Reverberation impairs brainstem temporal representations of voiced vowel sounds: challenging “periodicity-tagged” segregation of competing speech in rooms

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.

Statistics of natural reverberation enable perceptual separation of sound and space

“…Physiologically, responses to source direction (37), pitch (38,39), and amplitude modulation (40) are altered in the presence of reverberation, but in some cases there is evidence that reverberation is partially "removed" from the brain's representation of sound (40,41). Our results suggest that if these effects reflect the separation process that appears to be at work in human listeners, they should depend on whether the reverberation conforms to real-world IR regularities.…”

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

“…Contrast this with additive noise. Each indirect sound component in a reverberant space adds to the direct sound at the receiver with essentially random phase, reducing the depth of temporal-envelope modulation at the output of cochlear band-pass filters (Sabine 1922;Sayles and Winter 2008;Sayles et al 2015;Slama and Delgutte 2015). Perceptually, noise and reverberation can both decrease speech intelligibility (e.g., Nabelek 1993;Payton et al 1994), and can be particularly troublesome for cochlear-implant listeners (e.g., Qin and Oxenham 2005).…”

“…Details of our recording techniques are available elsewhere (Sayles and Winter 2008;Sayles et al 2015). Adult guinea pigs ( Cavia porcellus) were anesthetized with urethane and hypnorm (fentanyl/fluanisone).…”

“…2; Joris et al 2006;Sayles et al 2015). We computed the across-spike-train shuffled inter-spike-interval distribution in a short (30-ms duration) rectangular time window, slid in 5-ms steps through the 400-ms duration response.…”

Neural Representation of Concurrent Vowels in Macaque Primary Auditory Cortex

“…Alternatively, the high fidelity of spectral fine-structure encoding in A1 may reflect a transformation of temporal representations of individual vowel components in peripheral structures into a rate code at higher levels of the auditory pathway ( Buonomano and Merzenich, 1995 ; Wang et al, 2008 ). The auditory nerve and cochlear nucleus contain abundant temporal information from which, in principle, the pitch of vowels and other harmonic complex sounds may be derived (Palmer, 1990; Cariani and Delgutte, 1996 ; Keilson et al, 1997 ; Larsen et al, 2008 ; Cedolin and Delgutte, 2010 ; Sayles et al, 2015 ). Whereas peripheral auditory neurons can phase-lock to stimulus periodicities up to several thousand Hertz ( Rose et al, 1967 ; Langner, 1992 ), a purely temporal code for pitch is not viable at the cortical level, where upper limits of phase-locking are too low (generally <200 Hz) to account for the full range of F0s characteristic of human pitch perception, as determined in human and nonhuman primate studies ( Steinschneider et al, 1998 ; Brugge et al, 2009 ; Fishman et al, 2013, 2014 ; Steinschneider et al, 2013 ).…”