In auditory space, there exist zones of systematic ambiguity termed “cones of confusion” (Begault, 1994), where sounds create identical interaural time differences (ITD) and interaural level differences (ILD, see Section Materials and Methods). For instance, the Virtual Barber Shop ( 1996) illustrates a virtual auditory space, where the sounds of a haircut are modulated binaurally to create a surrounding percept.
This allows for the perception of auditory sound sources not only in the front or to the sides, but also behind the observer. For instance, in the Shepard Tone illusion (Shepard, 1964 Deutsch, 1992), the perceived pitch of a sound paradoxically rises or drops continuously.Ī foremost advantage of audition compared to vision is that it represents a three-dimensional sphere surrounding the listener, rather than being restricted to the frontal hemisphere. While most of these documented illusions are visual, multi-stable auditory illusions exist as well. In particular, bi-stable and multi-stable perceptual illusions (e.g., Rubin Vase, Necker Cube, Ames Window, the Spinning Dancer) have revealed how versatile and flexible human perception can be: when multiple interpretations of an external stimulus are possible, the observer might spontaneously switch between two representations of the same physical stimulus (Sterzer et al., 2009). Illusions are a delight to our playful minds, and artists, magicians, and scientists have long been searching for ways to create multiple meanings out of a single picture, sound, video or physical object. This multi-stable auditory illusion opens opportunities for studying the perceptual, cognitive, and neural representation of objects in motion, as well as exploring multimodal perceptual awareness. Participants were biased towards perceiving transverse compared to bouncing paths, and they became perceptually locked into alternating between front-to-back and back-to-front percepts, perhaps reflecting how auditory objects commonly move in the real world. In a second experiment, the illusory stimulus was looped continuously in headphones while participants identified its perceived path of motion to test properties of perceptual switching, locking, and biases. These results show that this illusion is effectively quadri-stable. Participants perceived all four illusory motion percepts, and could not distinguish the illusion from the unambiguous stimuli. When asked to rate their confidence in perceiving each sound’s motion, participants reported equal confidence for the illusory and unambiguous stimuli.
Here we demonstrate how human listeners experience this illusory phenomenon by comparing ambiguous and unambiguous stimuli for each of the four possible motion percepts.
The sound can be perceived as traveling transversely from front to back or back to front, or “bouncing” to remain exclusively in front of or behind the observer. Here, we introduce a novel quadri-stable illusion, the Transverse-and-Bounce Auditory Illusion, which combines front-back confusion with changes in volume levels of a nonspatial sound to create ambiguous percepts of an object approaching and withdrawing from the listener. However, the human auditory system has unequally distributed spatial resolution, including difficulty distinguishing sounds in front vs. One way we estimate the motion of an auditory object moving towards or away from us is from changes in volume intensity. In addition to vision, audition plays an important role in sound localization in our world.
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