Asymmetric effects of sudden changes in timbre on auditory stream segregation

Two experiments explored the effects of abrupt transitions in timbral properties [amplitude modulation (AM), pure tones vs narrow-band noises, and attack/decay envelope] on streaming. Listeners reported continuously the number of streams heard during 18-s-long alternating low- and high-frequency (LHL-) sequences (frequency separation: 2-6 semitones) that underwent a coherent transition at 6 s or remained unchanged. In experiment 1, triplets comprised unmodulated pure tones or 100%-depth AM was created using narrowly spaced tone pairs (dyads: 30- or 50-Hz modulation). In experiment 2, triplets comprised narrow-band noises, dyads, or pure tones with quasi-trapezoidal envelopes (10/80/10 ms), fast attacks and slow decays (10/90 ms), or vice versa (90/10 ms). Abrupt transitions led to direction-dependent changes in stream segregation. Transitions from modulated to unmodulated (or slower-modulated) tones, from noise bands to pure tones, or from slow- to fast-attack tones typically caused substantial loss of segregation (resetting), whereas transitions in the opposite direction mostly caused less or no resetting. Furthermore, for the smallest frequency separation, transitions in the latter direction usually led to increased segregation (overshoot). Overall, the results are reminiscent of the perceptual asymmetries found in auditory search for targets with or without a salient additional feature (or greater activation of that feature).