Cortical Tracking of Speech in Delta Band Relates to Individual Differences in Speech in Noise Comprehension in Older Adults.

OBJECTIVES: Understanding speech in adverse listening environments is challenging for older adults. Individual differences in pure tone averages and working memory are known to be critical indicators of speech in noise comprehension. Recent studies have suggested that tracking of the speech envelope in cortical oscillations <8 Hz may be an important mechanism related to speech comprehension by segmenting speech into words and phrases (delta, 1 to 4 Hz) or phonemes and syllables (theta, 4 to 8 Hz). The purpose of this study was to investigate the extent to which individual differences in pure tone averages, working memory, and cortical tracking of the speech envelope relate to speech in noise comprehension in older adults. DESIGN: Cortical tracking of continuous speech was assessed using electroencephalography in older adults (60 to 80 years). Participants listened to speech in quiet and in the presence of noise (time-reversed speech) and answered comprehension questions. Participants completed Forward Digit Span and Backward Digit Span as measures of working memory, and pure tone averages were collected. An index of reduction in noise (RIN) was calculated by normalizing the difference between raw cortical tracking in quiet and in noise. RESULTS: Comprehension question performance was greater for speech in quiet than for speech in noise. The relationship between RIN and speech in noise comprehension was assessed while controlling for the effects of individual differences in pure tone averages and working memory. Delta band RIN correlated with speech in noise comprehension, while theta band RIN did not. CONCLUSIONS: Cortical tracking by delta oscillations is robust to the effects of noise. These findings demonstrate that the magnitude of delta band RIN relates to individual differences in speech in noise comprehension in older adults. Delta band RIN may serve as a neural metric of speech in noise comprehension beyond the effects of pure tone averages and working memory.

Examining cortical tracking of the speech envelope in post-stroke aphasia.

Introduction: People with aphasia have been shown to benefit from rhythmic elements for language production during aphasia rehabilitation. However, it is unknown whether rhythmic processing is associated with such benefits. Cortical tracking of the speech envelope (CTenv) may provide a measure of encoding of speech rhythmic properties and serve as a predictor of candidacy for rhythm-based aphasia interventions. Methods: Electroencephalography was used to capture electrophysiological responses while Spanish speakers with aphasia (n = 9) listened to a continuous speech narrative (audiobook). The Temporal Response Function was used to estimate CTenv in the delta (associated with word- and phrase-level properties), theta (syllable-level properties), and alpha bands (attention-related properties). CTenv estimates were used to predict aphasia severity, performance in rhythmic perception and production tasks, and treatment response in a sentence-level rhythm-based intervention. Results: CTenv in delta and theta, but not alpha, predicted aphasia severity. Neither CTenv in delta, alpha, or theta bands predicted performance in rhythmic perception or production tasks. Some evidence supported that CTenv in theta could predict sentence-level learning in aphasia, but alpha and delta did not. Conclusion: CTenv of the syllable-level properties was relatively preserved in individuals with less language impairment. In contrast, higher encoding of word- and phrase-level properties was relatively impaired and was predictive of more severe language impairments. CTenv and treatment response to sentence-level rhythm-based interventions need to be further investigated.

A Linear Superposition Model of Envelope and Frequency Following Responses May Help Identify Generators Based on Latency.

Envelope and frequency-following responses (FFRENV and FFRTFS) are scalp-recorded electrophysiological potentials that closely follow the periodicity of complex sounds such as speech. These signals have been established as important biomarkers in speech and learning disorders. However, despite important advances, it has remained challenging to map altered FFRENV and FFRTFS to altered processing in specific brain regions. Here we explore the utility of a deconvolution approach based on the assumption that FFRENV and FFRTFS reflect the linear superposition of responses that are triggered by the glottal pulse in each cycle of the fundamental frequency (F0 responses). We tested the deconvolution method by applying it to FFRENV and FFRTFS of rhesus monkeys to human speech and click trains with time-varying pitch patterns. Our analyses show that F0ENV responses could be measured with high signal-to-noise ratio and featured several spectro-temporally and topographically distinct components that likely reflect the activation of brainstem (<5 ms; 200-1000 Hz), midbrain (5-15 ms; 100-250 Hz), and cortex (15-35 ms; ~90 Hz). In contrast, F0TFS responses contained only one spectro-temporal component that likely reflected activity in the midbrain. In summary, our results support the notion that the latency of F0 components map meaningfully onto successive processing stages. This opens the possibility that pathologically altered FFRENV or FFRTFS may be linked to altered F0ENV or F0TFS and from there to specific processing stages and ultimately spatially targeted interventions.

On the Role of Neural Oscillations Across Timescales in Speech and Music Processing.

This mini review is aimed at a clinician-scientist seeking to understand the role of oscillations in neural processing and their functional relevance in speech and music perception. We present an overview of neural oscillations, methods used to study them, and their functional relevance with respect to music processing, aging, hearing loss, and disorders affecting speech and language. We first review the oscillatory frequency bands and their associations with speech and music processing. Next we describe commonly used metrics for quantifying neural oscillations, briefly touching upon the still-debated mechanisms underpinning oscillatory alignment. Following this, we highlight key findings from research on neural oscillations in speech and music perception, as well as contributions of this work to our understanding of disordered perception in clinical populations. Finally, we conclude with a look toward the future of oscillatory research in speech and music perception, including promising methods and potential avenues for future work. We note that the intention of this mini review is not to systematically review all literature on cortical tracking of speech and music. Rather, we seek to provide the clinician-scientist with foundational information that can be used to evaluate and design research studies targeting the functional role of oscillations in speech and music processing in typical and clinical populations.

Neural tracking of the speech envelope is differentially modulated by attention and language experience.

The ability to selectively attend to a speech signal amid competing sounds is a significant challenge, especially for listeners trying to comprehend non-native speech. Attention is critical to direct neural processing resources to the most essential information. Here, neural tracking of the speech envelope of an English story narrative and cortical auditory evoked potentials (CAEPs) to non-speech stimuli were simultaneously assayed in native and non-native listeners of English. Although native listeners exhibited higher narrative comprehension accuracy, non-native listeners exhibited enhanced neural tracking of the speech envelope and heightened CAEP magnitudes. These results support an emerging view that although attention to a target speech signal enhances neural tracking of the speech envelope, this mechanism itself may not confer speech comprehension advantages. Our findings suggest that non-native listeners may engage neural attentional processes that enhance low-level acoustic features, regardless if the target signal contains speech or non-speech information.

The neural processing of pitch accents in continuous speech.

Pitch accents are local pitch patterns that convey differences in word prominence and modulate the information structure of the discourse. Despite the importance to discourse in languages like English, neural processing of pitch accents remains understudied. The current study investigates the neural processing of pitch accents by native and non-native English speakers while they are listening to or ignoring 45 min of continuous, natural speech. Leveraging an approach used to study phonemes in natural speech, we analyzed thousands of electroencephalography (EEG) segments time-locked to pitch accents in a prosodic transcription. The optimal neural discrimination between pitch accent categories emerged at latencies between 100 and 200 ms. During these latencies, we found a strong structural alignment between neural and phonetic representations of pitch accent categories. In the same latencies, native listeners exhibited more robust processing of pitch accent contrasts than non-native listeners. However, these group differences attenuated when the speech signal was ignored. We can reliably capture the neural processing of discrete and contrastive pitch accent categories in continuous speech. Our analytic approach also captures how language-specific knowledge and selective attention influences the neural processing of pitch accent categories.

Frequency-Following Responses to Speech Sounds Are Highly Conserved across Species and Contain Cortical Contributions.

Time-varying pitch is a vital cue for human speech perception. Neural processing of time-varying pitch has been extensively assayed using scalp-recorded frequency-following responses (FFRs), an electrophysiological signal thought to reflect integrated phase-locked neural ensemble activity from subcortical auditory areas. Emerging evidence increasingly points to a putative contribution of auditory cortical ensembles to the scalp-recorded FFRs. However, the properties of cortical FFRs and precise characterization of laminar sources are still unclear. Here we used direct human intracortical recordings as well as extracranial and intracranial recordings from macaques and guinea pigs to characterize the properties of cortical sources of FFRs to time-varying pitch patterns. We found robust FFRs in the auditory cortex across all species. We leveraged representational similarity analysis as a translational bridge to characterize similarities between the human and animal models. Laminar recordings in animal models showed FFRs emerging primarily from the thalamorecipient layers of the auditory cortex. FFRs arising from these cortical sources significantly contributed to the scalp-recorded FFRs via volume conduction. Our research paves the way for a wide array of studies to investigate the role of cortical FFRs in auditory perception and plasticity.

Cortical Tracking of the Speech Envelope in Logopenic Variant Primary Progressive Aphasia.

Logopenic variant primary progressive aphasia (lvPPA) is a neurodegenerative language disorder primarily characterized by impaired phonological processing. Sentence repetition and comprehension deficits are observed in lvPPA and linked to impaired phonological working memory, but recent evidence also implicates impaired speech perception. Currently, neural encoding of the speech envelope, which forms the scaffolding for perception, is not clearly understood in lvPPA. We leveraged recent analytical advances in electrophysiology to examine speech envelope encoding in lvPPA. We assessed cortical tracking of the speech envelope and in-task comprehension of two spoken narratives in individuals with lvPPA (n = 10) and age-matched (n = 10) controls. Despite markedly reduced narrative comprehension relative to controls, individuals with lvPPA had increased cortical tracking of the speech envelope in theta oscillations, which track low-level features (e.g., syllables), but not delta oscillations, which track speech units that unfold across a longer time scale (e.g., words, phrases, prosody). This neural signature was highly correlated across narratives. Results indicate an increased reliance on acoustic cues during speech encoding. This may reflect inefficient encoding of bottom-up speech cues, likely as a consequence of dysfunctional temporoparietal cortex.

Dichotic phase effects on frequency following responses reveal phase variant and invariant harmonic distortion products.

The dichotic frequency following responses (FFR) have been used in studies to infer about dichotic auditory processing. In the present study, we hypothesize that the proximity of the binaural neural generators of the FFR would result in interference of the volume-conducted electrical fields. This might lead to contamination of the scalp-recorded dichotic FFRs due to which it might be difficult to infer about true dichotic processing in the putative neural generators. We investigated this by recording FFRs to binaurally presented 200 Hz pure tone with graded dichotic phase offsets (0°, 90°, 180° and 270°) in normal hearing young adults. Spectral analysis of the FFRs was performed for the estimation of the magnitude and phase at the component frequencies. FFR spectra were compared using non-parametric paired randomizations within the subjects. We found that the brainstem responses to a 200 Hz pure tone consisted of prominent peaks at 200 Hz, and at frequencies corresponding to the harmonics of 200 Hz. The FFR spectral magnitude at 200 Hz diminished with a phase offset of 180°. Phase offsets of 90° and 270° showed reduced spectral magnitudes at 200 Hz than those in the 0° condition. Our findings, in line with the hypothesis, show that the dichotic FFRs do not reflect true dichotic processing and that they are contaminated during volume conduction. Additionally, we found harmonic distortion products (HDP) in the FFRs. We found that the response at 200 Hz and the 3rd HDP systematically varied with a change in phase of the stimulus, while the even HDPs (2nd and 4th) were phase-invariant. Based on our findings, and modeling FFRs using auditory models, we propose a rectification process as the contributors for the generation of HDPs. We also discuss the implications of this HDP generating mechanism in understanding the pitch represented in FFRs.

Novel Paradigm to Record Bilateral Click-Evoked Auditory Brainstem Responses Simultaneously (BiSi-ABR).

OBJECTIVES: The current study proposes a new and fast technique to record the auditory brainstem responses (ABRs) simultaneously (BiSi-ABR) from two ears. The BiSi-ABR technique can be used to record the ABRs two times faster than with a conventional ABR recording method. The objective of the study was to show the proof of concept and to compare the BiSi-ABR technique with that of a conventional ABRs recording method to test its clinical feasibility. MATERIALS AND METHODS: A repeated-measures design was used, wherein ABRs recorded in the BiSi-ABR were compared with that of conventional ABRs recordings. Twenty-five normal-hearing adults participated in the study. ABRs were recorded using the BiSi-ABR technique, as well as the conventional method. The peak latencies (in ms) of waves III and V between the new technique and conventional method were compared. The minimum intensity at which the wave V was present was tracked using both the methods. RESULTS: The wave latencies and thresholds of ABR using the BiSi-ABR technique were remarkably similar to those recorded in the conventional ABR technique. The ABR wave latencies and thresholds did not differ significantly between the new technique and the conventional method. CONCLUSION: ABRs recorded with the BiSi-ABR technique can be used to estimate ear-specific hearing thresholds with the same reliability as that of conventional ABRs, in half the recording time. The results of the study have strong implications for screening, diagnostic, and research purposes as they aid in cutting down the ABR testing time.

Gender-bias in the sensory representation of infant cry.

The auditory neural pathway in females appears to be more sensitive to the cry of an infant (De Pisapia et al., 2013; Messina et al., 2016). Cortical responses in females have shown a distinct advantage compared to males in the auditory processing of infant cry. Such gender-bias in the cortical responses might emanate either at higher levels of processing such as cognitive and emotional processing or at the lower level representation of stimulus features. We assessed for a difference if any, between the two genders, in the sensory representation of an infant's cry. We used frequency following responses (FFR) to assess the sensory representation of an infant cry. This was done in sixteen male and fifteen female non-parent adults. The FFR closely mimics the stimulus acoustics with fine temporal precision and is the measure of choice to assess the sensory encoding of sounds in the auditory system. We performed spectral analysis of the FFRs and compared the spectral magnitudes between males and females. We found significantly higher FFR spectral magnitudes in females compared to males. The gender differences found were not related to the confounding variables such as head size or differences in the volume-conducting media. By systematically controlling other influencing variables, we show that the bias in neural processing of infant cry in females emerges right at the sensory representation levels.

Functional Interplay Between the Putative Measures of Rostral and Caudal Efferent Regulation of Speech Perception in Noise.

Efferent modulation has been demonstrated to be very important for speech perception, especially in the presence of noise. We examined the functional relationship between two efferent systems: the rostral and caudal efferent pathways and their individual influences on speech perception in noise. Earlier studies have shown that these two efferent mechanisms were correlated with speech perception in noise. However, previously, these mechanisms were studied in isolation, and their functional relationship with each other was not investigated. We used a correlational design to study the relationship if any, between these two mechanisms in young and old normal hearing individuals. We recorded context-dependent brainstem encoding as an index of rostral efferent function and contralateral suppression of otoacoustic emissions as an index of caudal efferent function in groups with good and poor speech perception in noise. These efferent mechanisms were analysed for their relationship with each other and with speech perception in noise. We found that the two efferent mechanisms did not show any functional relationship. Interestingly, both the efferent mechanisms correlated with speech perception in noise and they even emerged as significant predictors. Based on the data, we posit that the two efferent mechanisms function relatively independently but with a common goal of fine-tuning the afferent input and refining auditory perception in degraded listening conditions.

Acoustic basis of context dependent brainstem encoding of speech.

The newfound context dependent brainstem encoding of speech is evidence of online regularity detection and modulation of the sub-cortical responses. We studied the influence of spectral structure of the contextual stimulus on context dependent encoding of speech at the brainstem, in an attempt to understand the acoustic basis for this effect. Fourteen normal hearing adults participated in a randomized true experimental design in whom brainstem responses were recorded. Brainstem responses for a high pass filtered /da/ in the context of syllables, that either had same or different spectral structure were compared with each other. The findings suggest that spectral structure is one of the parameters which cue the context dependent sub-cortical encoding of speech. Interestingly, the results also revealed that, brainstem can encode pitch even with negligible acoustic information below the second formant frequency.