Neural encoding and perception of speech signals in informational masking

Auditory Sensory Gating: Effects of Noise

Cortical auditory evoked potentials (CAEPs) indicate that noise degrades auditory neural encoding, causing decreased peak amplitude and increased peak latency. Different types of noise affect CAEP responses, with greater informational masking causing additional degradation. In noisy conditions, attention can improve target signals' neural encoding, reflected by an increased CAEP amplitude, which may be facilitated through various inhibitory mechanisms at both pre-attentive and attentive levels. While previous research has mainly focused on inhibition effects during attentive auditory processing in noise, the impact of noise on the neural response during the pre-attentive phase remains unclear. Therefore, this preliminary study aimed to assess the auditory gating response, reflective of the sensory inhibitory stage, to repeated vowel pairs presented in background noise. CAEPs were recorded via high-density EEG in fifteen normal-hearing adults in quiet and noise conditions with low and high informational masking. The difference between the average CAEP peak amplitude evoked by each vowel in the pair was compared across conditions. Scalp maps were generated to observe general cortical inhibitory networks in each condition. Significant gating occurred in quiet, while noise conditions resulted in a significantly decreased gating response. The gating function was significantly degraded in noise with less informational masking content, coinciding with a reduced activation of inhibitory gating networks. These findings illustrate the adverse effect of noise on pre-attentive inhibition related to speech perception.

Electrophysiological Measures of Listening-in-Noise With and Without Remote Microphone System Use in Autistic and Non-Autistic Youth

OBJECTIVES

This study examined the neural mechanisms by which remote microphone (RM) systems might lead to improved behavioral performance on listening-in-noise tasks in autistic and non-autistic youth.

DESIGN

Cortical auditory evoked potentials (CAEPs) were recorded in autistic (n = 25) and non-autistic (n = 22) youth who were matched at the group level on chronological age ( M = 14.21 ± 3.39 years) and biological sex. Potentials were recorded during an active syllable identification task completed in quiet and in multi-talker babble noise with and without the use of an RM system. The effects of noise and RM system use on speech-sound-evoked P1-N1-P2 responses and the associations between the cortical responses and behavioral performance on syllable identification were examined.

RESULTS

No group differences were observed for behavioral or CAEP measures of speech processing in quiet or in noise. In the combined sample, syllable identification in noise was less accurate and slower than in the quiet condition. The addition of the RM system to the noise condition restored accuracy, but not the response speed, to the levels observed in quiet. The CAEP analyses noted amplitude reductions and latency delays in the noise compared with the quiet condition. The RM system use increased the N1 amplitude as well as reduced and delayed the P2 response relative to the quiet and noise conditions. Exploratory brain-behavior correlations revealed that larger N1 amplitudes in the RM condition were associated with greater behavioral accuracy of syllable identification. Reduced N1 amplitude and accelerated P2 response were associated with shorter syllable identification response times when listening with the RM system.

CONCLUSIONS

Findings suggest that although listening-in-noise with an RM system might remain effortful, the improved signal to noise ratio facilitates attention to the sensory features of the stimuli and increases speech sound identification accuracy.

Modulation of auditory temporal processing, speech in noise perception, auditory-verbal memory, and reading efficiency by anodal tDCS in children with dyslexia

Dyslexia is a neurodevelopmental disorder that is prevalent in children. It is estimated that 30-50% of individuals diagnosed with dyslexia also manifest an auditory perceptual deficit characteristic of auditory processing disorder (APD). Some studies suggest that defects in basic auditory processing can lead to phonological defects as the most prominent cause of dyslexia. Thus, in some cases, there may be interrelationships between dyslexia and some of the aspects of central auditory processing. In recent years, transcranial direct current stimulation (tDCS) has been used as a safe method for the modulation of central auditory processing aspects in healthy adults and reading skills in children with dyslexia. Therefore, the objectives of our study were to investigate the effect of tDCS on the modulation of different aspects of central auditory processing, aspects of reading, and the relationship between these two domains in dyslexic children with APD. A within-subjects design was employed to investigate the effect of two electrode arrays (the anode on the left STG (AC)/cathode on the right shoulder and anode on the left STG/cathode on the right STG) on auditory temporal processing; speech-in-noise perception, short-term auditory memory; and high-frequency word, low-frequency word, pseudoword, and text reading. The results of this clinical trial showed the modulation of the studied variables in central auditory processing and the accuracy and speed of reading variables compared to the control and sham statuses in both electrode arrays. Our results also showed that the improvement of the accuracy and speed of text reading, as well as the accuracy of pseudoword reading were related to the improvement of speech in noise perception and temporal processing. The results of this research can be effective in clarifying the basis of the neurobiology of dyslexia and, in particular, the hypothesis of the role of basic auditory processing and subsequently the role of the auditory cortex in dyslexia. These results might provide a framework to facilitate behavioral rehabilitation in dyslexic children with APD.

Automatic Speech Discrimination Assessment Methods Based on Event-Related Potentials (ERP)

Speech discrimination is used by audiologists in diagnosing and determining treatment for hearing loss patients. Usually, assessing speech discrimination requires subjective responses. Using electroencephalography (EEG), a method that is based on event-related potentials (ERPs), could provide objective speech discrimination. In this work we proposed a visual-ERP-based method to assess speech discrimination using pictures that represent word meaning. The proposed method was implemented with three strategies, each with different number of pictures and test sequences. Machine learning was adopted to classify between the task conditions based on features that were extracted from EEG signals. The results from the proposed method were compared to that of a similar visual-ERP-based method using letters and a method that is based on the auditory mismatch negativity (MMN) component. The P3 component and the late positive potential (LPP) component were observed in the two visual-ERP-based methods while MMN was observed during the MMN-based method. A total of two out of three strategies of the proposed method, along with the MMN-based method, achieved approximately 80% average classification accuracy by a combination of support vector machine (SVM) and common spatial pattern (CSP). Potentially, these methods could serve as a pre-screening tool to make speech discrimination assessment more accessible, particularly in areas with a shortage of audiologists.

Informational Masking Effects of Similarity and Uncertainty on Early and Late Stages of Auditory Cortical Processing

PURPOSE

Understanding speech in a background of other people talking is a difficult listening situation for hearing-impaired individuals, and even for those with normal hearing. Speech-on-speech masking is known to contribute to increased perceptual difficulty over nonspeech background noise because of informational masking provided over and above the effects of energetic masking. While informational masking research has identified factors of similarity and uncertainty between target and masker that contribute to reduced behavioral performance in speech background noise, critical gaps in knowledge including the underlying neural-perceptual processes remain. By systematically manipulating aspects of acoustic similarity and uncertainty in the same auditory paradigm, the current study examined the time course and objectively quantified these informational masking effects at both early and late stages of auditory processing using auditory evoked potentials (AEPs).

METHOD

Thirty participants were included in a cross-sectional repeated measures design. Target-masker similarity was manipulated by varying the linguistic/phonetic similarity (i.e., language) of the talkers in the background. Specifically, four levels representing hypothesized increasing levels of informational masking were implemented: (1) no masker (quiet); (2) Mandarin; (3) Dutch; and (4) English. Stimulus uncertainty was manipulated by task complexity, specifically presentation of target-to-target interval (TTI) in the auditory evoked paradigm. Participants had to discriminate between English word stimuli (/bæt/ and /pæt/) presented in an oddball paradigm under each masker condition pressing buttons to either the target or standard stimulus. Responses were recorded simultaneously for P1-N1-P2 (standard waveform) and P3 (target waveform). This design allowed for simultaneous recording of multiple AEP peaks, as well as accuracy, reaction time, and d' behavioral discrimination to button press responses.

RESULTS

Several trends in AEP components were consistent with effects of increasing linguistic/phonetic similarity and stimulus uncertainty. All babble maskers significantly affected outcomes compared to quiet. In addition, the native language English masker had the largest effect on outcomes in the AEP paradigm, including reduced P3 amplitude and area, as well as decreased accuracy and d' behavioral discrimination to target word responses. AEP outcomes for the Mandarin and Dutch maskers, however, were not significantly different across any measured component. Latency outcomes for both N1 and P3 also supported an effect of stimulus uncertainty, consistent with increased processing time related to greater task complexity. An unanticipated result was the absence of the interaction of linguistic/phonetic similarity and stimulus uncertainty.

CONCLUSIONS

Observable effects of both similarity and uncertainty were evidenced at a level of the P3 more than the earlier N1 level of auditory cortical processing suggesting that higher-level active auditory processing may be more sensitive to informational masking deficits. The lack of significant interaction between similarity and uncertainty at either level of processing suggests that these informational masking factors operated independently. Speech babble maskers across languages altered AEP component measures, behavioral detection, and reaction time. Specifically, this occurred when the babble was in the native/same language as the target, while the effects of foreign language maskers did not differ. The objective results from this study provide a foundation for further investigation of how the linguistic content of target and masker and task difficulty contribute to difficulty understanding speech-in-noise.

Auditory cortex is susceptible to lexical influence as revealed by informational vs. energetic masking of speech categorization

Speech perception requires the grouping of acoustic information into meaningful phonetic units via the process of categorical perception (CP). Environmental masking influences speech perception and CP. However, it remains unclear at which stage of processing (encoding, decision, or both) masking affects listeners' categorization of speech signals. The purpose of this study was to determine whether linguistic interference influences the early acoustic-phonetic conversion process inherent to CP. To this end, we measured source level, event related brain potentials (ERPs) from auditory cortex (AC) and inferior frontal gyrus (IFG) as listeners rapidly categorized speech sounds along a /da/ to /ga/ continuum presented in three listening conditions: quiet, and in the presence of forward (informational masker) and time-reversed (energetic masker) 2-talker babble noise. Maskers were matched in overall SNR and spectral content and thus varied only in their degree of linguistic interference (i.e., informational masking). We hypothesized a differential effect of informational versus energetic masking on behavioral and neural categorization responses, where we predicted increased activation of frontal regions when disambiguating speech from noise, especially during lexical-informational maskers. We found (1) informational masking weakens behavioral speech phoneme identification above and beyond energetic masking; (2) low-level AC activity not only codes speech categories but is susceptible to higher-order lexical interference; (3) identifying speech amidst noise recruits a cross hemispheric circuit (AC_left → IFG_right) whose engagement varies according to task difficulty. These findings provide corroborating evidence for top-down influences on the early acoustic-phonetic analysis of speech through a coordinated interplay between frontotemporal brain areas.

Development of cortical auditory responses to speech in noise in unilaterally deaf adults following cochlear implantation

BACKGROUND

For patients with single-sided deafness (SSD), restoration of binaural function via cochlear implant (CI) has been shown to improve speech understanding in noise. The objective of this study was to investigate changes in behavioral performance and cortical auditory responses following cochlear implantation.

DESIGN

Prospective longitudinal study.

SETTING

Tertiary referral center.

METHODS

Six adults with SSD were tested before and 12 months post-activation of the CI. Six normal hearing (NH) participants served as experimental controls. Speech understanding in noise was evaluated for various spatial conditions. Cortical auditory evoked potentials were recorded with /ba/ stimuli in quiet and in noise. Global field power and responses at Cz were analyzed.

RESULTS

Speech understanding in noise significantly improved with the CI when speech was presented to the CI ear and noise to the normal ear (p<0.05), but remained poorer than that of NH controls (p<0.05). N1 peak amplitude measure in noise significantly increased after CI activation (p<0.05), but remained lower than that of NH controls (p<0.05) at 12 months. After 12 months of CI experience, cortical responses in noise became more comparable between groups.

CONCLUSION

Binaural restoration in SSD patients via cochlear implantation improved speech performance noise and cortical responses. While behavioral performance and cortical auditory responses improved, SSD-CI outcomes remained poorer than that of NH controls in most cases, suggesting only partial restoration of binaural hearing.

Investigating Influences of Medial Olivocochlear Efferent System on Central Auditory Processing and Listening in Noise: A Behavioral and Event-Related Potential Study

This electrophysiological study investigated the role of the medial olivocochlear (MOC) efferents in listening in noise. Both ears of eleven normal-hearing adult participants were tested. The physiological tests consisted of transient-evoked otoacoustic emission (TEOAE) inhibition and the measurement of cortical event-related potentials (ERPs). The mismatch negativity (MMN) and P300 responses were obtained in passive and active listening tasks, respectively. Behavioral responses for the word recognition in noise test were also analyzed. Consistent with previous findings, the TEOAE data showed significant inhibition in the presence of contralateral acoustic stimulation. However, performance in the word recognition in noise test was comparable for the two conditions (i.e., without contralateral stimulation and with contralateral stimulation). Peak latencies and peak amplitudes of MMN and P300 did not show changes with contralateral stimulation. Behavioral performance was also maintained in the P300 task. Together, the results show that the peripheral auditory efferent effects captured via otoacoustic emission (OAE) inhibition might not necessarily be reflected in measures of central cortical processing and behavioral performance. As the MOC effects may not play a role in all listening situations in adults, the functional significance of the cochlear effects of the medial olivocochlear efferents and the optimal conditions conducive to corresponding effects in behavioral and cortical responses remain to be elucidated.

Effects of Signal Type and Noise Background on Auditory Evoked Potential N1, P2, and P3 Measurements in Blast-Exposed Veterans

OBJECTIVES

Veterans who have been exposed to high-intensity blast waves frequently report persistent auditory difficulties such as problems with speech-in-noise (SIN) understanding, even when hearing sensitivity remains normal. However, these subjective reports have proven challenging to corroborate objectively. Here, we sought to determine whether use of complex stimuli and challenging signal contrasts in auditory evoked potential (AEP) paradigms rather than traditional use of simple stimuli and easy signal contrasts improved the ability of these measures to (1) distinguish between blast-exposed Veterans with auditory complaints and neurologically normal control participants, and (2) predict behavioral measures of SIN perception.

DESIGN

A total of 33 adults (aged 19-56 years) took part in this study, including 17 Veterans exposed to high-intensity blast waves within the past 10 years and 16 neurologically normal control participants matched for age and hearing status with the Veteran participants. All participants completed the following test measures: (1) a questionnaire probing perceived hearing abilities; (2) behavioral measures of SIN understanding including the BKB-SIN, the AzBio presented in 0 and +5 dB signal to noise ratios (SNRs), and a word-level consonant-vowel-consonant test presented at +5 dB SNR; and (3) electrophysiological tasks involving oddball paradigms in response to simple tones (500 Hz standard, 1000 Hz deviant) and complex speech syllables (/ba/ standard, /da/ deviant) presented in quiet and in four-talker speech babble at a SNR of +5 dB.

RESULTS

Blast-exposed Veterans reported significantly greater auditory difficulties compared to control participants. Behavioral performance on tests of SIN perception was generally, but not significantly, poorer among the groups. Latencies of P3 responses to tone signals were significantly longer among blast-exposed participants compared to control participants regardless of background condition, though responses to speech signals were similar across groups. For cortical AEPs, no significant interactions were found between group membership and either stimulus type or background. P3 amplitudes measured in response to signals in background babble accounted for 30.9% of the variance in subjective auditory reports. Behavioral SIN performance was best predicted by a combination of N1 and P2 responses to signals in quiet which accounted for 69.6% and 57.4% of the variance on the AzBio at 0 dB SNR and the BKB-SIN, respectively.

CONCLUSIONS

Although blast-exposed participants reported far more auditory difficulties compared to controls, use of complex stimuli and challenging signal contrasts in cortical and cognitive AEP measures failed to reveal larger group differences than responses to simple stimuli and easy signal contrasts. Despite this, only P3 responses to signals presented in background babble were predictive of subjective auditory complaints. In contrast, cortical N1 and P2 responses were predictive of behavioral SIN performance but not subjective auditory complaints, and use of challenging background babble generally did not improve performance predictions. These results suggest that challenging stimulus protocols are more likely to tap into perceived auditory deficits, but may not be beneficial for predicting performance on clinical measures of SIN understanding. Finally, these results should be interpreted with caution since blast-exposed participants did not perform significantly poorer on tests of SIN perception.

Impaired auditory processing and neural representation of speech in noise among symptomatic post-concussion adults

Background: The purpose of the study was to examine auditory event-related potential (AERP) evidence of changes in earlier and later stages of auditory processing in individuals with long-term post-concussion problems compared to healthy controls, with a secondary aim of comparing AERPs by functional auditory behavioral outcomes. Methods: P1-N1-P2 complex and P300 components recorded to speech in quiet and background noise conditions were completed in individuals with ongoing post-concussion symptoms following mTBI and healthy controls. AERPs were also examined between sub-groups with normal or impaired auditory processing by behavioral tests. Results: Group differences were present for later stages of auditory processing (P300). Earlier components did not significantly differ by group overall but were more affected by noise in the mTBI group. P2 amplitude in noise differed between mTBI sub-groups with normal or impaired auditory processing. Conclusion: AERPs revealed differences between healthy controls and those with chronic post-concussion symptoms following mTBI at a later stage of auditory processing (P300). Neural processing at the earlier stage (P1-N1-P2) was more affected by noise in the mTBI group. Preliminary evidence suggested that it may be only the proportion of individuals with functional evidence of central auditory dysfunction with changes in AERPs at earlier stages of processing.

Neuronal Correlates of Informational and Energetic Masking in the Human Brain in a Multi-Talker Situation

Human listeners can follow the voice of one speaker while several others are talking at the same time. This process requires segregating the speech streams from each other and continuously directing attention to the target stream. We investigated the functional brain networks underlying this ability. Two speech streams were presented simultaneously to participants, who followed one of them and detected targets within it (target stream). The loudness of the distractor speech stream varied on five levels: moderately softer, slightly softer, equal, slightly louder, or moderately louder than the attended. Performance measures showed that the most demanding task was the moderately softer distractors condition, which indicates that a softer distractor speech may receive more covert attention than louder distractors and, therefore, they require more cognitive resources. EEG-based measurement of functional connectivity between various brain regions revealed frequency-band specific networks: (1) energetic masking (comparing the louder distractor conditions with the equal loudness condition) was predominantly associated with stronger connectivity between the frontal and temporal regions at the lower alpha (8–10 Hz) and gamma (30–70 Hz) bands; (2) informational masking (comparing the softer distractor conditions with the equal loudness condition) was associated with a distributed network between parietal, frontal, and temporal regions at the theta (4–8 Hz) and beta (13–30 Hz) bands. These results suggest the presence of distinct cognitive and neural processes for solving the interference from energetic vs. informational masking.

Effect of competing noise on cortical auditory evoked potentials elicited by speech sounds in 7- to 25-year-old listeners

Child listeners have particular difficulty with speech perception when competing speech noise is present; this challenge is often attributed to their immature top-down processing abilities. The purpose of this study was to determine if the effects of competing speech noise on speech-sound processing vary with age. Cortical auditory evoked potentials (CAEPs) were measured during an active speech-syllable discrimination task in 58 normal-hearing participants (age 7-25 years). Speech syllables were presented in quiet and embedded in competing speech noise (4-talker babble, +15 dB signal-to-noise ratio; SNR). While noise was expected to similarly reduce amplitude and delay latencies of N1 and P2 peaks in all listeners, it was hypothesized that effects of noise on the P3b peak would be inversely related to age due to the maturation of top-down processing abilities throughout childhood. Consistent with previous work, results showed that a +15 dB SNR reduces amplitudes and delays latencies of CAEPs for listeners of all ages, affecting speech-sound processing, delaying stimulus evaluation, and causing a reduction in behavioral speech-sound discrimination. Contrary to expectations, findings suggest that competing speech noise at a +15 dB SNR may have similar effects on various stages of speech-sound processing for listeners of all ages. Future research directions should examine how more difficult listening conditions (poorer SNRs) might affect results across ages.

Compensatory and Serial Processing Models for Relating Electrophysiology, Speech Understanding, and Cognition

OBJECTIVES

The objective of this study was to develop a framework for investigating the roles of neural coding and cognition in speech perception.

DESIGN

N1 and P3 auditory evoked potentials, QuickSIN speech understanding scores, and the Digit Symbol Coding cognitive test results were used to test the accuracy of either a compensatory processing model or serial processing model.

RESULTS

The current dataset demonstrated that neither the compensatory nor the serial processing model were well supported. An additive processing model may best represent the relationships in these data.

CONCLUSIONS

With the outcome measures used in this study, it is apparent that an additive processing model, where exogenous neural coding and higher order cognition contribute independently, best describes the effects of neural coding and cognition on speech perception. Further testing with additional outcome measures and a larger number of subjects is needed to confirm and further clarify the relationships between these processing domains.

A perspective on brain-behavior relationships and effects of age and hearing using speech-in-noise stimuli

Understanding speech in background noise is often more difficult for individuals who are older and have hearing impairment than for younger, normal-hearing individuals. In fact, speech-understanding abilities among older individuals with hearing impairment varies greatly. Researchers have hypothesized that some of that variability can be explained by how the brain encodes speech signals in the presence of noise, and that brain measures may be useful for predicting behavioral performance in difficult-to-test patients. In a series of experiments, we have explored the effects of age and hearing impairment in both brain and behavioral domains with the goal of using brain measures to improve our understanding of speech-in-noise difficulties. The behavioral measures examined showed effect sizes for hearing impairment that were 6-10 dB larger than the effects of age when tested in steady-state noise, whereas electrophysiological age effects were similar in magnitude to those of hearing impairment. Both age and hearing status influence neural responses to speech as well as speech understanding in background noise. These effects can in turn be modulated by other factors, such as the characteristics of the background noise itself. Finally, the use of electrophysiology to predict performance on receptive speech-in-noise tasks holds promise, demonstrating root-mean-square prediction errors as small as 1-2 dB. An important next step in this field of inquiry is to sample the aging and hearing impairment variables continuously (rather than categorically) - across the whole lifespan and audiogram - to improve effect estimates.

Fatigue Related to Speech Processing in Children With Hearing Loss: Behavioral, Subjective, and Electrophysiological Measures

PURPOSE

The purpose of this study was to examine fatigue associated with sustained and effortful speech-processing in children with mild to moderately severe hearing loss.

METHOD

We used auditory P300 responses, subjective reports, and behavioral indices (response time, lapses of attention) to measure fatigue resulting from sustained speech-processing demands in 34 children with mild to moderately severe hearing loss (M = 10.03 years, SD = 1.93).

RESULTS

Compared to baseline values, children with hearing loss showed increased lapses in attention, longer reaction times, reduced P300 amplitudes, and greater reports of fatigue following the completion of the demanding speech-processing tasks.

CONCLUSIONS

Similar to children with normal hearing, children with hearing loss demonstrate reductions in attentional processing of speech in noise following sustained speech-processing tasks-a finding consistent with the development of fatigue.

Background Suppression and its Relation to Foreground Processing of Speech Versus Non-speech Streams

Since sound perception takes place against a background with a certain amount of noise, both speech and non-speech processing involve extraction of target signals and suppression of background noise. Previous works on early processing of speech phonemes largely neglected how background noise is encoded and suppressed. This study aimed to fill in this gap. We adopted an oddball paradigm where speech (vowels) or non-speech stimuli (complex tones) were presented with or without a background of amplitude-modulated noise and analyzed cortical responses related to foreground stimulus processing, including mismatch negativity (MMN), N2b, and P300, as well as neural representations of the background noise, that is, auditory steady-state response (ASSR). We found that speech deviants elicited later and weaker MMN, later N2b, and later P300 than non-speech ones, but N2b and P300 had similar strength, suggesting more complex processing of certain acoustic features in speech. Only for vowels, background noise enhanced N2b strength relative to silence, suggesting an attention-related speech-specific process to improve perception of foreground targets. In addition, noise suppression in speech contexts, quantified by ASSR amplitude reduction after stimulus onset, was lateralized towards the left hemisphere. The left-lateralized suppression following N2b was associated with the N2b enhancement in noise for speech, indicating that foreground processing may interact with background suppression, particularly during speech processing. Together, our findings indicate that the differences between perception of speech and non-speech sounds involve not only the processing of target information in the foreground but also the suppression of irrelevant aspects in the background.

Speech-Processing Fatigue in Children: Auditory Event-Related Potential and Behavioral Measures

PURPOSE

Fatigue related to speech processing is an understudied area that may have significant negative effects, especially in children who spend the majority of their school days listening to classroom instruction.

METHOD

This study examined the feasibility of using auditory P300 responses and behavioral indices (lapses of attention and self-report) to measure fatigue resulting from sustained listening demands in 27 children (M = 9.28 years).

RESULTS

Consistent with predictions, increased lapses of attention, longer reaction times, reduced P300 amplitudes to infrequent target stimuli, and self-report of greater fatigue were observed after the completion of a series of demanding listening tasks compared with the baseline values. The event-related potential responses correlated with the behavioral measures of performance.

CONCLUSION

These findings suggest that neural and behavioral responses indexing attention and processing resources show promise as effective markers of fatigue in children.

Cortical signal-in-noise coding varies by noise type, signal-to-noise ratio, age, and hearing status

The purpose of this study was to determine the effects of noise type, signal-to-noise ratio (SNR), age, and hearing status on cortical auditory evoked potentials (CAEPs) to speech sounds. This helps to explain the hearing-in-noise difficulties often seen in the aging and hearing impaired population. Continuous, modulated, and babble noise types were presented at varying SNRs to 30 individuals divided into three groups according to age and hearing status. Significant main effects of noise type, SNR, and group were found. Interaction effects revealed that the SNR effect varies as a function of noise type and is most systematic for continuous noise. Effects of age and hearing loss were limited to CAEP latency and were differentially modulated by energetic and informational-like masking. It is clear that the spectrotemporal characteristics of signals and noises play an important role in determining the morphology of neural responses. Participant factors such as age and hearing status, also play an important role in determining the brain's response to complex auditory stimuli and contribute to the ability to listen in noise.

Neural indices of phonemic discrimination and sentence-level speech intelligibility in quiet and noise: A mismatch negativity study

Successful speech communication requires the extraction of important acoustic cues from irrelevant background noise. In order to better understand this process, this study examined the effects of background noise on mismatch negativity (MMN) latency, amplitude, and spectral power measures as well as behavioral speech intelligibility tasks. Auditory event-related potentials (AERPs) were obtained from 15 normal-hearing participants to determine whether pre-attentive MMN measures recorded in response to a consonant (from /ba/ to /bu/) and vowel change (from /ba/ to /da/) in a double-oddball paradigm can predict sentence-level speech perception. The results showed that background noise increased MMN latencies and decreased MMN amplitudes with a reduction in the theta frequency band power. Differential noise-induced effects were observed for the pre-attentive processing of consonant and vowel changes due to different degrees of signal degradation by noise. Linear mixed-effects models further revealed significant correlations between the MMN measures and speech intelligibility scores across conditions and stimuli. These results confirm the utility of MMN as an objective neural marker for understanding noise-induced variations as well as individual differences in speech perception, which has important implications for potential clinical applications.

ERP responses to processing prosodic phrasing of sentences in amplitude modulated noise

Intonation phrase boundaries (IPBs) were hypothesized to be especially difficult to process in the presence of an amplitude modulated noise masker because of a potential rhythmic competition. In an event-related potential study, IPBs were presented in silence, stationary, and amplitude modulated noise. We elicited centro-parietal Closure Positive Shifts (CPS) in 23 young adults with normal hearing at IPBs in all acoustic conditions, albeit with some differences. CPS peak amplitudes were highest in stationary noise, followed by modulated noise, and lowest in silence. Both noise types elicited CPS delays, slightly more so in stationary compared to amplitude modulated noise. These data suggest that amplitude modulation is not tantamount to a rhythmic competitor for prosodic phrasing but rather supports an assumed speech perception benefit due to local release from masking. The duration of CPS time windows was, however, not only longer in noise compared to silence, but also longer for amplitude modulated compared to stationary noise. This is interpreted as support for additional processing load associated with amplitude modulation for the CPS component. Taken together, processing prosodic phrasing of sentences in amplitude modulated noise seems to involve the same issues that have been observed for the perception and processing of segmental information that are related to lexical items presented in noise: a benefit from local release from masking, even for prosodic cues, and a detrimental additional processing load that is associated with either stream segregation or signal reconstruction.

Diabetes-Associated Changes in Cortical Auditory-Evoked Potentials in Relation to Normal Aging

OBJECTIVES

(1) To characterize the influence of type 2 diabetes mellitus (DM) on cortical auditory-evoked potentials (CAEPs) separate from the effects of normal aging, and (2) to determine whether the disease-related effects are modified by insulin dependence.

DESIGN

A cross-sectional study was conducted in a large cohort of Veterans to investigate the relationships among type 2 DM, age, and CAEPs in randomly selected participants with (N = 108) and without (N = 114) the disease and who had no more than a moderate hearing loss. Participants with DM were classified as insulin-dependent (IDDM, N = 47) or noninsulin-dependent (NIDDM, N = 61). Other DM measures included concurrent serum glucose, HbA1c, and duration of disease. CAEPs were evoked using a passive homogeneous paradigm (single repeating stimulus) by suprathreshold tones presented to the right ear, left ear, or both ears. Outcome measures were adjusted for the pure-tone threshold average for frequencies of 0.5, 1, and 2 kHz and analyzed for differences in age effects between participant groups using multiple regression.

RESULTS

There is little variation across test ear conditions (left, right, binaural) on any CAEP peak in any of the groups. Among no-DM controls, P2 latency increases about 9 msec per decade of life. DM is associated with an additional delay in the P2 latency of 7 and 9 msec for the IDDM and NIDDM groups, respectively. Moreover, the slope of the function relating P2 latency with age is similar across participant groups and thus the DM effect appears constant across age. Effects on N1 latency are considerably weaker, with age effects of less than 4 msec per decade across all groups, and DM effects of only 2 (IDDM) or 3 msec (NIDDM). In the NIDDM group, the slope relating N1 latency to age is steeper relative to that observed for the no-DM group, providing some evidence of accelerated "aging" for this CAEP peak. DM does not substantially reduce N1-P2 amplitude and age relationships with N1-P2 amplitude are effectively absent. There is no association between pure-tone average at 0.5, 1, and 2 kHz and any aspect of CAEPs in this cohort.

CONCLUSIONS

In a large cohort of Veterans, we found that type 2 DM is associated with prolonged N1 and P2 latencies regardless of whether insulin is required to manage the disease and independent of peripheral hearing thresholds. The DM-related effects on CAEP latencies are threefold greater for P2 compared with N1, and there is little support that at the cortical level, IDDM participants had poorer responses compared with NIDDM participants, although their responses were more variable. Overall, these results indicate that DM is associated with slowed preattentive neural conduction. Moreover, the observed 7 to 9 msec P2 latency delay due to DM is substantial compared with normal age changes in P2, which are 9 msec per decade of life in this cohort. Results also suggest that whereas N1 latency changes with age are more pronounced among individuals with DM versus without DM, there was no evidence for more rapid aging of P2 among patients with DM. Thus, the damage responsible for the major DM-related differences may occur early in the DM disease process. These cross-sectional results should be verified using a longitudinal study design.

Functional changes in inter- and intra-hemispheric cortical processing underlying degraded speech perception

Previous studies suggest that at poorer signal-to-noise ratios (SNRs), auditory cortical event-related potentials are weakened, prolonged, and show a shift in the functional lateralization of cerebral processing from left to right hemisphere. Increased right hemisphere involvement during speech-in-noise (SIN) processing may reflect the recruitment of additional brain resources to aid speech recognition or alternatively, the progressive loss of involvement from left linguistic brain areas as speech becomes more impoverished (i.e., nonspeech-like). To better elucidate the brain basis of SIN perception, we recorded neuroelectric activity in normal hearing listeners to speech sounds presented at various SNRs. Behaviorally, listeners obtained superior SIN performance for speech presented to the right compared to the left ear (i.e., right ear advantage). Source analysis of neural data assessed the relative contribution of region-specific neural generators (linguistic and auditory brain areas) to SIN processing. We found that left inferior frontal brain areas (e.g., Broca's areas) partially disengage at poorer SNRs but responses do not right lateralize with increasing noise. In contrast, auditory sources showed more resilience to noise in left compared to right primary auditory cortex but also a progressive shift in dominance from left to right hemisphere at lower SNRs. Region- and ear-specific correlations revealed that listeners' right ear SIN advantage was predicted by source activity emitted from inferior frontal gyrus (but not primary auditory cortex). Our findings demonstrate changes in the functional asymmetry of cortical speech processing during adverse acoustic conditions and suggest that "cocktail party" listening skills depend on the quality of speech representations in the left cerebral hemisphere rather than compensatory recruitment of right hemisphere mechanisms.

Long-latency auditory evoked potentials with verbal and nonverbal stimuli

Introduction

Long-latency auditory evoked potentials represent the cortical activity related to attention, memory, and auditory discrimination skills. Acoustic signal processing occurs differently between verbal and nonverbal stimuli, influencing the latency and amplitude patterns.

Objective

To describe the latencies of the cortical potentials P1, N1, P2, N2, and P3, as well as P3 amplitude, with different speech stimuli and tone bursts, and to classify them in the presence and absence of these data.

Methods

A total of 30 subjects with normal hearing were assessed, aged 18–32 years old, matched by gender. Nonverbal stimuli were used (tone burst; 1000 Hz – frequent and 4000 Hz – rare); and verbal (/ba/ – frequent; /ga/, /da/, and /di/ – rare).

Results

Considering the component N2 for tone burst, the lowest latency found was 217.45 ms for the BA/DI stimulus; the highest latency found was 256.5 ms. For the P3 component, the shortest latency with tone burst stimuli was 298.7 with BA/GA stimuli, the highest, was 340 ms. For the P3 amplitude, there was no statistically significant difference among the different stimuli. For latencies of components P1, N1, P2, N2, P3, there were no statistical differences among them, regardless of the stimuli used.

Conclusion

There was a difference in the latency of potentials N2 and P3 among the stimuli employed but no difference was observed for the P3 amplitude.

Frontal midline θ power as an index of listening effort

Attempts to identify physiological correlates of listening effort have mainly focused on peripheral measures (e.g. pupillometry) and auditory-evoked/event-related potentials. Although nonauditory studies have suggested that sustained time-frequency electroencephalographic (EEG) features in the θ-band (4-7 Hz) are correlated with domain-general mental effort, little work has characterized such features during effortful listening. Here, high-density EEG data was collected while listeners performed a sentence-recognition task in noise, the signal-to-noise ratio (SNR) of which varied across blocks. Frontal midline θ (Fmθ), largely driven by sources localized in or near the medial frontal cortex, showed greater power with decreasing SNR and was positively correlated with self-reports of effort. Increased Fmθ was present before speech onset and during speech presentation. Fmθ power also differed across SNRs when including only trials in which all words were recognized, suggesting that the effects were unrelated to performance differences. Results suggest that frontal cortical networks play a larger role in listening as acoustic signals are increasingly masked. Further, sustained time-frequency EEG features may usefully supplement previously used peripheral and event-related potential measures in psychophysiological investigations of effortful listening.

Effects of background noise on inter-trial phase coherence and auditory N1-P2 responses to speech stimuli

This study investigated the effects of a speech-babble background noise on inter-trial phase coherence (ITPC, also referred to as phase locking value (PLV)) and auditory event-related responses (AERP) to speech sounds. Specifically, we analyzed EEG data from 11 normal hearing subjects to examine whether ITPC can predict noise-induced variations in the obligatory N1-P2 complex response. N1-P2 amplitude and latency data were obtained for the /bu/syllable in quiet and noise listening conditions. ITPC data in delta, theta, and alpha frequency bands were calculated for the N1-P2 responses in the two passive listening conditions. Consistent with previous studies, background noise produced significant amplitude reduction and latency increase in N1 and P2, which were accompanied by significant ITPC decreases in all the three frequency bands. Correlation analyses further revealed that variations in ITPC were able to predict the amplitude and latency variations in N1-P2. The results suggest that trial-by-trial analysis of cortical neural synchrony is a valuable tool in understanding the modulatory effects of background noise on AERP measures.

Physical activity enhances metabolic fitness independently of cardiorespiratory fitness in marathon runners

High levels of cardiovascular fitness (CRF) and physical activity (PA) are associated with decreased mortality and risk to develop metabolic diseases. The independent contributions of CRF and PA to metabolic disease risk factors are unknown. We tested the hypothesis that runners who run consistently >50 km/wk and/or >2 marathons/yr for the last 5 years have superior metabolic fitness compared to matched sedentary subjects (CRF, age, gender, and BMI). Case-control recruitment of 31 pairs of runner-sedentary subjects identified 10 matched pairs with similar VO_2max (mL/min/kg) (similar-VO_2max). The similar-VO_2max group was compared with a group of age, gender, and BMI matched pairs who had the largest difference in VO_2max (different-VO_2max). Primary outcomes that defined metabolic fitness including insulin response to an oral glucose tolerance test, fasting lipids, and fasting insulin were superior in runners versus sedentary controls despite similar VO_2max. Furthermore, performance (velocity at VO_2max, running economy), improved exercise metabolism (lactate threshold), and skeletal muscle levels of mitochondrial proteins were superior in runners versus sedentary controls with similar VO_2max. In conclusion subjects with a high amount of PA have more positive metabolic health parameters independent of CRF. PA is thus a good marker against metabolic diseases.

Background noise can enhance cortical auditory evoked potentials under certain conditions

OBJECTIVE

To use cortical auditory evoked potentials (CAEPs) to understand neural encoding in background noise and the conditions under which noise enhances CAEP responses.

METHODS

CAEPs from 16 normal-hearing listeners were recorded using the speech syllable/ba/presented in quiet and speech-shaped noise at signal-to-noise ratios of 10 and 30dB. The syllable was presented binaurally and monaurally at two presentation rates.

RESULTS

The amplitudes of N1 and N2 peaks were often significantly enhanced in the presence of low-level background noise relative to quiet conditions, while P1 and P2 amplitudes were consistently reduced in noise. P1 and P2 amplitudes were significantly larger during binaural compared to monaural presentations, while N1 and N2 peaks were similar between binaural and monaural conditions.

CONCLUSIONS

Methodological choices impact CAEP peaks in very different ways. Negative peaks can be enhanced by background noise in certain conditions, while positive peaks are generally enhanced by binaural presentations.

SIGNIFICANCE

Methodological choices significantly impact CAEPs acquired in quiet and in noise. If CAEPs are to be used as a tool to explore signal encoding in noise, scientists must be cognizant of how differences in acquisition and processing protocols selectively shape CAEP responses.

Attentional modulation of the early cortical representation of speech signals in informational or energetic masking

It is easier to recognize a masked speech when the speech and its masker are perceived as spatially segregated. Using event-related potentials, this study examined how the early cortical representation of speech is affected by different masker types and perceptual locations, when the listener is either passively or actively listening to the target speech syllable. The results showed that the two-talker-speech masker induced a much larger masking effect on the N1/P2 complex than either the steady-state-noise masker or the amplitude-modulated speech-spectrum-noise masker did. Also, a switch from the passive- to active-listening condition enhanced the N1/P2 complex only when the masker was speech. Moreover, under the active-listening condition, perceived separation between target and masker enhanced the N1/P2 complex only when the masker was speech. Thus, when a masker is present, the effect of selective attention to the target-speech signal on the early cortical representation of the speech signal is masker-type dependent.

Children's perception of speech produced in a two-talker background

PURPOSE

This study evaluated the degree to which children benefit from the acoustic modifications made by talkers when they produce speech in noise.

METHOD

A repeated measures design compared the speech perception performance of children (5-11 years) and adults in a 2-talker masker. Target speech was produced in a 2-talker background or in quiet. In Experiment 1, recognition with the 2 target sets was assessed using an adaptive spondee identification procedure. In Experiment 2, the benefit of speech produced in a 2-talker background was assessed using an open-set, monosyllabic word recognition task at a fixed signal-to-noise ratio (SNR).

RESULTS

Children performed more poorly than adults, regardless of whether the target speech was produced in quiet or in a 2-talker background. A small improvement in the SNR required to identify spondees was observed for both children and adults using speech produced in a 2-talker background (Experiment 1). Similarly, average open-set word recognition scores were 11 percentage points higher for both age groups using speech produced in a 2-talker background compared with quiet (Experiment 2).

CONCLUSION

The results indicate that children can use the acoustic modifications of speech produced in a 2-talker background to improve masked speech perception, as previously demonstrated for adults.

Predicting perception in noise using cortical auditory evoked potentials

Speech perception in background noise is a common challenge across individuals and health conditions (e.g., hearing impairment, aging, etc.). Both behavioral and physiological measures have been used to understand the important factors that contribute to perception-in-noise abilities. The addition of a physiological measure provides additional information about signal-in-noise encoding in the auditory system and may be useful in clarifying some of the variability in perception-in-noise abilities across individuals. Fifteen young normal-hearing individuals were tested using both electrophysiology and behavioral methods as a means to determine (1) the effects of signal-to-noise ratio (SNR) and signal level and (2) how well cortical auditory evoked potentials (CAEPs) can predict perception in noise. Three correlation/regression approaches were used to determine how well CAEPs predicted behavior. Main effects of SNR were found for both electrophysiology and speech perception measures, while signal level effects were found generally only for speech testing. These results demonstrate that when signals are presented in noise, sensitivity to SNR cues obscures any encoding of signal level cues. Electrophysiology and behavioral measures were strongly correlated. The best physiological predictors (e.g., latency, amplitude, and area of CAEP waves) of behavior (SNR at which 50 % of the sentence is understood) were N1 latency and N1 amplitude measures. In addition, behavior was best predicted by the 70-dB signal/5-dB SNR CAEP condition. It will be important in future studies to determine the relationship of electrophysiology and behavior in populations who experience difficulty understanding speech in noise such as those with hearing impairment or age-related deficits.

The influence of speech stimuli contrast in cortical auditory evoked potentials

Objective

Method

Results

Conclusion

Cortical encoding of signals in noise: effects of stimulus type and recording paradigm

OBJECTIVES

Perception-in-noise deficits have been demonstrated across many populations and listening conditions. Many factors contribute to successful perception of auditory stimuli in noise, including neural encoding in the central auditory system. Physiological measures such as cortical auditory-evoked potentials (CAEPs) can provide a view of neural encoding at the level of the cortex that may inform our understanding of listeners' abilities to perceive signals in the presence of background noise. To understand signal-in-noise neural encoding better, we set out to determine the effect of signal type, noise type, and evoking paradigm on the P1-N1-P2 complex.

DESIGN

Tones and speech stimuli were presented to nine individuals in quiet and in three background noise types: continuous speech spectrum noise, interrupted speech spectrum noise, and four-talker babble at a signal-to-noise ratio of -3 dB. In separate sessions, CAEPs were evoked by a passive homogenous paradigm (single repeating stimulus) and an active oddball paradigm.

RESULTS

The results for the N1 component indicated significant effects of signal type, noise type, and evoking paradigm. Although components P1 and P2 also had significant main effects of these variables, only P2 demonstrated significant interactions among these variables.

CONCLUSIONS

Signal type, noise type, and evoking paradigm all must be carefully considered when interpreting signal-in-noise evoked potentials. Furthermore, these data confirm the possible usefulness of CAEPs as an aid to understand perception-in-noise deficits.