsky may have intended the audience to hear the jumping, disjoint quality of his original scoring, and that Nikisch's change was motivated partly by tuning difficulties when violinists jump between notes rather than play a smooth scale.

Howlett notices that Tchaikovsky rescored the same theme during the recapitulation to match Nikisch's simpler arrangement, meaning Tchaikovsky expected listeners to hear it as the same theme both times despite two different underlying scorings. Barry Lam suggests this was deliberate, tied to the symphony's meditation on death, masking a theme at the beginning before revealing it unambiguously as the movement approaches its bleak ending.

Barry Lam notes a philosophical tradition that argues from auditory illusions to the conclusion that sounds are entirely mental constructions with no independent existence in the world. Casey O'Callaghan takes the opposing position, holding that sounds are real things in the environment with genuine physical properties like frequency and timbre, even while granting that some auditory features are constructed by the mind.

Barry Lam frames the historical dispute as unresolved: one of the two men was subject to an illusion while the other heard the piece as it truly was, but it is unclear which. Howlett's analysis suggests Tchaikovsky deliberately intended the disjoint, harder-to-hear version, implying Nikisch's rescoring, though practically identical in effect, departed from the composer's real intention.