PSYCHOACOUSTICS: a comprehensive MATLAB toolbox for auditory testing

PSYCHOACOUSTICS is a new MATLAB toolbox which implements three classic adaptive procedures for auditory threshold estimation. The first includes those of the Staircase family (method of limits, simple up-down and transformed up-down); the second is the Parameter Estimation by Sequential Testing (PEST); and the third is the Maximum Likelihood Procedure (MLP). The toolbox comes with more than twenty built-in experiments each provided with the recommended (default) parameters. However, if desired, these parameters can be modified through an intuitive and user friendly graphical interface and stored for future use (no programming skills are required). Finally, PSYCHOACOUSTICS is very flexible as it comes with several signal generators and can be easily extended for any experiment

EFFECTS OF AGING ON VOICE-PITCH PROCESSING: THE ROLE OF SPECTRAL AND TEMPORAL CUES

Declines in auditory temporal processing are a common consequence of natural aging. Interactions between aging and spectro-temporal pitch processing have yet to be thoroughly investigated in humans, though recent neurophysiologic and electrophysiologic data lend support to the notion that periodicity coding using only unresolved harmonics (i.e., those available via the temporal envelope) is negatively affected as a consequence of age. Individuals with cochlear implants (CIs) must rely on the temporal envelope of speech to glean information about voice pitch [coded through the fundamental frequency (f0)], as spectral f0 cues are not available. While cochlear implants have been shown to be efficacious in older adults, it is hypothesized that they would experience difficulty perceiving spectrally-degraded voice-pitch information. The current experiments were aimed at quantifying the ability of younger and older listeners to utilize spectro-temporal cues to obtain voice pitch information when performing simple and complex auditory tasks. Experiment 1 measured the ability of younger and older NH listeners to perceive a difference in the frequency of amplitude modulated broad-band noise, thereby exploiting only temporal envelope cues to perform the task. Experiment 2 measured age-related differences in f0 difference limens as the degree of spectral degradation was manipulated to approximate CI processing. Results from Experiments 1 and 2 demonstrated that spectro-temporal processing of f0 information in non-speech stimuli is affected in older adults. Experiment 3 showed that age-related performances observed in Experiments 1 and 2 translated to voice gender identification using a natural speech stimulus. Experiment 4 attempted to estimate how younger and older NH listeners are able to utilize differences in voice pitch information in everyday listening environments (i.e., speech in noise) and how such abilities are affected by spectral degradation. Comprehensive results provide further insight on pitch coding in both normal and impaired auditory systems, and demonstrate that spectro-temporal pitch processing is dependent upon the age of the listener. Results could have important implications for elderly cochlear implant recipients

Neural coding of pitch cues in the auditory midbrain of unanesthetized rabbits

Pitch is an important attribute of auditory perception that conveys key features in music, speech, and helps listeners extract useful information from complex auditory environments. Although the psychophysics of pitch perception has been extensively studied for over a century, the underlying neural mechanisms are still poorly understood. This thesis examines pitch cues in the inferior colliculus (IC), which is the core processing center in the mammalian auditory midbrain that relays and transforms convergent inputs from peripheral brainstem nuclei to the auditory cortex. Previous studies have shown that IC can encode low-frequency fluctuations in stimulus envelope that are related to pitch, but most experiments were conducted in anesthetized animals using stimuli that only evoked weak pitch sensations and only investigated a limited frequency range. Here, we used single-neuron recordings from the IC in normal hearing, unanesthetized rabbits in response to a comprehensive set of complex auditory stimuli to explore the role of IC in the neural processing of pitch. We characterized three neural codes for pitch cues: a temporal code for the stimulus envelope repetition rate (ERR) below 900 Hz, a rate code for ERR between 60 and 1600 Hz, and a rate-place code for frequency components individually resolved by the cochlea that is mainly available above 800 Hz. While the temporal code and the rate-place code are inherited from the auditory periphery, the rate code for ERR has not been currently characterized in processing stages prior to the IC. To help interpret our experimental findings, we used computational models to show that the IC rate code for ERR likely arises via temporal interaction of multiple synaptic inputs, and thus the IC performs a temporal-to-rate code transformation from peripheral to cortical representations of pitch cues. We also show that the IC rate-place code is robust across a 40 dB range of sound levels, and is likely strengthened by inhibitory synaptic inputs. Together, these three codes could provide neural substrates for pitch of stimuli with various temporal and spectral compositions over the entire frequency range

Behavioral and neural selectivity for acoustic signatures of vocalizations

Vocal communication relies on the ability of listeners to identify, process, and respond to vocal sounds produced by others in complex environments. In order to accurately recognize these signals, animals’ auditory systems must robustly represent acoustic features that distinguish vocal sounds from other environmental sounds. In this dissertation, I describe experiments combining acoustic, behavioral, and neurophysiological approaches to identify behaviorally relevant vocalization features and understand how they are represented in the brain. First, I show that vocal responses to communication sounds in songbirds depend on the presence of specific spectral signatures of vocalizations. Second, I identify an anatomically localized neural population in the auditory cortex that shows selective responses for behaviorally relevant sounds. Third, I show that these neurons’ spectral selectivity is robust to acoustic context, indicating that they could function as spectral signature detectors in a variety of listening conditions. Last, I deconstruct neural selectivity for behaviorally relevant sounds and show that it is driven by a sensitivity to deep fluctuations in power along the sound frequency spectrum. Together, these results show that the processing of behaviorally relevant spectral features engages a specialized neural population in the auditory cortex, and elucidate an acoustic driver of vocalization selectivity

Investigating the build-up of precedence effect using reflection masking

The auditory processing level involved in the build‐up of precedence [Freyman et al., J. Acoust. Soc. Am. 90, 874–884 (1991)] has been investigated here by employing reflection masked threshold (RMT) techniques. Given that RMT techniques are generally assumed to address lower levels of the auditory signal processing, such an approach represents a bottom‐up approach to the buildup of precedence. Three conditioner configurations measuring a possible buildup of reflection suppression were compared to the baseline RMT for four reflection delays ranging from 2.5–15 ms. No buildup of reflection suppression was observed for any of the conditioner configurations. Buildup of template (decrease in RMT for two of the conditioners), on the other hand, was found to be delay dependent. For five of six listeners, with reflection delay=2.5 and 15 ms, RMT decreased relative to the baseline. For 5‐ and 10‐ms delay, no change in threshold was observed. It is concluded that the low‐level auditory processing involved in RMT is not sufficient to realize a buildup of reflection suppression. This confirms suggestions that higher level processing is involved in PE buildup. The observed enhancement of reflection detection (RMT) may contribute to active suppression at higher processing levels

Investigation of Auditory Encoding and the Use of Auditory Feedback During Speech Production

Responses to altered auditory feedback during speech production are highly variable. The extent to which auditory encoding influences this varied use is not well understood. Thirty-nine normal hearing adults completed a first formant (F1) manipulation paradigm where F1 of the vowel /ε/ was shifted upwards in frequency towards an /æ/–like vowel in real-time. Frequency following responses (FFRs) and envelope following responses (EFRs) were used to measure neuronal activity to the same vowels produced by the participant and a prototypical talker. Cochlear tuning, measured by SFOAEs and a psychophysical method, was also recorded. Results showed that average F1 production changed to oppose the manipulation. Three metrics of EFR and FFR encoding were evaluated. No reliable relationship was found between speech compensation and evoked response measures or measures of cochlear tuning. Differences in brainstem encoding of vowels and sharpness of cochlear tuning do not appear to explain the variability observed in speech production

The use of acoustic cues in phonetic perception: Effects of spectral degradation, limited bandwidth and background noise

Hearing impairment, cochlear implantation, background noise and other auditory degradations result in the loss or distortion of sound information thought to be critical to speech perception. In many cases, listeners can still identify speech sounds despite degradations, but understanding of how this is accomplished is incomplete. Experiments presented here tested the hypothesis that listeners would utilize acoustic-phonetic cues differently if one or more cues were degraded by hearing impairment or simulated hearing impairment. Results supported this hypothesis for various listening conditions that are directly relevant for clinical populations. Analysis included mixed-effects logistic modeling of contributions of individual acoustic cues for various contrasts. Listeners with cochlear implants (CIs) or normal-hearing (NH) listeners in CI simulations showed increased use of acoustic cues in the temporal domain and decreased use of cues in the spectral domain for the tense/lax vowel contrast and the word-final fricative voicing contrast. For the word-initial stop voicing contrast, NH listeners made less use of voice-onset time and greater use of voice pitch in conditions that simulated high-frequency hearing impairment and/or masking noise; influence of these cues was further modulated by consonant place of articulation. A pair of experiments measured phonetic context effects for the "s/sh" contrast, replicating previously observed effects for NH listeners and generalizing them to CI listeners as well, despite known deficiencies in spectral resolution for CI listeners. For NH listeners in CI simulations, these context effects were absent or negligible. Audio-visual delivery of this experiment revealed enhanced influence of visual lip-rounding cues for CI listeners and NH listeners in CI simulations. Additionally, CI listeners demonstrated that visual cues to gender influence phonetic perception in a manner consistent with gender-related voice acoustics. All of these results suggest that listeners are able to accommodate challenging listening situations by capitalizing on the natural (multimodal) covariance in speech signals. Additionally, these results imply that there are potential differences in speech perception by NH listeners and listeners with hearing impairment that would be overlooked by traditional word recognition or consonant confusion matrix analysis

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

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.This work was supported by a grant from the BBSRC to Ian M. Winter. Mark Sayles received a University of Cambridge MB/PhD studentship. Tony Watkins (University of Reading, UK) provided the real-room impulse responses. Portions of the data analysis and manuscript preparation were performed by Mark Sayles during the course of an Action on Hearing Loss funded UK–US Fulbright Commission professional scholarship held in the Auditory Neurophysiology and Modeling Laboratory at Purdue University, USA. Mark Sayles is currently supported by a post-doctoral fellowship from Fonds Wetenschappelijk Onderzoek—Vlaanderen, held in the Laboratory of Auditory Neurophysiology at KU Leuven, Belgium.This paper was originally published in Frontiers in Systems Neuroscience (Sayles M, Stasiak A, Winter IM, Frontiers in Systems Neuroscience 2015, 8, 248, doi:10.3389/fnsys.2014.00248)

The Role of Age-Related Declines in Subcortical Auditory Processing in Speech Perception in Noise

Older adults, even those without hearing impairment, often experience increased difficulties understanding speech in the presence of background noise. This study examined the role of age-related declines in subcortical auditory processing in the perception of speech in different types of background noise. Participants included normal-hearing young (19 – 29 years) and older (60 – 72 years) adults. Normal hearing was defined as pure-tone thresholds of 25 dB HL or better at octave frequencies from 0.25 to 4 kHz in both ears and at 6 kHz in at least one ear. Speech reception thresholds (SRTs) to sentences were measured in steady-state (SS) and 10-Hz amplitude-modulated (AM) speech-shaped noise, as well as two-talker babble. In addition, click-evoked auditory brainstem responses (ABRs) and envelope following responses (EFRs) in response to the vowel /ɑ/ in quiet, SS, and AM noise were measured. Of primary interest was the relationship between the SRTs and EFRs. SRTs were significantly higher (i.e., worse) by about 1.5 dB for older adults in two-talker babble but not in AM and SS noise. In addition, the EFRs of the older adults were less robust compared to the younger participants in quiet, AM, and SS noise. Both young and older adults showed a “neural masking release,” indicated by a more robust EFR at the trough compared to the peak of the AM masker. The amount of neural masking release did not differ between the two age groups. Variability in SRTs was best accounted for by audiometric thresholds (pure-tone average across 0.5–4 kHz) and not by the EFR in quiet or noise. Aging is thus associated with a degradation of the EFR, both in quiet and noise. However, these declines in subcortical neural speech encoding are not necessarily associated with impaired perception of speech in noise, as measured by the SRT, in normal-hearing older adults