The role of spectral and periodicity cues in auditory stream segregation, measured using a temporal discrimination task.

In a previous paper, it was shown that sequential stream segregation could be based on both spectral information and periodicity information, if listeners were encouraged to hear segregation [Vliegen and Oxenham, J. Acoust. Soc. Am. 105, 339-346 (1999)]. The present paper investigates whether segregation based on periodicity information alone also occurs when the task requires integration. This addresses the question: Is segregation based on periodicity automatic and obligatory? A temporal discrimination task was used, as there is evidence that it is difficult to compare the timing of auditory events that are perceived as being in different perceptual streams. An ABA ABA ABA... sequence was used, in which tone B could be either exactly at the temporal midpoint between two successive tones A or slightly delayed. The tones A and B were of three types: (1) both pure tones; (2) both complex tones filtered through a fixed passband so as to contain only harmonics higher than the 10th, thereby eliminating detectable spectral differences, where only the fundamental frequency (f0) was varied between tones A and B; and (3) both complex tones with the same f0, but where the center frequency of the spectral passband varied between tones. Tone A had a fixed frequency of 300 Hz (when A and B were pure tones) or a fundamental frequency (f0) of 100 Hz (when A and B were complex tones). Five different intervals, ranging from 1 to 18 semitones, were used. The results for all three conditions showed that shift thresholds increased with increasing interval between tones A and B, but the effect was largest for the conditions where A and B differed in spectrum (i.e., the pure-tone and the variable-center-frequency conditions). The results suggest that spectral information is dominant in inducing (involuntary) segregation, but periodicity information can also play a role.