Neural Segregation of Concurrent Speech: Effects of Background Noise and Reverberation on Auditory Scene Analysis in the Ventral Cochlear Nucleus

Abstract

Concurrent complex sounds (e.g., two voices speaking at once) are perceptually disentangled into separate "auditory objects". This neural processing often occurs in the presence of acoustic-signal distortions from noise and reverberation (e.g., in a busy restaurant). A difference in periodicity between sounds is a strong segregation cue under quiet, anechoic conditions. However, noise and reverberation exert differential effects on speech intelligibility under "cocktail-party" listening conditions. Previous neurophysiological studies have concentrated on understanding auditory scene analysis under ideal listening conditions. Here, we examine the effects of noise and reverberation on periodicity-based neural segregation of concurrent vowels /a/ and /i/, in the responses of single units in the guinea-pig ventral cochlear nucleus (VCN): the first processing station of the auditory brain stem. In line with human psychoacoustic data, we find reverberation significantly impairs segregation when vowels have an intonated pitch contour, but not when they are spoken on a monotone. In contrast, noise impairs segregation independent of intonation pattern. These results are informative for models of speech processing under ecologically valid listening conditions, where noise and reverberation abound.

Cortical tracking of speakers’ spectral changes predicts selective listening

“…Lexical processing of speech depends too on the precise encoding of vocal spectral modulations [48], [49], [50], [51], [52]. Relative temporal displacements between spectral components of speech may affect intelligibility performance [53]; this may naturally arise from surface reverberation effects that lead to weaker segregation in multispeaker conditions [54], [55]. In another example of dependence on spectral encoding, bandwidth tuning loss at higher frequencies under presbycusis severely limits the ability to target speech in multispeaker conditions.…”

Subcortical pathways: Towards a better understanding of auditory disorders

“…Additionally, response fidelity in the IC is greater in reverberant conditions compared to anechoic stimuli that are matched in modulation depth. These findings suggest that subcortical processing that occurs between the cochlear nucleus and auditory cortex may compensate for the degradation of periodicity pitch processing induced by challenging listening conditions, in part by filtering out noise (Sayles et al, 2016). Furthermore, these potential mechanisms may contribute to speech representations in the auditory cortex that are relatively invariant with respect to competing noise and reverberation (Mesgarani et al, 2014), in which normal-hearing individuals have considerable listening abilities in moderately challenging conditions (Poissant et al, 2006).…”

“…However, when the listening environment is contaminated by extraneous noise and reverberation, AVCN pitch processing is degraded (Sayles and Winter, 2008b). These findings suggest that early brainstem networks are not fully capable of faithfully representing ongoing speech sounds in the acoustically challenging conditions that we often experience (Sayles et al, 2016).…”

Phonetics in the Brain

“…Voice-onset time appears to be represented in a similar fashion to the auditory nerve, that is, through a pause in neuronal spiking corresponding to the VOT (Young, 2008). The inferior colliculus plays a crucial role in the process of filtering and sharpening the signal, as well as compensating for the effects of reverberation on the amplitude envelope of the speech signal (Slama & Delgutte, 2015;Suga, 1995), for example when the system perceives vowels such as /a/ and /i/ (Sayles, Stasiak, & Winter, 2016). This early filtering and compensation system appears to help the primary auditory cortex further up in the hierarchy fulfil important functions, such as processing speech sounds as robust and invariant categories in conditions marked by noise or reverberation , which may occur in a loud restaurant or cocktail party where we may hear many people speaking at once (Cherry, 1953).…”