Hemispheric asymmetry in auditory processing of speech envelope modulations in prereading children.

The temporal envelope of speech is an important cue contributing to speech intelligibility. Theories about the neural foundations of speech perception postulate that the left and right auditory cortices are functionally specialized in analyzing speech envelope information at different time scales: the right hemisphere is thought to be specialized in processing syllable rate modulations, whereas a bilateral or left hemispheric specialization is assumed for phoneme rate modulations. Recently, it has been found that this functional hemispheric asymmetry is different in individuals with language-related disorders such as dyslexia. Most studies were, however, performed in adults and school-aged children, and only a little is known about how neural auditory processing at these specific rates manifests and develops in very young children before reading acquisition. Yet, studying hemispheric specialization for processing syllable and phoneme rate modulations in preliterate children may reveal early neural markers for dyslexia. In the present study, human cortical evoked potentials to syllable and phoneme rate modulations were measured in 5-year-old children at high and low hereditary risk for dyslexia. The results demonstrate a right hemispheric preference for processing syllable rate modulations and a symmetric pattern for phoneme rate modulations, regardless of hereditary risk for dyslexia. These results suggest that, while hemispheric specialization for processing syllable rate modulations seems to be mature in prereading children, hemispheric specialization for phoneme rate modulation processing may still be developing. These findings could have important implications for the development of phonological and reading skills.

A tractography study in dyslexia: neuroanatomic correlates of orthographic, phonological and speech processing.

Diffusion tensor imaging tractography is a structural magnetic resonance imaging technique allowing reconstruction and assessment of the integrity of three dimensional white matter tracts, as indexed by their fractional anisotropy. It is assumed that the left arcuate fasciculus plays a crucial role for reading development, as it connects two regions of the reading network, the left temporoparietal region and the left inferior frontal gyrus, for which atypical functional activation and lower fractional anisotropy values have been reported in dyslexic readers. In addition, we explored the potential role of the left inferior fronto-occipital fasciculus, which might connect a third region of the reading network, the left ventral occipitotemporal region with the left inferior frontal gyrus. In the present study, 20 adults with dyslexia and 20 typical reading adults were scanned using diffusion tensor imaging, and the bilateral arcuate fasciculus and the left inferior fronto-occipital fasciculus were delineated. Group comparisons show a significantly reduced fractional anisotropy in the left arcuate fasciculus of adults with dyslexia, in particular in the segment that directly connects posterior temporal and frontal areas. This fractional anisotropy reduction might reflect a lower degree of myelination in the dyslexic sample, as it co-occurred with a group difference in radial diffusivity. In contrast, no significant group differences in fractional anisotropy were found in the right arcuate fasciculus or in the left inferior fronto-occipital fasciculus. Correlational analyses (controlled for reading status) demonstrated a specific relation between performance on phoneme awareness and speech perception and the integrity of left arcuate fasciculus as indexed by fractional anisotropy, and between orthographic processing and fractional anisotropy values in left inferior fronto-occipital fasciculus. The present study reveals structural anomalies in the left arcuate fasciculus in adults with dyslexia. This finding corroborates current hypotheses of dyslexia as a disorder of network connections. In addition, our study demonstrates a correlational double dissociation, which might reflect neuroanatomical correlates of the dual route reading model: the left arcuate fasciculus seems to sustain the dorsal phonological route underlying grapheme-phoneme decoding, while the left inferior fronto-occipital fasciculus seems to sustain the ventral orthographic route underlying reading by direct word access.

Adults with dyslexia are impaired in categorizing speech and nonspeech sounds on the basis of temporal cues.

Developmental dyslexia is characterized by severe reading and spelling difficulties that are persistent and resistant to the usual didactic measures and remedial efforts. It is well established that a major cause of these problems lies in poorly specified representations of speech sounds. One hypothesis states that this phonological deficit results from a more fundamental deficit in auditory processing. Despite substantial research effort, the specific nature of these auditory problems remains debated. A first controversy concerns the speech specificity of the auditory processing problems: Can they be reduced to more basic auditory processing, or are they specific to the perception of speech sounds? A second topic of debate concerns the extent to which the auditory problems are specific to the processing of rapidly changing temporal information or whether they encompass a broader range of complex spectro-temporal processing. By applying a balanced design with stimuli that were adequately controlled for acoustic complexity, we show that adults with dyslexia are specifically impaired at categorizing speech and nonspeech sounds that differ in terms of rapidly changing acoustic cues (i.e., temporal cues), but that they perform adequately when categorizing steady-state speech and nonspeech sounds. Thus, we show that individuals with dyslexia have an auditory temporal processing deficit that is not speech-specific.

Auditory steady state cortical responses indicate deviant phonemic-rate processing in adults with dyslexia.

OBJECTIVES: Speech intelligibility is strongly influenced by the ability to process temporal modulations. It is hypothesized that in dyslexia, deficient processing of rapidly changing auditory information underlies a deficient development of phonological representations, causing reading and spelling problems. Low-frequency modulations between 4 and 20 Hz correspond to the processing rate of important phonological segments (syllables and phonemes, respectively) in speech and therefore provide a bridge between low-level auditory and phonological processing. In the present study, temporal modulation processing was investigated by auditory steady state responses (ASSRs) in normal-reading and dyslexic adults. DESIGN: Multichannel ASSRs were recorded in normal-reading and dyslexic adults in response to speech-weighted noise stimuli amplitude modulated at 80, 20, and 4 Hz. The 80 Hz modulation is known to be primarily generated by the brainstem, whereas the 20 and 4 Hz modulations are mainly generated in the cortex. Furthermore, the 20 and 4 Hz modulations provide an objective auditory performance measure related to phonemic- and syllabic-rate processing. In addition to neurophysiological measures, psychophysical tests of speech-in-noise perception and phonological awareness were assessed. RESULTS: On the basis of response strength and phase coherence measures, normal-reading and dyslexic participants showed similar processing at the brainstem level. At the cortical level of the auditory system, dyslexic subjects demonstrated deviant phonemic-rate responses compared with normal readers, whereas no group differences were found for the syllabic rate. Furthermore, a relationship between phonemic-rate ASSRs and psychophysical tests of speech-in-noise perception and phonological awareness was obtained. CONCLUSIONS: The results suggest reduced cortical processing for phonemic-rate modulations in dyslexic adults, presumably resulting in limited integration of temporal information in the dorsal phonological pathway.

Investigation of a significant increase in referrals during neonatal hearing screening: a comparison of Natus ALGO Portable and ALGO 3i.

OBJECTIVE: Neonatal hearing screening programs in Flanders and the Netherlands use Natus ALGO screening devices. Since 2006 in Flanders and 2009 in the Netherlands, both programs have replaced the older ALGO Portable devices with the newer ALGO 3i devices. However, in both countries, ALGO 3i devices have a significantly higher rate of referrals than ALGO Portable devices. In Flanders, the refer rate has more than doubled with the switch to ALGO 3i devices. In the Netherlands, screening centers which used ALGO 3i devices also showed a significant increase in referrals. In both countries, the percentage of children diagnosed with permanent hearing loss remained approximately the same. DESIGN: A technical comparison of both device types was carried out to identify possible causes for the increase in referrals. The stimulus output of two ALGO Portable and three ALGO 3i devices was recorded and analysed for stimulus level, spectral properties, and stimulus irregularities. RESULTS: ALGO 3i devices stimulate at a peak level 4.6 dB lower than ALGO Portable devices, have a different stimulus spectrum and show unexplained stimulus irregularities during 4% of the stimulation time. CONCLUSIONS: A number of technical differences were found between both device types which could explain the increase in referrals.

Deviant neurophysiological responses to phonological regularities in speech in dyslexic children.

Developmental dyslexia is strongly associated with a phonological deficit. Yet, implicit phonological processing (in)capacities in dyslexia remain relatively unexplored. Here we use a neurophysiological response sensitive to experience-dependent auditory memory traces, the mismatch negativity (MMN), to investigate implicit phonological processing of natural speech in dyslexic and normally reading children. In a modified passive oddball design that minimizes the contribution of acoustic processes, we presented non-words that differed by the degree of phonotactic probability, i.e. the distributional frequency of phoneme combinations in a given language. Overall morphology of ERP responses to the non-words indicated comparable processing of acoustic-phonetic stimulus differences in both children groups. Consistent with previous findings in adults, normally reading children showed a significantly stronger MMN response to the non-word with high phonotactic probability (notsel) as compared to the non-word with low phonotactic probability (notkel), suggesting auditory cortical tuning to statistical regularities of phoneme combinations. In contrast, dyslexic children did not show this sensitivity to phonotactic probability. These findings indicate that the phonological problems often reported in dyslexia relate to a subtle deficit in the implicit phonetic-phonological processing of natural speech.

Do prereaders' auditory processing and speech perception predict later literacy?

Developmental dyslexia has frequently been linked to deficits in auditory processing and speech perception. However, the presence and precise nature of these deficits and the direction of their relation with reading, remains debated. In this longitudinal study, 87 five-year-olds at high and low family risk for dyslexia were followed before and during different stages of reading acquisition. The processing of different auditory cues was investigated, together with performance on speech perception and phonology and reading. Results show no effect of family risk for dyslexia on prereading auditory processing and speech perception skills. However, a relation is present between the performance on these skills in kindergarten and later phonology and literacy. In particular, links are found with the auditory processing of cues characteristic for the temporal speech amplitude envelope, rather than with other auditory cues important for speech intelligibility. Hereby, cues embedded in the speech amplitude envelope show to be related to a broad range of phonological precursors for reading. In addition, speech-in-noise perception demonstrates to operate as the most contributing factor for later phonological awareness and to be a predictor for reading mediated by the association with phonology. This study provides behavioral support for the link between prereading speech amplitude envelope processing and speech perception for future phonology and reading.

Auditory cortical tuning to statistical regularities in phonology.

OBJECTIVE: Ample behavioral evidence suggests that distributional properties of the language environment influence the processing of speech. Yet, how these characteristics are reflected in neural processes remains largely unknown. The present ERP study investigates neurophysiological correlates of phonotactic probability: the distributional frequency of phoneme combinations. METHODS: We employed an ERP measure indicative of experience-dependent auditory memory traces, the mismatch negativity (MMN). We presented pairs of non-words that differed by the degree of phonotactic probability in a modified passive oddball design that minimizes the contribution of acoustic processes. RESULTS: In Experiment 1 the non-word with high phonotactic probability (notsel) elicited a significantly enhanced MMN as compared to the non-word with low phonotactic probability (notkel). In Experiment 2 this finding was replicated with a non-word pair with a smaller acoustic difference (notsel-notfel). An MMN enhancement was not observed in a third acoustic control experiment with stimuli having comparable phonotactic probability (so-fo). CONCLUSIONS: Our data suggest that auditory cortical responses to phoneme clusters are modulated by statistical regularities of phoneme combinations. SIGNIFICANCE: This study indicates that the language environment is relevant in shaping the neural processing of speech. Furthermore, it provides a potentially useful design for investigating implicit phonological processing in children with anomalous language functions like dyslexia.

Theta, beta and gamma rate modulations in the developing auditory system.

In the brain, the temporal analysis of many important auditory features relies on the synchronized firing of neurons to the auditory input rhythm. These so-called neural oscillations play a crucial role in sensory and cognitive processing and deviances in oscillatory activity have shown to be associated with neurodevelopmental disorders. Given the importance of neural auditory oscillations in normal and impaired sensory and cognitive functioning, there has been growing interest in their developmental trajectory from early childhood on. In the present study, neural auditory processing was investigated in typically developing young children (n = 40) and adults (n = 27). In all participants, auditory evoked theta, beta and gamma responses were recorded. The results of this study show maturational differences between children and adults in neural auditory processing at cortical as well as at brainstem level. Neural background noise at cortical level was shown to be higher in children compared to adults. In addition, higher theta response amplitudes were measured in children compared to adults. For beta and gamma rate modulations, different processing asymmetry patterns were observed between both age groups. The mean response phase was also shown to differ significantly between children and adults for all rates. Results suggest that cortical auditory processing of beta develops from a general processing pattern into a more specialized asymmetric processing preference over age. Moreover, the results indicate an enhancement of bilateral representation of monaural sound input at brainstem with age. A dissimilar efficiency of auditory signal transmission from brainstem to cortex along the auditory pathway between children and adults is suggested. These developmental differences might be due to both functional experience-dependent as well as anatomical changes. The findings of the present study offer important information about maturational differences between children and adults for responses to theta, beta and gamma rates. The current study can have important implications for the understanding of developmental disorders which are known to be associated with deviances in neural auditory processing.

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.

Hemispheric asymmetry of auditory steady-state responses to monaural and diotic stimulation.

Amplitude modulations in the speech envelope are crucial elements for speech perception. These modulations comprise the processing rate at which syllabic (~3-7 Hz), and phonemic transitions occur in speech. Theories about speech perception hypothesize that each hemisphere in the auditory cortex is specialized in analyzing modulations at different timescales, and that phonemic-rate modulations of the speech envelope lateralize to the left hemisphere, whereas right lateralization occurs for slow, syllabic-rate modulations. In the present study, neural processing of phonemic- and syllabic-rate modulations was investigated with auditory steady-state responses (ASSRs). ASSRs to speech-weighted noise stimuli, amplitude modulated at 4, 20, and 80 Hz, were recorded in 30 normal-hearing adults. The 80 Hz ASSR is primarily generated by the brainstem, whereas 20 and 4 Hz ASSRs are mainly cortically evoked and relate to speech perception. Stimuli were presented diotically (same signal to both ears) and monaurally (one signal to the left or right ear). For 80 Hz, diotic ASSRs were larger than monaural responses. This binaural advantage decreased with decreasing modulation frequency. For 20 Hz, diotic ASSRs were equal to monaural responses, while for 4 Hz, diotic responses were smaller than monaural responses. Comparison of left and right ear stimulation demonstrated that, with decreasing modulation rate, a gradual change from ipsilateral to right lateralization occurred. Together, these results (1) suggest that ASSR enhancement to binaural stimulation decreases in the ascending auditory system and (2) indicate that right lateralization is more prominent for low-frequency ASSRs. These findings may have important consequences for electrode placement in clinical settings, as well as for the understanding of low-frequency ASSR generation.

Impairments in speech and nonspeech sound categorization in children with dyslexia are driven by temporal processing difficulties.

Auditory processing problems in persons with dyslexia are still subject to debate, and one central issue concerns the specific nature of the deficit. In particular, it is questioned whether the deficit is specific to speech and/or specific to temporal processing. To resolve this issue, a categorical perception identification task was administered in thirteen 11-year old dyslexic readers and 25 matched normal readers using 4 sound continua: (1) a speech contrast exploiting temporal cues (/bA/-/dA/), (2) a speech contrast defined by nontemporal spectral cues (/u/-/y/), (3) a nonspeech temporal contrast (spectrally rotated/bA/-/da/), and (4) a nonspeech nontemporal contrast (spectrally rotated/u/-/y/). Results indicate that children with dyslexia are less consistent in classifying speech and nonspeech sounds on the basis of rapidly changing (i.e., temporal) information whereas they are unimpaired in steady-state speech and nonspeech sounds. The deficit is thus restricted to categorizing sounds on the basis of temporal cues and is independent of the speech status of the stimuli. The finding of a temporal-specific but not speech-specific deficit in children with dyslexia is in line with findings obtained in adults using the same paradigm (Vandermosten et al., 2010, Proceedings of the National Academy of Sciences of the United States of America, 107: 10389-10394). Comparison of the child and adult data indicates that the consistency of categorization considerably improves between late childhood and adulthood, particularly for the continua with temporal cues. Dyslexic and normal readers show a similar developmental progress with the dyslexic readers lagging behind both in late childhood and in adulthood.

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.

Preschool impairments in auditory processing and speech perception uniquely predict future reading problems.

Developmental dyslexia is characterized by severe reading and spelling difficulties that are persistent and resistant to the usual didactic measures and remedial efforts. It is well established that a major cause of these problems lies in poorly specified phonological representations. Many individuals with dyslexia also present impairments in auditory temporal processing and speech perception, but it remains debated whether these more basic perceptual impairments play a role in causing the reading problem. Longitudinal studies may help clarifying this issue by assessing preschool children before they receive reading instruction and by following them up through literacy development. The current longitudinal study shows impairments in auditory frequency modulation (FM) detection, speech perception and phonological awareness in kindergarten and in grade 1 in children who receive a dyslexia diagnosis in grade 3. FM sensitivity and speech-in-noise perception in kindergarten uniquely contribute to growth in reading ability, even after controlling for letter knowledge and phonological awareness. These findings indicate that impairments in auditory processing and speech perception are not merely an epiphenomenon of reading failure. Although no specific directional relations were observed between auditory processing, speech perception and phonological awareness, the highly significant concurrent and predictive correlations between all these variables suggest a reciprocal association and corroborate the evidence for the auditory deficit theory of dyslexia.

Detection and identification of monaural and binaural pitch contours in dyslexic listeners.

The use of binaural pitch stimuli to test for the presence of binaural auditory impairment in reading-disabled subjects has so far led to contradictory outcomes. While some studies found that a majority of dyslexic subjects was unable to perceive binaural pitch, others obtained a clear response of dyslexic listeners to Huggins' pitch (HP). The present study clarified whether impaired binaural pitch perception is found in dyslexia. Results from a pitch contour identification test, performed in 31 dyslexic listeners and 31 matched controls, clearly showed that dyslexics perceived HP as well as the controls. Both groups also showed comparable results with a similar-sounding, but monaurally detectable, pitch-evoking stimulus. However, nine of the dyslexic subjects were found to have difficulty identifying pitch contours both in the binaural and the monaural conditions. The ability of subjects to correctly identify pitch contours was found to be significantly correlated to measures of frequency discrimination. This correlation may be attributed to the similarity of the experimental tasks and probably reflects impaired cognitive mechanisms related to auditory memory or auditory attention rather than impaired low-level auditory processing per se.