Stability of Perceptual Organisation in Auditory Streaming
In everyday situations, we perceive sounds organised according to their source, and can follow someone’s speech or a musical piece in the presence of other sounds without apparent effort. Thus, it is surprising that recent evidence obtained in the most widely used experimental test-bed of auditory scene analysis, the two-tone streaming paradigm, demonstrated extensive bistability even in regions of the parameter space previously thought to be strongly biased towards a particular organisation. This raises the question of what aspects of the rich natural input allow the auditory system to form stable representations of concurrently active sound sources. Here, we report the results of perceptual studies aimed at testing this issue. It is possible that the extreme repetitiveness of the alternating two-tone sequence, i.e. lack of change, causes perceptual instability. Our first experiment addressed this hypothesis by introducing random changes in the stimulation. It is also possible that under natural conditions, multiple redundant cues stabilise perception. The second experiment tested this hypothesis by adding a second cue which favoured one organisation. Much to our surprise, neither one of these manipulations stabilised the perception of two-tone streaming sequences. We discuss these experimental results in the light of our previous theoretical proposals and findings of significant differences between the first and later perceptual phases. We argue that multi-stability is inherent in perception. However, it is normally hidden by switches of attention, which allow the return of the dominant perceptual organisation resulting in the subjective experience of perceptual stability. In our third experiment, we explored this possibility by inserting short gaps into the sequences, since gaps have been shown to reset auditory streaming in a manner similar to switches in attention.
KeywordsAuditory streaming Bistability Perceptual switching Auditory scene analysis
This work was supported by the European Research Area Specific Targeted Projects EmCAP (IST-FP6-013123), and SCANDLE (IST-FP7-231168).
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