User:3d4u5e/Multistable Auditory Perception

Multistable perceptions are a thoroughly investigated cognitive phenomena in which a certain stimuli can be perceived in multiple ways. While the most commonly studied form of this is in the visual domain, this phenomena also has been observed in the auditory and olfactory domains. 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 is characterized by three things; exclusivity, implying that the multiple perceptions cannot simultaneously occur, randomness, which is indicative of the duration of perceived times following a random law, and inevitability, meaning that subjects are unable to completely block out one perception indefinitely. (Blake & Logothetis 2002)

History
While binocular rivalry has been studied since the 16th century, multistable auditory perception is relatively new. (Blake 2001) Diana Deutsch was the first to discover multistability in our auditory perception, in the form of auditory illusions involving periodically oscillating tones (Deutsch 1974).

Experimental Findings
Several experimental paradigms have been used to study multistable perception in the auditory modality. One is auditory stream segregation, in which a Tone A and a tone of different frequency Tone B 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. 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 common across modalities (Pressnitzer and Hupe 2006). 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 (Sterzer et al 2009).

Principles of Perceptual Bistability
Pressnitzer & Hupe 2006 analyzed results of an auditory streaming experiment and demonstrated that the perceptual experience that occurred exhibited the all properties of multistable perception found in the visual modality. Exclusivity was met, as there was “spontaneous alternation between mutually exclusive percepts,” and little time was spent in an “indeterminate” experience. Randomness also characterized the phenomenon, as the first phase of perception is longer in duration than subsequent phases, and then the “steady-state of the temporal dynamics of auditory streaming is purely stochastic with no long-term trend.” Lastly, the percept alternation was inevitable; even though volitional control did reduce suppression of the specified percept, it did not exclude perception of the alternative percept altogether. The similarities between the temporal dynamics of perceptual bistability in the visual and auditory modalities raise the possibility of a common mechanism governing the phenomenon. Pressnitzer & Hupe’s analysis of the distribution of phase durations for the two modalities

Sequential Grouping
A problem of large behavioral importance is the question of how to group auditory stimuli. When a continuous stream of auditory information is received, numerous alternative interpretations are possible, but individuals are only consciously aware of one (or very few) percepts at a time. For this to occur, the auditory system must segregate and group incoming sounds, the goal being to “construct, modify, and maintain dynamic representations of putative objects within its environment” (Winkler et al 2012). It has been suggested that this process of binding sound events into groups is driven by different levels of similarities. One principle for binding is based on the perceptual similarity between individual events. Sounds that share many or all of their acoustic features are more likely to have been emitted by the same source, and thus are more likely to be linked to form a “proto-object” (Winkler et al 2012). The other principle for binding is based on the sequential predictability of sound events. If events reliably follow each other, it is also more likely that they have a common underlying cause.

Competition
A theory explaining the alternation of auditory percepts is that different interpretations are neurally represented simultaneously, but all but the dominant one at the time are suppressed. This idea of competition among parallel hypotheses might provide an explanation for the temporal dynamics observed in auditory stream segregation. The initial perceptual phase is held longer than the subsequent ones, “with the duration of the first phase being stimulus-parameter dependent and an order of magnitude longer in duration than parameter-independent subsequent phases” (Denham et al 2010). At stimulus onset, the first percept might be the easiest to discover, based on featural proximity (consistent with stimulus-parameter dependence), and it is held for relatively longer because time is required for other hypotheses to form. As more sensory information is received and processed, the “neural associations underlying the alternative sound organizations become strong and start to vie for dominance” and “the probabilities of perceiving different organizations tend to become more balanced with time” (Winkler et. 2012).