Scalp potentials evoked by amplitude-modulated tones in dyslexia

Rapid and objective assessment of auditory temporal processing using dynamic amplitude-modulated stimuli

Auditory neural coding of speech-relevant temporal cues can be noninvasively probed using envelope following responses (EFRs), neural ensemble responses phase-locked to the stimulus amplitude envelope. EFRs emphasize different neural generators, such as the auditory brainstem or auditory cortex, by altering the temporal modulation rate of the stimulus. EFRs can be an important diagnostic tool to assess auditory neural coding deficits that go beyond traditional audiometric estimations. Existing approaches to measure EFRs use discrete amplitude modulated (AM) tones of varying modulation frequencies, which is time consuming and inefficient, impeding clinical translation. Here we present a faster and more efficient framework to measure EFRs across a range of AM frequencies using stimuli that dynamically vary in modulation rates, combined with spectrally specific analyses that offer optimal spectrotemporal resolution. EFRs obtained from several species (humans, Mongolian gerbils, Fischer-344 rats, and Cba/CaJ mice) showed robust, high-SNR tracking of dynamic AM trajectories (up to 800Hz in humans, and 1.4 kHz in rodents), with a fivefold decrease in recording time and thirtyfold increase in spectrotemporal resolution. EFR amplitudes between dynamic AM stimuli and traditional discrete AM tokens within the same subjects were highly correlated (94% variance explained) across species. Hence, we establish a time-efficient and spectrally specific approach to measure EFRs. These results could yield novel clinical diagnostics for precision audiology approaches by enabling rapid, objective assessment of temporal processing along the entire auditory neuraxis.

Development of Atypical Reading at Ages 5 to 9 Years and Processing of Speech Envelope Modulations in the Brain

Different studies have suggested that during speech processing readers with dyslexia present atypical levels of neural entrainment as well as atypical functional hemispherical asymmetries in comparison with typical readers. In this study, we evaluated these differences in children and the variation with age before and after starting with formal reading instruction. Synchronized neural auditory processing activity was quantified based on auditory steady-state responses (ASSRs) from EEG recordings. The stimulation was modulated at syllabic and phonemic fluctuation rates present in speech. We measured the brain activation patterns and the hemispherical asymmetries in children at three age points (5, 7, and 9 years old). Despite the well-known heterogeneity during developmental stages, especially in children and in dyslexia, we could extract meaningful common oscillatory patterns. The analyses included (1) the estimations of source localization, (2) hemispherical preferences using a laterality index, measures of neural entrainment, (3) signal-to-noise ratios (SNRs), and (4) connectivity using phase coherence measures. In this longitudinal study, we confirmed that the existence of atypical levels of neural entrainment and connectivity already exists at pre-reading stages. Overall, these measures reflected a lower ability of the dyslectic brain to synchronize with syllabic rate stimulation. In addition, our findings reinforced the hypothesis of a later maturation of the processing of beta rhythms in dyslexia. This investigation emphasizes the importance of longitudinal studies in dyslexia, especially in children, where neural oscillatory patterns as well as differences between typical and atypical developing children can vary in the span of a year.

Atypical processing in neural source analysis of speech envelope modulations in adolescents with dyslexia

Different studies have suggested that language and developmental disorders such as dyslexia are associated with a disturbance of auditory entrainment and of the functional hemispheric asymmetries during speech processing. These disorders typically result from an issue in the phonological component of language that causes problems to represent and manipulate the phonological structure of words at the syllable and/or phoneme level. We used Auditory Steady-State Responses (ASSRs) in EEG recordings to investigate the brain activation and hemisphere asymmetry of theta, alpha, beta and low-gamma range oscillations in typical readers and readers with dyslexia. The aim was to analyse whether the group differences found in previous electrode level studies were caused by a different source activation pattern or conversely was an effect that could be found on the active brain sources. We could not find differences in the brain locations of the main active brain sources. However, we observed differences in the extracted waveforms. The group average of the first DSS component of all signal-to-noise ratios of ASSR at source level was higher than the group averages at the electrode level. These analyses included a lower alpha synchronisation in adolescents with dyslexia and the possibility of compensatory mechanisms in theta, beta and low-gamma frequency bands. The main brain auditory sources were located in cortical regions around the auditory cortex. Thus, the differences observed in auditory EEG experiments would, according to our findings, have their origin in the intrinsic oscillatory mechanisms of the brain cortical sources related to speech perception.

Effect of Listening Biographies on Frequency Following Response Responses of Vocalists, Violinists, and Non-Musicians to Indian Carnatic Music Stimuli

Background and Objectives

The current study investigates pitch coding using frequency following response (FFR) among vocalists, violinists, and non-musicians for Indian Carnatic transition music stimuli and assesses whether their listening biographies strengthen their F0 neural encoding for these stimuli.

Subjects and Methods

Three participant groups in the age range of 18-45 years were included in the study. The first group of participants consisted of 20 trained Carnatic vocalists, the second group consisted of 13 trained violinists, and the third group consisted of 22 non-musicians. The stimuli consisted of three Indian Carnatic raga notes (/S-R2-G3/), which was sung by a trained vocalist and played by a trained violinist. For the purposes of this study, the two transitions between the notes T1=/S-R2/ and T2=/R2-G3/ were analyzed, and FFRs were recorded binaurally at 80 dB SPL using neuroscan equipment.

Results

Overall average responses of the participants were generated. To assess the participants’ pitch tracking to the Carnatic music stimuli, stimulus to response correlation (CC), pitch strength (PS), and pitch error (PE) were measured. Results revealed that both the vocalists and violinists had better CC and PS values with lower PE values, as compared to non-musicians, for both vocal and violin T1 and T2 transition stimuli. Between the musician groups, the vocalists were found to perform superiorly to the violinists for both vocal and violin T1 and T2 transition stimuli.

Conclusions

Listening biographies strengthened F0 neural coding, with respect to the vocalists for vocal stimulus at the brainstem level. The violinists, on the other hand, did not show such preference.

Neural entrainment to speech and nonspeech in dyslexia: Conceptual replication and extension of previous investigations

Whether phonological deficits in developmental dyslexia are associated with impaired neural sampling of auditory information is still under debate. Previous findings suggested that dyslexic participants showed atypical neural entrainment to slow and/or fast temporal modulations in speech, which might affect prosodic/syllabic and phonemic processing respectively. However, the large methodological variations across these studies do not allow us to draw clear conclusions on the nature of the entrainment deficit in dyslexia. Using magnetoencephalography, we measured neural entrainment to nonspeech and speech in both groups. We first aimed to conceptually replicate previous studies on auditory entrainment in dyslexia, using the same measurement methods as in previous studies, and also using new measurement methods (cross-correlation analyses) to better characterize the synchronization between stimulus and brain response. We failed to observe any of the significant group differences that had previously been reported in delta, theta and gamma frequency bands, whether using speech or nonspeech stimuli. However, when analyzing amplitude cross-correlations between noise stimuli and brain responses, we found that control participants showed larger responses than dyslexic participants in the delta range in the right hemisphere and in the gamma range in the left hemisphere. Overall, our results are weakly consistent with the hypothesis that dyslexic individuals show an atypical entrainment to temporal modulations. Our attempt at replicating previously published results highlights the multiple weaknesses of this research area, particularly low statistical power due to small sample size, and the lack of methodological standards inducing considerable heterogeneity of measurement and analysis methods across studies.

Reading ability and phoneme categorization

Dyslexia is associated with abnormal performance on many auditory psychophysics tasks, particularly those involving the categorization of speech sounds. However, it is debated whether those apparent auditory deficits arise from (a) reduced sensitivity to particular acoustic cues, (b) the difficulty of experimental tasks, or (c) unmodeled lapses of attention. Here we investigate the relationship between phoneme categorization and reading ability, with special attention to the nature of the cue encoding the phoneme contrast (static versus dynamic), differences in task paradigm difficulty, and methodological details of psychometric model fitting. We find a robust relationship between reading ability and categorization performance, show that task difficulty cannot fully explain that relationship, and provide evidence that the deficit is not restricted to dynamic cue contrasts, contrary to prior reports. Finally, we demonstrate that improved modeling of behavioral responses suggests that performance does differ between children with dyslexia and typical readers, but that the difference may be smaller than previously reported.

Temporal plasticity in auditory cortex improves neural discrimination of speech sounds

BACKGROUND

Many individuals with language learning impairments exhibit temporal processing deficits and degraded neural responses to speech sounds. Auditory training can improve both the neural and behavioral deficits, though significant deficits remain. Recent evidence suggests that vagus nerve stimulation (VNS) paired with rehabilitative therapies enhances both cortical plasticity and recovery of normal function.

OBJECTIVE/HYPOTHESIS

We predicted that pairing VNS with rapid tone trains would enhance the primary auditory cortex (A1) response to unpaired novel speech sounds.

METHODS

VNS was paired with tone trains 300 times per day for 20 days in adult rats. Responses to isolated speech sounds, compressed speech sounds, word sequences, and compressed word sequences were recorded in A1 following the completion of VNS-tone train pairing.

RESULTS

Pairing VNS with rapid tone trains resulted in stronger, faster, and more discriminable A1 responses to speech sounds presented at conversational rates.

CONCLUSION

This study extends previous findings by documenting that VNS paired with rapid tone trains altered the neural response to novel unpaired speech sounds. Future studies are necessary to determine whether pairing VNS with appropriate auditory stimuli could potentially be used to improve both neural responses to speech sounds and speech perception in individuals with receptive language disorders.

Neural Mechanisms of Language-Based Learning Impairments: Insights from Human Populations and Animal Models

The acquisition of speech perception and consequent expression of language represent fundamental aspects of human functioning. Yet roughly 7% to 8% of children who are otherwise healthy and of normal intelligence exhibit unexplained delays and impairments in acquiring these skills. Ongoing research has revealed several key features of language disability that may pro-vide more direct insight into underlying anomalous neural functioning. For example, evidence supports a strong association between basic defects in processing rapidly changing acoustic information and emergent disruptions in speech perception, as well as cascading effects on other forms of language development (including reading). Considerable neurobiological research has thus focused on developmental factors that might deleteriously influence rapid sensory processing. Additional research focuses on mechanisms of neural plasticity, including how such brains might be “retrained” for improved processing of language. These and related findings from human clinical studies, electrophysiological studies, neuroimaging studies, and animal models are reviewed.

Objective measurement of high-level auditory cortical function in children

OBJECTIVE

This study examined whether the N2 latency of the cortical auditory evoked potential (CAEP) could be used as an objective indicator of temporal processing ability in normally hearing children.

METHODS

The N2 latency was evoked using three temporal processing paradigms: (1) differences in voice-onset-times (VOTs); (2) speech-in-noise using the CV/da/embedded in broadband noise (BBN) with varying signal-to-noise ratios (SNRs); and (3) 16Hz amplitude-modulated (AM) BBN presented (i) alone and (ii) following an unmodulated BBN, using four modulation depths. Thirty-four school-aged children with normal hearing, speech, language and reading were stratified into two groups: 5-7 years (n=13) and 8-12 years (n=21).

RESULTS

The N2 latency shifted significantly and systematically with differences in VOT and SNR, and was significantly different in the two AM-BBN conditions.

CONCLUSIONS

For children without an N1 peak in the cortical waveform, the N2 peak can be used as a sensitive measure of temporal processing for these stimuli.

SIGNIFICANCE

N2 latency of the CAEP can be used as an objective measure of temporal processing ability in a paediatric population with temporal processing disorder who are difficult to assess via behavioural response.

Strategy choice mediates the link between auditory processing and spelling

Relations among linguistic auditory processing, nonlinguistic auditory processing, spelling ability, and spelling strategy choice were examined. Sixty-three undergraduate students completed measures of auditory processing (one involving distinguishing similar tones, one involving distinguishing similar phonemes, and one involving selecting appropriate spellings for individual phonemes). Participants also completed a modified version of a standardized spelling test, and a secondary spelling test with retrospective strategy reports. Once testing was completed, participants were divided into phonological versus nonphonological spellers on the basis of the number of words they spelled using phonological strategies only. Results indicated a) moderate to strong positive correlations among the different auditory processing tasks in terms of reaction time, but not accuracy levels, and b) weak to moderate positive correlations between measures of linguistic auditory processing (phoneme distinction and phoneme spelling choice in the presence of foils) and spelling ability for phonological spellers, but not for nonphonological spellers. These results suggest a possible explanation for past contradictory research on auditory processing and spelling, which has been divided in terms of whether or not disabled spellers seemed to have poorer auditory processing than did typically developing spellers, and suggest implications for teaching spelling to children with good versus poor auditory processing abilities.

Age-related changes in the relationship between auditory brainstem responses and envelope-following responses

Hearing thresholds and wave amplitudes measured using auditory brainstem responses (ABRs) to brief sounds are the predominantly used clinical measures to objectively assess auditory function. However, frequency-following responses (FFRs) to tonal carriers and to the modulation envelope (envelope-following responses or EFRs) to longer and spectro-temporally modulated stimuli are rapidly gaining prominence as a measure of complex sound processing in the brainstem and midbrain. In spite of numerous studies reporting changes in hearing thresholds, ABR wave amplitudes, and the FFRs and EFRs under neurodegenerative conditions, including aging, the relationships between these metrics are not clearly understood. In this study, the relationships between ABR thresholds, ABR wave amplitudes, and EFRs are explored in a rodent model of aging. ABRs to broadband click stimuli and EFRs to sinusoidally amplitude-modulated noise carriers were measured in young (3-6 months) and aged (22-25 months) Fischer-344 rats. ABR thresholds and amplitudes of the different waves as well as phase-locking amplitudes of EFRs were calculated. Age-related differences were observed in all these measures, primarily as increases in ABR thresholds and decreases in ABR wave amplitudes and EFR phase-locking capacity. There were no observed correlations between the ABR thresholds and the ABR wave amplitudes. Significant correlations between the EFR amplitudes and ABR wave amplitudes were observed across a range of modulation frequencies in the young. However, no such significant correlations were found in the aged. The aged click ABR amplitudes were found to be lower than would be predicted using a linear regression model of the young, suggesting altered gain mechanisms in the relationship between ABRs and FFRs with age. These results suggest that ABR thresholds, ABR wave amplitudes, and EFRs measure complementary aspects of overlapping neurophysiological processes and the relationships between these measurements changes asymmetrically with age. Hence, measuring all three metrics provides a more complete assessment of auditory function, especially under pathological conditions like aging.

The visual magnocellular deficit in Chinese-speaking children with developmental dyslexia

Many alphabetic studies have evidenced that individuals with developmental dyslexia (DD) have deficits in visual magnocellular (M) pathway. However, there are few studies to investigate the M function of Chinese DD. Chinese is a logographic language, and Chinese characters are complicated in structure. Visual skills and orthographic processing abilities are particularly important for efficient reading in Chinese as compared to alphabetic languages. Therefore, it is necessary to investigate the visual M function of Chinese DD and whether the M function was associated with orthographic skills. In the present study, 26 dyslexic children (mean age: 10.03 years) and 27 age-matched normal children (mean age: 10.37 years) took part in a coherent motion (CM) detection task and an orthographic awareness test. The results showed that dyslexic children had a significantly higher threshold than age-matched children in CM detection task. Meanwhile, children with DD responded more slowly in orthographic awareness test, although the group difference was marginally significant. The results suggested that Chinese dyslexics had deficits both in visual M pathway processing and orthographic processing. In order to investigate the relationship between M function and orthographic skills, we made a correlation analysis between CM threshold and orthographic awareness by merging performance of dyslexic children and age-matched children. The results revealed that CM thresholds were positively correlated with reaction times in orthographic awareness test, suggesting that better M function was related to better orthographic processing skills.

Subcortical encoding of speech cues in children with attention deficit hyperactivity disorder

OBJECTIVE

There is little information about processing of nonspeech and speech stimuli at the subcortical level in individuals with attention deficit hyperactivity disorder (ADHD). The auditory brainstem response (ABR) provides information about the function of the auditory brainstem pathways. We aim to investigate the subcortical function in neural encoding of click and speech stimuli in children with ADHD.

METHODS

The subjects include 50 children with ADHD and 34 typically developing (TD) children between the ages of 8 and 12 years. Click ABR (cABR) and speech ABR (sABR) with 40 ms synthetic /da/ syllable stimulus were recorded.

RESULTS

Latencies of cABR in waves of III and V and duration of V-Vn (P⩽0.027), and latencies of sABR in waves A, D, E, F and O and duration of V-A (P⩽0.034) were significantly longer in children with ADHD than in TD children. There were no apparent differences in components the sustained frequency following response (FFR).

CONCLUSIONS

We conclude that children with ADHD have deficits in temporal neural encoding of both nonspeech and speech stimuli.

SIGNIFICANCE

There is a common dysfunction in the processing of click and speech stimuli at the brainstem level in children with suspected ADHD.

Impaired extraction of speech rhythm from temporal modulation patterns in speech in developmental dyslexia

Dyslexia is associated with impaired neural representation of the sound structure of words (phonology). The “phonological deficit” in dyslexia may arise in part from impaired speech rhythm perception, thought to depend on neural oscillatory phase-locking to slow amplitude modulation (AM) patterns in the speech envelope. Speech contains AM patterns at multiple temporal rates, and these different AM rates are associated with phonological units of different grain sizes, e.g., related to stress, syllables or phonemes. Here, we assess the ability of adults with dyslexia to use speech AMs to identify rhythm patterns (RPs). We study 3 important temporal rates: “Stress” (~2 Hz), “Syllable” (~4 Hz) and “Sub-beat” (reduced syllables, ~14 Hz). 21 dyslexics and 21 controls listened to nursery rhyme sentences that had been tone-vocoded using either single AM rates from the speech envelope (Stress only, Syllable only, Sub-beat only) or pairs of AM rates (Stress + Syllable, Syllable + Sub-beat). They were asked to use the acoustic rhythm of the stimulus to identity the original nursery rhyme sentence. The data showed that dyslexics were significantly poorer at detecting rhythm compared to controls when they had to utilize multi-rate temporal information from pairs of AMs (Stress + Syllable or Syllable + Sub-beat). These data suggest that dyslexia is associated with a reduced ability to utilize AMs <20 Hz for rhythm recognition. This perceptual deficit in utilizing AM patterns in speech could be underpinned by less efficient neuronal phase alignment and cross-frequency neuronal oscillatory synchronization in dyslexia. Dyslexics' perceptual difficulties in capturing the full spectro-temporal complexity of speech over multiple timescales could contribute to the development of impaired phonological representations for words, the cognitive hallmark of dyslexia across languages.

Effects of congenital blindness on the subcortical representation of speech cues

Human modalities play a vital role in the way the brain produces mental representations of the world around us. Although congenital blindness limits the understanding of the environment in some aspects, blind individuals may have other superior capabilities from long-term experience and neural plasticity. This study investigated the effects of congenital blindness on temporal and spectral neural encoding of speech at the subcortical level. The study included 26 congenitally blind individuals and 24 normal-sighted individuals with normal hearing. Auditory brainstem response (ABR) was recorded with both click and speech synthetic 40-ms /da/ stimuli. No significant difference was observed between the two groups in wave latencies or amplitudes of click ABR. Latencies of speech ABR D (p=0.012) and O (p=0.014) waves were significantly shorter in blind individuals than in normal-sighted individuals. Amplitudes of the A (p<0.001) and E (p=0.001) speech ABR (sABR) waves were also significantly higher in blind subjects. Blind individuals had significantly better results for duration (p<0.001) amplitude (p=0.015) and slope of the V-A complex (p=0.004), signal-to-noise ratio (p<0.001), and amplitude of the stimulus fundamental frequency (F0) (p=0.009), first formant (F1) (p<0.001) and higher-frequency region (HF) (p<0.001) ranges. Results indicate that congenitally blind subjects have improved hearing function in response to the /da/ syllable in both source and filter classes of sABR. It is possible that these subjects have enhanced neural representation of vocal cord vibrations and improved neural synchronization in temporal encoding of the onset and offset parts of speech stimuli at the brainstem level. This may result from the compensatory mechanism of neural reorganization in blind subjects influenced from top-down corticofugal connections with the auditory cortex.

Early neural disruption and auditory processing outcomes in rodent models: implications for developmental language disability

Most researchers in the field of neural plasticity are familiar with the "Kennard Principle," which purports a positive relationship between age at brain injury and severity of subsequent deficits (plateauing in adulthood). As an example, a child with left hemispherectomy can recover seemingly normal language, while an adult with focal injury to sub-regions of left temporal and/or frontal cortex can suffer dramatic and permanent language loss. Here we present data regarding the impact of early brain injury in rat models as a function of type and timing, measuring long-term behavioral outcomes via auditory discrimination tasks varying in temporal demand. These tasks were created to model (in rodents) aspects of human sensory processing that may correlate-both developmentally and functionally-with typical and atypical language. We found that bilateral focal lesions to the cortical plate in rats during active neuronal migration led to worse auditory outcomes than comparable lesions induced after cortical migration was complete. Conversely, unilateral hypoxic-ischemic (HI) injuries (similar to those seen in premature infants and term infants with birth complications) led to permanent auditory processing deficits when induced at a neurodevelopmental point comparable to human "term," but only transient deficits (undetectable in adulthood) when induced in a "preterm" window. Convergent evidence suggests that regardless of when or how disruption of early neural development occurs, the consequences may be particularly deleterious to rapid auditory processing (RAP) outcomes when they trigger developmental alterations that extend into subcortical structures (i.e., lower sensory processing stations). Collective findings hold implications for the study of behavioral outcomes following early brain injury as well as genetic/environmental disruption, and are relevant to our understanding of the neurologic risk factors underlying developmental language disability in human populations.

Basic auditory processing deficits in dyslexia: systematic review of the behavioral and event-related potential/ field evidence

A review of research that uses behavioral, electroencephalographic, and/or magnetoencephalographic methods to investigate auditory processing deficits in individuals with dyslexia is presented. Findings show that measures of frequency, rise time, and duration discrimination as well as amplitude modulation and frequency modulation detection were most often impaired in individuals with dyslexia. Less consistent findings were found for intensity and gap perception. Additional factors that mediate auditory processing deficits in individuals with dyslexia and their implications are discussed.

White matter lateralization and interhemispheric coherence to auditory modulations in normal reading and dyslexic adults

Neural activation of slow acoustic variations that are important for syllable identification is more lateralized to the right hemisphere than activation of fast acoustic changes that are important for phoneme identification. It has been suggested that this complementary function at different hemispheres is rooted in a different degree of white matter myelination in the left versus right hemisphere. The present study will investigate this structure-function relationship with Diffusion Tensor Imaging (DTI) and Auditory Steady-State Responses (ASSR), respectively. With DTI we examined white matter lateralization in the cortical auditory and language regions (i.e. posterior region of the superior temporal gyrus and the arcuate fasciculus) and white matter integrity in the splenium of the corpus callosum. With ASSR we examined interhemispheric coherence to slow, syllabic-rate (i.e. 4 Hz) and fast, phonemic-rate (i.e. 20 Hz) modulations. These structural and functional techniques were applied in a group of normal reading adults and a group of dyslexic adults for whom previously reduced functional interhemispheric connectivity at 20 Hz has been reported (Poelmans et al. (2012). Ear and Hearing, 33, 134-143). This sample was chosen since it is hypothesized that in dyslexic readers insufficient hemispheric asymmetry in myelination might relate to their auditory and phonological problems. Results demonstrate reduced white matter lateralization in the posterior superior temporal gyrus and the arcuate fasciculus in the dyslexic readers. Additionally, white matter lateralization in the posterior superior temporal gyrus and white matter integrity in the splenium of the corpus callosum related to interhemispheric coherence to phonemic-rate modulations (i.e. 20 Hz). Interestingly, this correlation pattern was opposite in normal versus dyslexic readers. These results might imply that less pronounced left white matter dominance in dyslexic adults might relate to their problems to process phonemic-rate acoustic information and to integrate them into the phonological system.

Mismatch negativity responses in children with a diagnosis of childhood apraxia of speech (CAS)

PURPOSE

To evaluate whether a hypothesis suggesting that apraxia of speech results from phonological overspecification could be relevant for childhood apraxia of speech (CAS).

METHOD

High-density EEG was recorded from 5 children with CAS and 5 matched controls, ages 5-8 years, with and without CAS, as they listened to randomized sequences of CV syllables in two oddball paradigms: phonemic (/ba/, /pa/) and allophonic (/pa/, /p(h)a/).

RESULTS

In the phonemic contrast condition, mismatch negativity (MMN) responses to oddball sounds were observed for the typically developing (comparison) group but not the CAS group, although a component similar to an immature mismatch response was apparent. The allophonic contrast did not elicit MMN responses in the comparison group, but in the CAS group, an MMN-like response was observed.

CONCLUSION

The authors propose that these preliminary findings are consistent with a view of CAS as a disorder that not only affects motor planning but also has a phonological component.

Pairing tone trains with vagus nerve stimulation induces temporal plasticity in auditory cortex

The selectivity of neurons in sensory cortex can be modified by pairing neuromodulator release with sensory stimulation. Repeated pairing of electrical stimulation of the cholinergic nucleus basalis, for example, induces input specific plasticity in primary auditory cortex (A1). Pairing nucleus basalis stimulation (NBS) with a tone increases the number of A1 neurons that respond to the paired tone frequency. Pairing NBS with fast or slow tone trains can respectively increase or decrease the ability of A1 neurons to respond to rapidly presented tones. Pairing vagus nerve stimulation (VNS) with a single tone alters spectral tuning in the same way as NBS-tone pairing without the need for brain surgery. In this study, we tested whether pairing VNS with tone trains can change the temporal response properties of A1 neurons. In naïve rats, A1 neurons respond strongly to tones repeated at rates up to 10 pulses per second (pps). Repeatedly pairing VNS with 15 pps tone trains increased the temporal following capacity of A1 neurons and repeatedly pairing VNS with 5 pps tone trains decreased the temporal following capacity of A1 neurons. Pairing VNS with tone trains did not alter the frequency selectivity or tonotopic organization of auditory cortex neurons. Since VNS is well tolerated by patients, VNS-tone train pairing represents a viable method to direct temporal plasticity in a variety of human conditions associated with temporal processing deficits.

Reduced sensitivity to slow-rate dynamic auditory information in children with dyslexia

The etiology of developmental dyslexia remains widely debated. An appealing theory postulates that the reading and spelling problems in individuals with dyslexia originate from reduced sensitivity to slow-rate dynamic auditory cues. This low-level auditory deficit is thought to provoke a cascade of effects, including inaccurate speech perception and eventually unspecified phoneme representations. The present study investigated sensitivity to frequency modulation and amplitude rise time, speech-in-noise perception and phonological awareness in 11-year-old children with dyslexia and a matched normal-reading control children. Group comparisons demonstrated that children with dyslexia were less sensitive than normal-reading children to slow-rate dynamic auditory processing, speech-in-noise perception, phonological awareness and literacy abilities. Correlations were found between slow-rate dynamic auditory processing and phonological awareness, and speech-in-noise perception and reading. Yet, no significant correlation between slow-rate dynamic auditory processing and speech-in-noise perception was obtained. Together, these results indicate that children with dyslexia have difficulties with slow-rate dynamic auditory processing and speech-in-noise perception and that these problems persist until sixth grade.

Perception of speech in noise: neural correlates

The presence of irrelevant auditory information (other talkers, environmental noises) presents a major challenge to listening to speech. The fundamental frequency (F(0)) of the target speaker is thought to provide an important cue for the extraction of the speaker's voice from background noise, but little is known about the relationship between speech-in-noise (SIN) perceptual ability and neural encoding of the F(0). Motivated by recent findings that music and language experience enhance brainstem representation of sound, we examined the hypothesis that brainstem encoding of the F(0) is diminished to a greater degree by background noise in people with poorer perceptual abilities in noise. To this end, we measured speech-evoked auditory brainstem responses to /da/ in quiet and two multitalker babble conditions (two-talker and six-talker) in native English-speaking young adults who ranged in their ability to perceive and recall SIN. Listeners who were poorer performers on a standardized SIN measure demonstrated greater susceptibility to the degradative effects of noise on the neural encoding of the F(0). Particularly diminished was their phase-locked activity to the fundamental frequency in the portion of the syllable known to be most vulnerable to perceptual disruption (i.e., the formant transition period). Our findings suggest that the subcortical representation of the F(0) in noise contributes to the perception of speech in noisy conditions.

Sensory-based learning disability: Insights from brainstem processing of speech sounds

Speech-evoked auditory brainstem responses (speech-ABR) provide a reliable marker of learning disability in a substantial subgroup of individuals with language-based learning problems (LDs). Here we review work describing the properties of the speech-ABR in typically developing children and in children with LD. We also review studies on the relationships between speech-ABR and the commonly used click-ABR, and between speech-ABR and auditory processing at the level of the cortex. In a critical examination of previously published data, we conclude that as many as 40% of LDs have abnormal speech-ABRs and that these individuals are also likely to exhibit abnormal cortical processing. Yet, the profile of learning problems these individuals exhibit is unspecific. Leaving open the question of causality, these data suggest that speech-ABR can be used to identify a large sub-population of LDs, those with abnormal auditory physiological function. Further studies are required to determine the functional relationships among abnormal speech-ABR, speech perception, and the pattern of literacy-related and cognitive deficits in LD.

Detection of sinusoidal amplitude modulated sounds: deficits after bilateral lesions of auditory cortex in the rat

The ability of rats to detect the presence of sinusoidal amplitude modulation (AM) of a broadband noise carrier was determined before and after bilateral ablation of auditory cortex. The rats were trained to withdraw from a drinking spout to avoid a shock when they detected a modulation of the sound. Sensitivity was evaluated by testing the rats at progressively smaller depths of modulation. Psychophysical curves were produced to describe the limits of detection at modulation rates of 10, 100 and 1000Hz. Performance scores were based on the probability of withdrawal from the spout during AM (warning periods) relative to withdrawal during the un-modulated noise (safe periods). A threshold was defined as the depth of modulation that produced a score halfway between perfect avoidance and no avoidance (performance score=0.5). Bilateral auditory cortical lesions resulted in significant elevations in threshold for detection of AM at rates of 100 and 1000Hz. No significant shift was found at a modulation rate of 10Hz. The magnitude of the deficit for AM rates of 100 and 1000Hz was positively correlated with the size of the cortical lesion. Substantial deficits were found only in animals with lesions that included secondary as well as primary auditory cortical areas. The results show that the rat's auditory cortex is important for processing sinusoidal AM and that its contribution is most apparent at high modulation rates. The data suggest that the auditory cortex is a crucial structure for maintaining normal sensitivity to temporal modulation of an auditory stimulus.

Infant information processing and family history of specific language impairment: converging evidence for RAP deficits from two paradigms

An infant's ability to process auditory signals presented in rapid succession (i.e. rapid auditory processing abilities [RAP]) has been shown to predict differences in language outcomes in toddlers and preschool children. Early deficits in RAP abilities may serve as a behavioral marker for language-based learning disabilities. The purpose of this study is to determine if performance on infant information processing measures designed to tap RAP and global processing skills differ as a function of family history of specific language impairment (SLI) and/or the particular demand characteristics of the paradigm used. Seventeen 6- to 9-month-old infants from families with a history of specific language impairment (FH+) and 29 control infants (FH-) participated in this study. Infants' performance on two different RAP paradigms (head-turn procedure [HT] and auditory-visual habituation/recognition memory [AVH/RM]) and on a global processing task (visual habituation/recognition memory [VH/RM]) was assessed at 6 and 9 months. Toddler language and cognitive skills were evaluated at 12 and 16 months. A number of significant group differences were seen: FH+ infants showed significantly poorer discrimination of fast rate stimuli on both RAP tasks, took longer to habituate on both habituation/recognition memory measures, and had lower novelty preference scores on the visual habituation/recognition memory task. Infants' performance on the two RAP measures provided independent but converging contributions to outcome. Thus, different mechanisms appear to underlie performance on operantly conditioned tasks as compared to habituation/recognition memory paradigms. Further, infant RAP processing abilities predicted to 12- and 16-month language scores above and beyond family history of SLI. The results of this study provide additional support for the validity of infant RAP abilities as a behavioral marker for later language outcome. Finally, this is the first study to use a battery of infant tasks to demonstrate multi-modal processing deficits in infants at risk for SLI.

On the relationship between speech- and nonspeech-evoked auditory brainstem responses

Auditory brainstem response (ABR) reflects activation of the neural generators along the ascending auditory pathway when a sound is heard. In this study, we explored the relationship between brainstem encoding of click and speech signals in normal-learning children and in those with language-based learning problems. To that end, ABR was recorded from both types of stimuli. We found that the normal pattern of correlation between click- and speech-evoked ABRs was disrupted when speech-evoked ABRs were delayed. Thus, delayed responses to speech were not indicative of clinically abnormal responses to clicks. We conclude that these two responses reflect largely separate neural processes and that only processes involved in encoding complex signals such as speech are impaired in children with learning problems.

Auditory temporal information processing in preschool children at family risk for dyslexia: relations with phonological abilities and developing literacy skills

In this project, the hypothesis of an auditory temporal processing deficit in dyslexia was tested by examining auditory processing in relation to phonological skills in two contrasting groups of five-year-old preschool children, a familial high risk and a familial low risk group. Participants were individually matched for gender, age, non-verbal IQ, school environment, and parental educational level. Psychophysical thresholds were estimated for gap-detection, frequency modulation detection, and tone-in-noise detection using a three-interval forced-choice adaptive staircase paradigm embedded within a computer game. Phonological skills were measured by tasks assessing phonological awareness, rapid serial naming, and verbal short-term memory. Significant group differences were found for phonological awareness and letter knowledge. In contrast, none of the auditory tasks differentiated significantly between both groups. However, both frequency modulation and tone-in-noise detection were significantly related to phonological awareness. This relation with phonological skills was not present for gap-detection.

The infant as a prelinguistic model for language learning impairments: predicting from event-related potentials to behavior

Associations between efficient processing of brief, rapidly presented, successive stimuli and language learning impairments (LLI) in older children and adults have been well documented. In this paper we examine the role that impaired rapid auditory processing (RAP) might play during early language acquisition. Using behavioral measures we have demonstrated that RAP abilities in infancy are critically linked to later language abilities for both non-speech and speech stimuli. Variance in infant RAP thresholds reliably predict language outcome at 3 years-of-age for infants at risk for LLI and control infants. We present data here describing patterns of electrocortical (EEG/ERP) activation at 6 month-of-age to the same non-verbal stimuli used in our behavioral studies. Well-defined differences were seen between infants from families with a history of LLI (FH+) and FH- controls in the amplitude of the mismatch response (MMR) as well as the latency of the N250 component in the 70 ms ISI condition only. Smaller mismatch responses and delayed onsets of the N250 component were seen in the FH+ group. The latency differences in the N250 component, but not the MMR amplitude variation, were significantly related to 24-month language outcome. Such converging tasks provide the opportunity to examine early precursors of LLI and allow the opportunity for earlier identification and intervention.

Human auditory steady-state responses

Steady-state evoked potentials can be recorded from the human scalp in response to auditory stimuli presented at rates between 1 and 200 Hz or by periodic modulations of the amplitude and/or frequency of a continuous tone. Responses can be objectively detected using frequency-based analyses. In waking subjects, the responses are particularly prominent at rates near 40 Hz. Responses evoked by more rapidly presented stimuli are less affected by changes in arousal and can be evoked by multiple simultaneous stimuli without significant loss of amplitude. Response amplitude increases as the depth of modulation or the intensity increases. The phase delay of the response increases as the intensity or the carrier frequency decreases. Auditory steady-state responses are generated throughout the auditory nervous system, with cortical regions contributing more than brainstem generators to responses at lower modulation frequencies. These responses are useful for objectively evaluating auditory thresholds, assessing suprathreshold hearing, and monitoring the state of arousal during anesthesia.

Brain responses to changes in speech sound durations differ between infants with and without familial risk for dyslexia

A specific learning disability, developmental dyslexia, is a language-based disorder that is shown to be strongly familial. Therefore, infants born to families with a history of the disorder are at an elevated risk for the disorder. However, little is known of the potential early markers of dyslexia. Here we report differences between 6-month-old infants with and without high risk of familial dyslexia in brain electrical activation generated by changes in the temporal structure of speech sounds, a critical cueing feature in speech. We measured event-related brain responses to consonant duration changes embedded in ata pseudowords applying an oddball paradigm, in which pseudoword tokens with varying /t/ duration were presented as frequent standard (80%) or as rare deviant stimuli (each 10%) with an interval of 610 msec between the stimuli. The infants at risk differ from control infants in both their initial responsiveness to sounds per se and in their change-detection responses dependent on the stimulus context. These results show that infants at risk due to a familial background of reading problems process auditory temporal cues of speech sounds differently from infants without such a risk even before they learn to speak.

The sensory basis of reading problems

Learning to read is much more difficult than learning to speak. Most children teach themselves to speak with little or no difficulty. Yet a few years later when they come to learn to read they have to be taught how to do it; they do not pick up reading by themselves. This is because we speak in words and syllables, but we write in phonemes. Syllables do not naturally break down into the sounds of letters and letter units (i.e., phonemes) because these do not correspond to physiologically distinct articulatory gestures (Liberman, Shankweiler, & Studdert-Kennedy, 1967). Alphabetic writing was only invented when people realized that syllables could be artificially divided into smaller acoustically distinguishable phonemes that could be represented by a small number of letters. But these distinctions are arbitrary cultural artifacts, and their mastery was originally confined to a select social class. And until about 100 years ago it did not matter much if the majority of people could not read; the acquisition of reading probably had no serious disadvantages. Reading requires the integration of at least two kinds of analysis (Castles & Coltheart, 1993; Ellis, 1984; Manis, Seidenberg, Doi, McBride-Chang, & Petersen, 1996; Morton, 1969; Seidenburg, 1993). First, the visual form of words, the shape of letters, their order in words, and common spelling patterns, which is termed their orthography, has to be processed visually. Their orthography yields the meaning of familiar words very rapidly without needing to sound them out. But for unfamiliar words, and all words are fairly unfamiliar to the beginning reader, the letters have to be translated into the speech sounds (i.e., phonemes) that they stand for, and then those sounds have to be melded together in inner speech to yield the word and its meaning. Reading exclusively by the phonological route is more time consuming than if words can be accessed directly without requiring phonological mediation.

Temporal envelope perception in dyslexic children

Speech intelligibility depends heavily on the accurate perception of auditory temporal envelope cues, that is the slower amplitude modulations present in the speech waveform. In a previous study, McAnally and Stein demonstrated that dyslexics may show impaired audibility (i.e. detectability) of these envelope cues. In the present psychophysical study, the ability to process temporal envelope cues was further investigated in dyslexic children by measuring detection thresholds of sinusoidal amplitude-modulation (SAM) and discrimination thresholds of SAM depth and SAM rate. Each threshold was measured at slow and fast SAM rates of 4 and 128 Hz, respectively. Overall, SAM thresholds were higher in dyslexics than in controls at both rates. The strongest deficit was observed at 4 Hz in the SAM detection task, but a deficit was also apparent at 128 Hz in the SAM discrimination tasks. Therefore, these results reveal that, in addition to reduced audibility of slow and fast envelope cues, some dyslexic children show poor encoding fidelity for these cues (as measured by the discrimination tasks). Overall, these findings are consistent with Tallal's hypothesis according to which the speech and reading deficits in some dyslexics may be caused by impaired temporal processes.

Separate influences of acoustic AM and FM sensitivity on the phonological decoding skills of impaired and normal readers

Developmental dyslexia is associated with deficits in the processing of basic auditory stimuli. Yet it is unclear how these sensory impairments might contribute to poor reading skills. This study better characterizes the relationship between phonological decoding skills, the lack of which is generally accepted to comprise the core deficit in reading disabilities, and auditory sensitivity to amplitude modulation (AM) and frequency modulation (FM). Thirty-eight adult subjects, 17 of whom had a history of developmental dyslexia, completed a battery of psychophysical measures of sensitivity to FM and AM at different modulation rates, along with a measure of pseudoword reading accuracy and standardized assessments of literacy and cognitive skills. The subjects with a history of dyslexia were significantly less sensitive than controls to 2-Hz FM and 20-Hz AM only. The absence of a significant group difference for 2-Hz AM shows that the dyslexics do not have a general deficit in detecting all slow modulations. Thresholds for detecting 2-Hz and 240-Hz FM and 20-Hz AM correlated significantly with pseudoword reading accuracy. After accounting for various cognitive skills, however, multiple regression analyses showed that detection thresholds for both 2-Hz FM and 20-Hz AM were significant and independent predictors of pseudoword reading ability in the entire sample. Thresholds for 2-Hz AM and 240-Hz FM did not explain significant additional variance in pseudoword reading skill. It is therefore possible that certain components of auditory processing of modulations are related to phonological decoding skills, whereas others are not.

The importance of rapid auditory processing abilities to early language development: evidence from converging methodologies

The ability to process two or more rapidly presented, successive, auditory stimuli is believed to underlie successful language acquisition. Likewise, deficits in rapid auditory processing of both verbal and nonverbal stimuli are characteristic of individuals with developmental language disorders such as Specific Language Impairment. Auditory processing abilities are well developed in infancy, and thus such deficits should be detectable in infants. In the studies presented here, converging methodologies are used to examine such abilities in infants with and without a family history of language disorder. Behavioral measures, including assessments of infant information processing, and an EEG/event-related potential (ERP) paradigm are used concurrently. Results suggest that rapid auditory processing skills differ as a function of family history and are predictive of later language outcome. Further, these paradigms may prove to be sensitive tools for identifying children with poor processing skills in infancy and thus at a higher risk for developing a language disorder.

Neurobiologic responses to speech in noise in children with learning problems: deficits and strategies for improvement

OBJECTIVES

Some children with learning problems (LP) experience speech-sound perception deficits that worsen in background noise. The first goal was to determine whether these impairments are associated with abnormal neurophysiologic representation of speech features in noise reflected at brain-stem and cortical levels. The second goal was to examine the perceptual and neurophysiological benefits provided to an impaired system by acoustic cue enhancements.

METHODS

Behavioral speech perception measures (just noticeable difference scores), auditory brain-stem responses, frequency-following responses and cortical-evoked potentials (P1, N1, P1', N1') were studied in a group of LP children and compared to responses in normal children.

RESULTS

We report abnormalities in the fundamental sensory representation of sound at brain-stem and cortical levels in the LP children when speech sounds were presented in noise, but not in quiet. Specifically, the neurophysiologic responses from these LP children displayed a different spectral pattern and lacked precision in the neural representation of key stimulus features. Cue enhancement benefited both behavioral and neurophysiological responses.

CONCLUSIONS

Overall, these findings contribute to our understanding of the preconscious biological processes underlying perception deficits and may assist in the design of effective intervention strategies.

Use of temporal envelope cues by children with developmental dyslexia

This study evaluates the ability to process auditory temporal-envelope cues in a group of 6 children with dyslexia (mean age: 10;10 years;months). To address this issue, we measured (a) temporal modulation transfer functions (TMTFs), that is, the detection thresholds of sinusoidal amplitude modulation (SAM) applied to a white noise carrier, as a function of modulation frequency, fm (fm was 4, 16, 64, 256, and 1,024 Hz) and (b) identification performance for vowel-consonant-vowel (VCV) stimuli over 5 sessions. VCV stimuli were either unprocessed or digitally processed to remove the original spectral information, resulting in a time-varying speech envelope amplitude modulating a noise carrier. The same tests were conducted in 6 normal control children (mean age: 11;6 years;months) and 6 normal control adults (mean age: 24;8 years;months). SAM thresholds were similar in normal children and adults. For both normal groups, TMTFs were low pass in shape and showed low between-listener variability. TMTFs measured in children with dyslexia showed higher between-listener variability: TMTFs were band pass in 2 children, flat in 1 child, and low pass in the 3 others. Overall, SAM thresholds were higher in children with dyslexia than in normal children at fm = 4 and 1,024 Hz. Unprocessed-speech identification performance was nearly perfect in normal children and adults, and impaired in children with dyslexia. "Speech-envelope noise" identification performance was poorer in normal children and children with dyslexia than in normal adults. Performance improved across sessions in normal children and adults, but remained constant in children with dyslexia. Compared to normal children, children with dyslexia showed poorer reception of voicing, manner, and place of articulation for unprocessed speech and poorer reception of voicing for "speech-envelope noise." Taken together, these results support the hypothesis that some children with dyslexia may show abnormal auditory temporal-envelope processing. Such a deficit, in turn, may explain the difficulties of children with dyslexia with speech perception.

Auditory scene analysis in dyslexics

It has been argued that dyslexics suffer from temporal sensory processing deficits which affect their ability to discriminate speech in quiet environments. The impact of auditory deficits on non-language aspects of perception, however, is poorly understood. In almost every natural-listening environment, one must constantly construct scenes of the auditory world by grouping and analyzing sounds generated by multiple sources. We investigated whether dyslexics have difficulties grouping sounds. The results demonstrate that dyslexics have an impairment in grouping auditory objects that depends both on the sounds' frequency and presentation rate (i.e. the spectrotemporal context of the sound). We conclude that dyslexics have difficulty constructing scenes of the auditory world, and that these deficits can contribute to learning impairments.

Psychophysical sensitivity and physiological response to amplitude modulation in adult dyslexic listeners

This study reports two experiments conducted to assess the sensitivity of dyslexic listeners to amplitude modulation (AM) of acoustic stimuli. The smallest detectable depth of AM of white noise was measured as a function of modulation frequency. Dyslexic listeners had significantly higher thresholds of AM depth than did matched control listeners. We also recorded the scalp potential evoked by AM of white noise (the amplitude modulation following response, AMFR). Dyslexic listeners had significantly smaller AMFRs than did matched control listeners. The reduced AMFR is consistent with reduced sensitivity to AM, and there was a strong association between these psychophysical and physiological measures. This deficit in AM sensitivity may result in impaired perception of the AM present in speech.

Auditory evoked responses to single tones and closely spaced tone pairs in children grouped by reading or matrices abilities

Long latency auditory evoked responses (AER) were formed to single tones and rapid tone pairs. Using the t-statistic SPM technique, children with poorer WIAT reading scores demonstrated group difference overlying the left parietal and frontal language regions but just for AER to tone pair stimuli. Variables derived from these regions were not significantly different when the same subjects were grouped by K-BIT Matrices scores. When the same children were regrouped by Matrices scores and compared using the SPM technique, differences were now seen over the right hemisphere, especially in the parietal and frontotemporal regions, for both single and two-tone derived AERs. Variables derived from these regions were not significantly different for children when grouped by reading score. AER data support a specific deficit in two-tone stimulation for poorer reading children over the left hemisphere and also a deficit to both single and two-tone stimulation over the right hemisphere for children with poorer Matrices scores.

Cortical auditory signal processing in poor readers

Magnetoencephalographic responses recorded from auditory cortex evoked by brief and rapidly successive stimuli differed between adults with poor vs. good reading abilities in four important ways. First, the response amplitude evoked by short-duration acoustic stimuli was stronger in the post-stimulus time range of 150-200 ms in poor readers than in normal readers. Second, response amplitude to rapidly successive and brief stimuli that were identical or that differed significantly in frequency were substantially weaker in poor readers compared with controls, for interstimulus intervals of 100 or 200 ms, but not for an interstimulus interval of 500 ms. Third, this neurological deficit closely paralleled subjects' ability to distinguish between and to reconstruct the order of presentation of those stimulus sequences. Fourth, the average distributed response coherence evoked by rapidly successive stimuli was significantly weaker in the beta- and gamma-band frequency ranges (20-60 Hz) in poor readers, compared with controls. These results provide direct electrophysiological evidence supporting the hypothesis that reading disabilities are correlated with the abnormal neural representation of brief and rapidly successive sensory inputs, manifested in this study at the entry level of the cortical auditory/aural speech representational system(s).

Effect of time and frequency manipulation on syllable perception in developmental dyslexics

Many people with developmental dyslexia have difficulty perceiving stop consonant contrasts as effectively as other people and it has been suggested that this may be due to perceptual limitations of a temporal nature. Accordingly, we predicted that perception of such stimuli by listeners with dyslexia might be improved by stretching them in time-equivalent to speaking slowly. Conversely, their perception of the same stimuli ought to be made even worse by compressing them in time-equivalent to speaking quickly. We tested 15 children with dyslexia on their ability to identify correctly consonant-vowel-consonant (CVC) stimuli that had been stretched or compressed in the time domain. We also tested their perception of the same CVC stimuli after the formant transitions had been stretched or compressed in the frequency domain. Contrary to our predictions, we failed to find any systematic improvement in their performance with either manipulation. We conclude that simple manipulations in the time and frequency domains are unlikely to benefit the ability of people with dyslexia to discriminate between CVCs containing stop consonants.