Abstract
Three experiments explored the effects of abrupt changes in stimulus properties on streaming dynamics. Listeners monitored 20-s-long low- and high-frequency (LHL-) tone sequences and reported the number of streams heard throughout. Experiments 1 and 2 used pure tones and examined the effects of changing triplet base frequency and level, respectively. Abrupt changes in base frequency (±3-12 semitones) caused significant magnitude-related falls in segregation (resetting), regardless of transition direction, but an asymmetry occurred for changes in level (±12 dB). Rising-level transitions usually decreased segregation significantly, whereas falling-level transitions had little or no effect. Experiment 3 used pure tones (unmodulated) and narrowly spaced (±25 Hz) tone pairs (dyads); the two evoke similar excitation patterns, but dyads are strongly modulated with a distinctive timbre. Dyad-only sequences induced a strongly segregated percept, limiting scope for further build-up. Alternation between groups of pure tones and dyads produced large, asymmetric changes in streaming. Dyad-to-pure transitions caused substantial resetting, but pure-to-dyad transitions sometimes elicited even greater segregation than for the corresponding interval in dyad-only sequences (overshoot). The results indicate that abrupt changes in timbre can strongly affect the likelihood of stream segregation without introducing significant peripheral-channeling cues. These asymmetric effects of transition direction are reminiscent of subtractive adaptation in vision.
Highlights
Auditory stream segregation is the process by which sounds are grouped perceptually to form coherent representations of objects and events in the auditory scene (Bregman, 1990)
Our pilot observations suggested that tone sequences whose triplet base frequency changed in this way were broadly as effective at inducing build-up as tone sequences with constant base frequency, despite the differences in peripheral channeling between the two types of stimulus
Experiment 1 explored the effects of sudden changes in triplet base frequency and found that part of the build-up of stream segregation prior to a transition can transfer over a wider frequency region, and more for sudden rises and falls, than had been suggested by the results for the particular set of values tested by Anstis and Saida (1985)
Summary
Auditory stream segregation is the process by which sounds are grouped perceptually to form coherent representations of objects and events in the auditory scene (Bregman, 1990). It has long been known that sequences with a greater frequency separation (Df) or faster presentation rate are more likely to be heard as segregated (Miller and Heise, 1950; Bregman and Campbell, 1971; van Noorden, 1975) Perception of these sequences is bistable, involving stochastic switching between one and two streams (Denham and Winkler, 2006; Pressnitzer and Hupe, 2006), but averaging over several trials can be used to reveal the probability of hearing a segregated percept and how this changes over time (Carlyon et al, 2001; Roberts et al, 2002). The experiments reported here used tone sequences whose properties were changed at one or more time points and for which the consequences of those changes were tracked over time (cf. Haywood and Roberts, 2013; Rajasingam et al, 2018)
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