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.

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.

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.

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.

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.