Dorea R. Ruggles

Normal hearing is not enough to guarantee robust encoding of suprathreshold features important in everyday communication

“Normal hearing” is typically defined by threshold audibility, even though everyday communication relies on extracting key features of easily audible sound, not on sound detection. Anecdotally, many normal-hearing listeners report difficulty communicating in settings where there are competing sound sources, but the reasons for such difficulties are debated: Do these difficulties originate from deficits in cognitive processing, or differences in peripheral, sensory encoding? Here we show that listeners with clinically normal thresholds exhibit very large individual differences on a task requiring them to focus spatial selective auditory attention to understand one speech stream when there are similar, competing speech streams coming from other directions. These individual differences in selective auditory attention ability are unrelated to age, reading span (a measure of cognitive function), and minor differences in absolute hearing threshold; however, selective attention ability correlates with the ability to detect simple frequency modulation in a clearly audible tone. Importantly, we also find that selective attention performance correlates with physiological measures of how well the periodic, temporal structure of sounds above the threshold of audibility are encoded in early, subcortical portions of the auditory pathway. These results suggest that the fidelity of early sensory encoding of the temporal structure in suprathreshold sounds influences the ability to communicate in challenging settings. Tests like these may help tease apart how peripheral and central deficits contribute to communication impairments, ultimately leading to new approaches to combat the social isolation that often ensues.

Influence of musical training on understanding voiced and whispered speech in noise.

This study tested the hypothesis that the previously reported advantage of musicians over non-musicians in understanding speech in noise arises from more efficient or robust coding of periodic voiced speech, particularly in fluctuating backgrounds. Speech intelligibility was measured in listeners with extensive musical training, and in those with very little musical training or experience, using normal (voiced) or whispered (unvoiced) grammatically correct nonsense sentences in noise that was spectrally shaped to match the long-term spectrum of the speech, and was either continuous or gated with a 16-Hz square wave. Performance was also measured in clinical speech-in-noise tests and in pitch discrimination. Musicians exhibited enhanced pitch discrimination, as expected. However, no systematic or statistically significant advantage for musicians over non-musicians was found in understanding either voiced or whispered sentences in either continuous or gated noise. Musicians also showed no statistically significant advantage in the clinical speech-in-noise tests. Overall, the results provide no evidence for a significant difference between young adult musicians and non-musicians in their ability to understand speech in noise.

How Early Aging and Environment Interact in Everyday Listening: From Brainstem to Behavior Through Modeling

We recently showed that listeners with normal hearing thresholds vary in their ability to direct spatial attention and that ability is related to the fidelity of temporal coding in the brainstem. Here, we recruited additional middle-aged listeners and extended our analysis of the brainstem response, measured using the frequency-following response (FFR). We found that even though age does not predict overall selective attention ability, middle-aged listeners are more susceptible to the detrimental effects of reverberant energy than young adults. We separated the overall FFR into orthogonal envelope and carrier components and used an existing model to predict which auditory channels drive each component. We find that responses in mid- to high-frequency auditory channels dominate envelope FFR, while lower-frequency channels dominate the carrier FFR. Importantly, we find that which component of the FFR predicts selective attention performance changes with age. We suggest that early aging degrades peripheral temporal coding in mid-to-high frequencies, interfering with the coding of envelope interaural time differences. We argue that, compared to young adults, middle-aged listeners, who do not have strong temporal envelope coding, have more trouble following a conversation in a reverberant room because they are forced to rely on fragile carrier ITDs that are susceptible to the degrading effects of reverberation.

Intelligibility of voiced and whispered speech in noise in listeners with and without musical training

Musicians have been shown to exhibit more robust neural coding of periodicity than non-musicians; they have also been reported to exhibit an advantage in understanding speech in noise. This study tested the hypothesis that the musicians’ speech intelligibility advantage arises from more efficient coding of voiced (periodic) speech. This was tested by comparing intelligibility of normal speech in noise with that of whispered (unvoiced) speech in musicians and non-musicians. Listeners with less than 2 years of formal musical training were categorized as non-musicians; listeners who began musical training before age 10 and who currently play more than 10 h/wk were included as musicians. Listeners heard grammatically correct nonsense sentences that were either (1) voiced, (2) whispered, or (3) whispered with subband amplitude distributions matched to the voiced speech. Masking noise was either continuous or gated with a 16 Hz, 50% duty cycle. In contrast to the earlier study, preliminary data suggest no advan...

Sustained Cortical and Subcortical Measures of Auditory and Visual Plasticity following Short-Term Perceptual Learning

Short-term training can lead to improvements in behavioral discrimination of auditory and visual stimuli, as well as enhanced EEG responses to those stimuli. In the auditory domain, fluency with tonal languages and musical training has been associated with long-term cortical and subcortical plasticity, but less is known about the effects of shorter-term training. This study combined electroencephalography (EEG) and behavioral measures to investigate short-term learning and neural plasticity in both auditory and visual domains. Forty adult participants were divided into four groups. Three groups trained on one of three tasks, involving discrimination of auditory fundamental frequency (F0), auditory amplitude modulation rate (AM), or visual orientation (VIS). The fourth (control) group received no training. Pre- and post-training tests, as well as retention tests 30 days after training, involved behavioral discrimination thresholds, steady-state visually evoked potentials (SSVEP) to the flicker frequencies of visual stimuli, and auditory envelope-following responses simultaneously evoked and measured in response to rapid stimulus F0 (EFR), thought to reflect subcortical generators, and slow amplitude modulation (ASSR), thought to reflect cortical generators. Enhancement of the ASSR was observed in both auditory-trained groups, not specific to the AM-trained group, whereas enhancement of the SSVEP was found only in the visually-trained group. No evidence was found for changes in the EFR. The results suggest that some aspects of neural plasticity can develop rapidly and may generalize across tasks but not across modalities. Behaviorally, the pattern of learning was complex, with significant cross-task and cross-modal learning effects.

Sustained periodic phase locking during a perceptual streaming task using resolved and unresolved harmonic tones

Perceptual object formation and streaming are critical aspects of auditory processing. Previous studies have found cortical markers of streaming and attention, but it is unclear if these depend on tonotopic separation or whether other features, such as fundamental frequency (F0), are represented by these cortical markers. In this study, we used complex tones comprised of resolved or unresolved harmonics and simultaneously measured cortical and subcortical steady-state EEG responses during a behavioral streaming task. Subjects attended to either a fast stream of high-F0 harmonic complexes or a slower stream of lower-F0 complexes, filtered into the same spectral region. Subjects maintained directed attention during 1-minute blocks while reporting level oddballs in the attended stream and ignoring oddballs in the unattended stream. During the task, sustained envelope following responses phase locked to the complex F0 (nominally subcortical) and to the presentation rates (nominally cortical) were recorded. Ph...

How individual differences in sensory coding and attentional control impact understanding speech in noise

Historically, the majority of psychoacoustic studies of hearing ability have viewed individual differences as noise: a nuisance that makes it difficult to see the effects that different acoustic conditions have on auditory perception. This talk reviews how we have begun to use individual differences to tease apart the processes that affect perception, with a particular focus on how listeners understand speech when there are competing sound sources. We find that individual subjects show consistent differences in their ability to understand speech in noise. These consistent differences can come both from differences in the fidelity of sensory coding and from differences in the ability to focus selectively on important sound and suppress unimportant sound. Importantly, which of these factors predicts performance depends greatly on the details of the stimuli used in a given task, and what stage of processing is the resulting bottleneck, determining performance. When fine differences in the sound content, such...

The influence of short term perceptual learning of pitch discrimination and modulation discrimination on subcortical envelope-following and cortical steady-state EEG responses

Many perceptual skills improve with training, and research suggests that long- and short-term experiences modify auditory neural structures and function. Long-term cortical and subcortical plasticity has been associated with musical training and fluency in tonal languages, and short-term training effects have been regularly observed in cortical responses. Less is known about short-term subcortical plasticity, or the simultaneous relationships between subcortical and cortical responses under training conditions. The current study examines short-term learning and neural plasticity, using behavioral measures in combination with simultaneous subcortical and cortical steady-state EEG responses. Untrained, naive subjects were randomly assigned to groups that were trained on fundamental-frequency (F0) discrimination, amplitude-modulation rate discrimination, or visual orientation discrimination. All auditory stimuli consisted of unresolved harmonic complexes with a nominal F0 of 137 Hz that were sinusoidally amp...

Relationships linking age, selective attention, and frequency following in the brainstem.

Several studies have demonstrated that increasing age leads to a decrease in the ability of the brainstem to phase‐lock to periodic stimuli. This frequency following response has additionally been related to several other metrics, including musical training, experience with a tonal language, and ability to understand speech in noise. The current study explored a potential relationship between brainstem responses and large, observed individual differences in the ability of normal‐hearing listeners to deploy selective spatial auditory attention. Previously, we reported that the ability to selectively attend varies enormously (from near chance to 90% correct) in normal‐hearing listeners, but is uncorrelated with age (ranging from 20–55 years). Here, we recruited a subset of the original listeners and measured the brainstem’s frequency‐following response (FFR). Although selective‐attention performance was correlated with the strength of the FFR, there was an interaction between the FFR strength and age. For y...

Reverberation disrupts spatial selective auditory attention.

When there are multiple, competing speech streams, listeners can selectively attend to a desired target using spatial cues like interaural time differences (ITDs). However, reverberation smears fine temporal information needed to encode ITDs. Reverberation may therefore interfere with the ability to selectively attend to a target from a particular direction. Subjects were asked to report a stream of digits simulated to be directly in front of them in the presence of two masker streams symmetrically positioned at 15 deg left and right. Because the maskers were statistically identical to the target (except in direction), listeners had to focus spatial attention on the target while ignoring the masker streams. Three levels of reverberation were simulated using a rectangular room model to measure the influence of reverberation on performance. Initial results showed that spatial selective attention degraded as reverberation increased. However, overall performance varied dramatically from listener to listener i...