Multistable auditory perception

Multistable auditory perception is a thoroughly investigated cognitive phenomena in which certain auditory stimuli can be perceived in multiple ways. While it has been most commonly studied in the visual domain, this phenomenon also has been observed in the auditory and olfactory modalities. In the olfactory domain, different scents are piped to the two nostrils, while in the auditory domain, researchers often examine the effects of binaural sequences of pure tones. Generally speaking, multistable perception has three main characteristics: exclusivity, implying that the multiple perceptions cannot simultaneously occur; randomness, indicating that the duration of perceptual phases follows a random law, and inevitability, meaning that subjects are unable to completely block out one percept indefinitely.

While binocular rivalry has been studied since the 16th century, multistable auditory perception is relatively new. Diana Deutsch was the first to discover multistability in human auditory perception, in the form of auditory illusions involving periodically oscillating tones.

Different experimental paradigms have since been used to study multistable perception in the auditory modality. One is auditory stream segregation, in which two different frequencies are presented in a temporal pattern. Listeners experience alternating percepts: one percept is of a single stream fluctuating between frequencies, and the alternative percept is of two separate streams repeating single frequencies each.

Other experimental findings demonstrate the verbal transformation effect. In this paradigm, the input is a speech form repeated rapidly and continuously. The alternating percepts here are words—for example, continuous repetition of the word “life” results in the bistability of “life” and “fly.” Prefrontal activation is implicated with such fluctuations in percept, and not with changes in the physical stimulus, and there is also a possible inverse relationship between left inferior frontal and cingulate activation involved in this percept alternation.

The temporal dynamics observed in auditory stream segregation are similar to those of bistable visual perception, suggesting that the mechanisms mediating multistable perception, the alternating dominance and suppression of multiple competing interpretations of ambiguous sensory input, might be shared across modalities. Pressnitzer and Hupe analyzed results of an auditory streaming experiment and demonstrated that the perceptual experience that occurred exhibited all three properties of multistable perception found in the visual modality—exclusivity, randomness, and inevitability.

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