Abstract
The auditory system typically processes information from concurrently active sound sources (e.g., two voices speaking at once), in the presence of multiple delayed, attenuated and distorted sound-wave reflections (reverberation). Brainstem circuits help segregate these complex acoustic mixtures into “auditory objects.” Psychophysical studies demonstrate a strong interaction between reverberation and fundamental-frequency (F0) modulation, leading to impaired segregation of competing vowels when segregation is on the basis of F0 differences. Neurophysiological studies of complex-sound segregation have concentrated on sounds with steady F0s, in anechoic environments. However, F0 modulation and reverberation are quasi-ubiquitous. We examine the ability of 129 single units in the ventral cochlear nucleus (VCN) of the anesthetized guinea pig to segregate the concurrent synthetic vowel sounds /a/ and /i/, based on temporal discharge patterns under closed-field conditions. We address the effects of added real-room reverberation, F0 modulation, and the interaction of these two factors, on brainstem neural segregation of voiced speech sounds. A firing-rate representation of single-vowels' spectral envelopes is robust to the combination of F0 modulation and reverberation: local firing-rate maxima and minima across the tonotopic array code vowel-formant structure. However, single-vowel F0-related periodicity information in shuffled inter-spike interval distributions is significantly degraded in the combined presence of reverberation and F0 modulation. Hence, segregation of double-vowels' spectral energy into two streams (corresponding to the two vowels), on the basis of temporal discharge patterns, is impaired by reverberation; specifically when F0 is modulated. All unit types (primary-like, chopper, onset) are similarly affected. These results offer neurophysiological insights to perceptual organization of complex acoustic scenes under realistically challenging listening conditions.
Highlights
Multiple concurrent sound sources are usually present in everyday listening environments
Reverberation is a common feature of most man-made and natural real-world spaces: acoustic pressure waves first arrive at the ears directly from their source(s), followed by many delayed, attenuated and distorted indirect waves reflected from nearby surfaces (Sabine, 1922)
A group of units with very low best frequency (BF), which cannot be classified into the above categories due to strong phase locking at BF, despite randomization of stimulus starting phase, are termed “low-frequency” units (LF, 19)
Summary
Multiple concurrent sound sources are usually present in everyday listening environments. The higher F0 of a young child) is a strong cue for this “scene analysis” problem; allowing a target sound to be followed against an interferer under quiet, anechoic conditions (Cherry, 1953; Brokx and Nooteboom, 1982; Scheffers, 1983; Bregman, 1990; Micheyl and Oxenham, 2010) This F0-benefit for perceptual segregation is severely diminished in reverberant spaces, especially when F0 is time-varying, as in naturally intonated speech (Culling et al, 1994, 2003; Darwin and Hukin, 2000; Deroche and Culling, 2008, 2011; Lavandier and Culling, 2008). The F0 of steady-state and time-varying complex sounds is represented in the temporal pattern of actionpotential discharge of neurons in the ventral cochlear nucleus (VCN); the first brainstem processing station of the ascending
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