Rivalry between pitch and timbre in auditory stream segregation

Abstract

Two rapidly alternating tones with different pitches may be perceived as one integrated stream when the pitch differences are small or two separated streams when the pitch differences are large. Likewise, timbre differences between two tones may also cause such sequential stream segregation. Moreover, the effects of pitch and timbre on stream segregation may cancel each other out, which is called a trade-off. However, how timbre differences caused by specific patterns of spectral shapes interact with pitch differences and affect stream segregation has been largely unexplored. Therefore, we used stripe tones, in which stripe-like spectral patterns of harmonic complex tones were realized by grouping harmonic components into several bands based on harmonic numbers and removing harmonic components in every other band. Here, we show that 2- and 4-band stimuli elicited distinctive stream segregation against pitch proximity. By contrast, pitch separations dominated stream segregation for 16-band stimuli. The results for 8-band stimuli most clearly showed the trade-off between pitch and timbre on stream segregation. These results suggest that the stimuli with a small number (≤4) of bands elicit strong stream segregation due to sharp timbral contrasts between stripe-like spectral patterns, and that the auditory system looks to be limited in integrating blocks of frequency components dispersed over frequency and time.