Modifying Deutsch's scale illusion for application in music

Deutsch's scale illusion demonstrates that the overall pitch range is the preferred organization when in competition with both local (note-to-note) pitch proximity and laterality (differences in the input ear). Such intricate factors can make it difficult to mimic this illusion. If a note is under a condition in which grouping by the overall pitch range and the local pitch proximity do not conflict, we hypothesized that an illusion would be perceived simply as the result of the competition between pitch proximity and laterality. In this paper, we aimed to replicate such a condition by modifying Deutsch's scale illusion. Psychophysical studies were conducted with healthy subjects. In the first half of the study, the C major scale with successive tones was presented in ascending form, alternating between the right and left ears; counterpart notes were simultaneously presented to the opposite ear, and the subjects were asked to listen to these dichotic tone patterns. Several counterpart notes were applied; we found that when the sequences of counterpart notes were close in note-to-note pitch proximity and were not overlapped with the ascending scale in pitch, the subjects appeared to perceive the scale clearly. In the latter half of the study, we applied this condition in music and devised auditory illusions such that melodies of the passages of "Lightly Row," "Cherry Blossoms," and "Jingle Bells" were perceived by listening to "jagged" dichotic tone patterns. The method we described in this paper is simple, and it is possible to easily create auditory illusions in music by applying our method.