Neural Encoding of Speech and Music: Implications for Hearing Speech in Noise

Reconsidering commonly used stimuli in speech perception experiments

This paper examines some commonly used stimuli in speech perception experiments and raises questions about their use, or about the interpretations of previous results. The takeaway messages are: 1) the Hillenbrand vowels represent a particular dialect rather than a gold standard, and English vowels contain spectral dynamics that have been largely underappreciated, 2) the /ɑ/ context is very common but not clearly superior as a context for testing consonant perception, 3) /ɑ/ is particularly problematic when testing voice-onset-time perception because it introduces strong confounds in the formant transitions, 4) /dɑ/ is grossly overrepresented in neurophysiological studies and yet is insufficient as a generalized proxy for "speech perception," and 5) digit tests and matrix sentences including the coordinate response measure are systematically insensitive to important patterns in speech perception. Each of these stimulus sets and concepts is described with careful attention to their unique value and also cases where they might be misunderstood or over-interpreted.

Enhancement of loudness discrimination acuity for self-generated sound is independent of musical experience

Musicians tend to have better auditory and motor performance than non-musicians because of their extensive musical experience. In a previous study, we established that loudness discrimination acuity is enhanced when sound is produced by a precise force generation task. In this study, we compared the enhancement effect between experienced pianists and non-musicians. Without the force generation task, loudness discrimination acuity was better in pianists than non-musicians in the condition. However, the force generation task enhanced loudness discrimination acuity similarly in both pianists and non-musicians. The reaction time was also reduced with the force control task, but only in the non-musician group. The results suggest that the enhancement of loudness discrimination acuity with the precise force generation task is independent of musical experience and is, therefore, a fundamental function in auditory-motor interaction.

Efferent unmasking of speech-in-noise encoding?

OBJECTIVE

The medial olivocochlear (MOC) reflex provides efferent feedback from the brainstem to cochlear outer hair cells. Physiologic studies have demonstrated that the MOC reflex is involved in "unmasking" of signals-in-noise at the level of the auditory nerve; however, its functional importance in human hearing remains unclear.

DESIGN

This study examined relationships between pre-neural measurements of MOC reflex strength (click-evoked otoacoustic emission inhibition; CEOAE) and neural measurements of speech-in-noise encoding (speech frequency following response; sFFR) in four conditions (Quiet, Contralateral Noise, Ipsilateral Noise, and Ipsilateral + Contralateral Noise). Three measures of CEOAE inhibition (amplitude reduction, effective attenuation, and input-output slope inhibition) were used to quantify pre-neural MOC reflex strength. Correlations between pre-neural MOC reflex strength and sFFR "unmasking" (i.e. response recovery from masking effects with activation of the MOC reflex in time and frequency domains) were assessed.

STUDY SAMPLE

18 young adults with normal hearing.

RESULTS

sFFR unmasking effects were insignificant, and there were no correlations between pre-neural MOC reflex strength and sFFR unmasking in the time or frequency domain.

CONCLUSION

Our results do not support the hypothesis that the MOC reflex is involved in speech-in-noise neural encoding, at least for features that are represented in the sFFR at the SNR tested.

Auditory and frontal anatomic correlates of pitch discrimination in musicians, non-musicians, and children without musical training

Individual differences in pitch discrimination have been associated with the volume of both the bilateral Heschl's gyrus and the right inferior frontal gyrus (IFG). However, most of these studies used samples composed of individuals with different amounts of musical training. Here, we investigated the relationship between pitch discrimination and individual differences in the gray matter (GM) volume of these brain structures in 32 adult musicians, 28 adult non-musicians, and 32 children without musical training. The results showed that (i) the individuals without musical training (whether children or adults) who were better at pitch discrimination had greater volume of auditory regions, whereas (ii) musicians with better pitch discrimination had greater volume of the IFG. These results suggest that the relationship between pitch discrimination and the volume of auditory regions is innately established early in life, and that musical training modulates the volume of the IFG, probably improving audio-motor connectivity. This is the first study to detect a relationship between pitch discrimination ability and GM volume before beginning any musical training in children and adults.

Speech-evoked auditory brainstem response; electrophysiological evidence of upper brainstem facilitative role on sound lateralization in noise

BACKGROUND AND AIM

Sound lateralization/localization is one of the most important auditory processing abilities, which plays approved role in auditory streaming and speech perception in challenging situations like noisy places. In addition to the main role of lower brainstem centers like superior olivary complex in sound lateralization, efferent auditory system effects on improving auditory skills in everyday auditory challenging positions were revealed. This study evaluated noise effects on lateralization scores in correlation with an objective electrophysiologic test (Speech-ABR in noise), which objectively shows cumulative effects of the afferent and efferent auditory systems at the inferior colliculus and upper brainstem pathway.

METHOD

Fourteen normal-hearing subjects in the age range of 18 to 25 participated in this study. Lateralization scores in the quiet and noisy modes were evaluated. Speech-ABR in both ears for quiet mode and three different contralateral noise levels (SNR = + 5, 0, - 5) were recorded, too. Correlation of lateralization scores and Speech-ABR changes in noise was studied.

RESULTS

Significant decrease of lateralization scores with latency increase and amplitude decrease of Speech-ABR transient peaks (V, A, O) was seen with noise presentation. A high positive correlation between lateralization decrease with latency increase of onset peaks (V, A) and amplitude decrease of transient peaks (V, A, O) was found in low signal-to-noise ratios.

CONCLUSION

The study revealed that in high challenging auditory situations like auditory lateralization in noise, upper brainstem centers and pathways play a facilitative role for main auditory lateralization centers in lower levels.

Sensitivity to Melody, Rhythm, and Beat in Supporting Speech-in-Noise Perception in Young Adults

OBJECTIVES

Musicians appear to have an enhanced ability to perceive speech-in-noise, prompting suggestions that musical training could be used to help people who struggle to communicate in noisy environments. This study assessed the role of sensitivity to beat, rhythm, and melody in supporting speech-in-noise perception.

DESIGN

This is an exploratory study based on correlation. The study included 24 normally hearing young adult participants with a wide range of musical training and experience. Formal and informal musical experience was measured with the training subscale of the Goldsmiths' Musical Sophistication Index. Speech reception thresholds (SRT) were measured using the Matrix Sentence Test and three different speech-spectrum-shaped noise maskers: unmodulated and sinusoidally amplitude-modulated (modulation frequency, fm = 8 Hz; modulation depths: 60 and 80%). Primary predictors were measures of sensitivity to beat, rhythm, and melody. Secondary predictors were pure-tone frequency discrimination and auditory working memory (digit span). Any contributions from these two predictors were to be controlled for as appropriate.

RESULTS

Participants with more musical experience and greater sensitivity to rhythm, beat, and melody had better SRTs. Sensitivity to beat was more strongly linked with SRT than sensitivity to either rhythm or melody. This relationship remained strong even after factoring out contributions from frequency discrimination and auditory working memory.

CONCLUSIONS

Sensitivity to beat predicted SRTs in unmodulated and modulated noise. We propose that this sensitivity maximizes benefit from fluctuations in signal-to-noise ratio through temporal orienting of attention to perceptually salient parts of the signal. Beat perception may be a good candidate for targeted training aimed at enhancing speech perception when listening in noise.

Separate Contribution of Striatum Volume and Pitch Discrimination to Individual Differences in Music Reward

Individual differences in the level of pleasure induced by music have been associated with the response of the striatum and differences in functional connectivity between the striatum and the auditory cortex. In this study, we tested whether individual differences in music reward are related to the structure of the striatum and the ability to discriminate pitch. We acquired a 3-D magnetization-prepared rapid-acquisition gradient-echo image for 32 musicians and 26 nonmusicians who completed a music-reward questionnaire and a test of pitch discrimination. The analysis of both groups together showed that sensitivity to music reward correlated negatively with the volume of both the caudate and nucleus accumbens and correlated positively with pitch-discrimination abilities. Moreover, musicianship, pitch discrimination, and caudate volume significantly predicted individual differences in music reward. These results are consistent with the proposal that individual differences in music reward depend on the interplay between auditory abilities and the reward network.

Brainstem Correlates of Comodulation Masking Release for Speech in Normal Hearing Adults

Background and Objectives

Weak signals embedded in fluctuating masker can be perceived more efficiently than similar signals embedded in unmodulated masker. This release from masking is known as comodulation masking release (CMR). In this paper, we investigate, neural correlates of CMR in the human auditory brainstem.

Subjects and Methods

A total of 26 normal hearing subjects aged 18-30 years participated in this study. First, the impact of CMR was quantified by a behavioral experiment. After that, the brainstem correlates of CMR was investigated by the auditory brainstem response to complex sounds (cABR) in comodulated (CM) and unmodulated (UM) masking conditions.

Results

The auditory brainstem responses are less susceptible to degradation in response to the speech syllable /da/ in the CM noise masker in comparison with the UM noise masker. In the CM noise masker, frequency-following response (FFR) and fundamental frequency (F0) were correlated with better behavioral CMR. Furthermore, the subcortical response timing of subjects with higher CMR was less affected by the CM noise masker, having higher stimulus-to-noise response correlations over the FFR range.

Conclusions

The results of the present study revealed a significant link between brainstem auditory processes and CMR. The findings of the present study show that cABR provides objective information about the neural correlates of CMR for speech stimulus.

Pitch Discrimination in Musicians and Non-Musicians: Effects of Harmonic Resolvability and Processing Effort

Musicians typically show enhanced pitch discrimination abilities compared to non-musicians. The present study investigated this perceptual enhancement behaviorally and objectively for resolved and unresolved complex tones to clarify whether the enhanced performance in musicians can be ascribed to increased peripheral frequency selectivity and/or to a different processing effort in performing the task. In a first experiment, pitch discrimination thresholds were obtained for harmonic complex tones with fundamental frequencies (F0s) between 100 and 500 Hz, filtered in either a low- or a high-frequency region, leading to variations in the resolvability of audible harmonics. The results showed that pitch discrimination performance in musicians was enhanced for resolved and unresolved complexes to a similar extent. Additionally, the harmonics became resolved at a similar F0 in musicians and non-musicians, suggesting similar peripheral frequency selectivity in the two groups of listeners. In a follow-up experiment, listeners' pupil dilations were measured as an indicator of the required effort in performing the same pitch discrimination task for conditions of varying resolvability and task difficulty. Pupillometry responses indicated a lower processing effort in the musicians versus the non-musicians, although the processing demand imposed by the pitch discrimination task was individually adjusted according to the behavioral thresholds. Overall, these findings indicate that the enhanced pitch discrimination abilities in musicians are unlikely to be related to higher peripheral frequency selectivity and may suggest an enhanced pitch representation at more central stages of the auditory system in musically trained listeners.

The investigation of cortical auditory evoked potentials responses in young adults having musical education

BACKGROUND

In the literature, music education has been shown to enhance auditory perception for children and young adults. When compared to young adult non-musicians, young adult musicians demonstrate increased auditory processing, and enhanced sensitivity to acoustic changes. The evoked response potentials associated with the interpretation of sound are enhanced in musicians. Studies show that training also changes sound perception and cortical responses. The earlier training appears to lead to larger changes in the auditory cortex.

AIMS

Most cortical studies in the literature have used pure tones or musical instrument sounds as stimuli signals. The aim of those studies was to investigate whether musical education would enhance auditory cortical responses when speech signals were used. In this study, the speech sounds extracted from running speech were used as sound stimuli.

STUDY DESIGN

Non-randomized controlled study.

METHODS

The experimental group consists of young adults up to 21 years-old, all with a minimum of 4 years of musical education. The control group was selected from young adults of the same age without any musical education. The experiments were conducted by using a cortical evoked potential analyser and /m/, /t/ /g/ sound stimulation at the level of 65 dB SPL. In this study, P1 / N1 / P2 amplitude and latency values were measured.

RESULTS

Significant differences were found in the amplitude values of P1 and P2 (p<0.05). The differences among the latencies were not found to be significantly important (p>0.05).

CONCLUSION

The results obtained in our study indicate that musical experience has an effect on the nervous system and this can be seen in cortical auditory evoked potentials recorded when the subjects hear speech.