Illusions contain ambiguous images that allow for multiple distinct interpretations or that can lure the brain into making perceptual errors (Kandel, 2012, p. One of the most intriguing psychoacoustic effects employed in musical composition is that of auditory illusions. However, a purely mathematical approach to sound synthesis is not enough to manufacture artistically satisfying sounds, instead one has to take into account the relation between the physical structure-which the composer controls when he specifies the synthesis data-and the aural effect-which determines the musical impact (Risset, 1988). Thanks to the flexibility of programming, the physical parameters of acoustic sounds can be controlled directly and sounds can be manufactured with unprecedented reproducibility and precision. Vice versa, the study and exploitation of cognitive principles and effects have made their way into the realm of musical composition.

The digital resources of synthesis initially developed for artistically reasons have contributed significantly to our understanding of the perception of musical sound. These findings suggest that musical paradoxes may be of interest not only for the insights they provide on our perceptual system, but also for the richness of the emotional experience elicited during listening. Moreover, results indicated higher (Bonferroni-uncorrected) neuroticism for those who experienced an equilibrium disturbance relative to subjects who did not have this experience during listening to the Computer Suite. Openness to experience correlated positively with positive emotion ratings for the Computer Suite, while agreeableness correlated negatively with positive emotion ratings for the Matlab stimulus. Finally, we found a complex pattern of relationships between personality and the subjective experience of the glissando. Moreover, generally for the Shepard-Risset glissando illusion, higher negative emotional ratings were given by individuals who had experienced a feeling of disturbance of equilibrium relative to those who had not had this experience. We also found that the Shepard-Risset glissando illusion, both within the aesthetic context of a musical composition and on its own, was capable of evoking disruption of equilibrium, frequently leading to the associated feeling of falling. The results show that negative emotions were most strongly evoked during listening to each of the stimuli. Personality was also measured with the Five-Factor Model of personality traits. Seventy-three volunteers completed a listening experiment during which they rated their emotional response to these stimuli on a seven-point Likert scale and indicated whether they had experienced a disruption of equilibrium. For this purpose we chose three musical stimuli: a Matlab-generated Shepard Risset glissando, Jean-Claude Risset's Computer Suite from Little Boy, which presents a Shepard-Risset glissando integrated in the aesthetic context of a composition, and an ordinary orchestral glissando taken from the opening of Iannis Xenakis's Metastasis.

In the present study, we investigated the emotional states induced by the Shepard-Risset glissando, a variant of the Shepard scale. Music psychology and neuroscience has been interested in this particular psychoacoustic phenomenon mainly for studying the cognitive processes of pitch perception involved. The endless scale illusion, obtained by cyclically repeating a chromatic scale made up of Shepard tones, has been used in a variety of musical works.

Shepard tone illusion