Deficits in auditory temporal and spectral resolution in language-impaired children

Temporal resolution of auditory perception in relation to perception, memory, and language skills in typical children

This study examined temporal resolution of auditory perception (TRAP) in relation to speech perception and phonological working memory in one experiment and in relation to measures of rapid naming, understanding of grammatical contrasts, and phonological awareness in a second experiment. In the TRAP test, 6- to 9-year-old children discriminated four pairs of two sine tones, 878 Hz and 1350 Hz, presented in blocks with stepwise decreasing interstimulus intervals from 256 ms to 8 ms. In Experiment 1, weak but significant correlations between TRAP and speech perception and phonological working memory were found, as well as a significantly lower TRAP performance by girls. Experiment 2 indicated a significant relation between TRAP and the phonological awareness task. The results give qualified support to the notion of a weak relationship between TRAP and speech perception and language skills among typical children. However, the gender differences in TRAP performance, the limited variance explained by the regression models, and the finding that only one of the language parameters correlated with TRAP do not support the notion of a causal relationship between language skills and TRAP. The possibility that TRAP is connected to general neurological maturation rather than specifically to language abilities is discussed.

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.

Neuropsychologische Interventionsstrategien am Beispiel der umschriebenen Entwicklungsstörungen der Sprache und der Motorik

Zusammenfassung. Für höhere mentale Funktionen ist das unbeeinträchtigte Zusammenwirken mehrerer einzelner Prozesse erforderlich. Mittels neuropsychologischer Interventionsansätze bei Kindern wird versucht, bei Funktionsbeeinträchtigungen, die sich auf in Entwicklung befindliche Systeme auswirken, gestörte Prozesse sowie deren Zusammenwirken spezifisch zu beeinflussen. Eine neuropsychologische Interventionsstrategie bei umschriebenen Entwicklungsstörungen beinhaltet die Erfassung des Profils von Stärken und Schwächen, die Identifizierung des oder der gestörten Prozesse des betreffenden funktionellen Systems, die Anwendung spezifischer neuropsychologischer Interventionen unter Verwendung geeigneter Instruktionsstrategien und verhaltensmodifzierender Maßnahmen sowie den kompensatorischen Einsatz intakter Prozesse oder Funktionen. Bei Patienten mit umschriebenen Entwicklungsstörungen der Sprache wurde eine Beeinträchtigung der auditiven Verarbeitung kurzer bzw. in schneller Abfolge präsentierter Stimuli gezeigt. Mittels computergestützter neuropsychologischer Trainingsprogramme konnten auditive Diskrimination und Segmentierung, Phonemverarbeitung und Sprachverständnis deutlich verbessert werden ( Merzenich et al., 1996 ; Tallal et al., 1996 ). Zur Behandlung von Kindern mit umschriebenen Entwicklungsstörungen der Motorik wurden sensorische Integration und Kinästhetisches Training eingesetzt. Für beide Behandlungsverfahren wird zwar postuliert, daß Funktionsverbesserungen über die Veränderung spezifischer Prozesse erreicht werden, es konnte auch eine gewisse Effektivität belegt werden, welche jedoch weitgehend unspezifisch ist und wahrscheinlich auf allgemeinen Wirkfaktoren beruht.

Relating selective brain damage to impairments with voicing contrasts

Research is reviewed concerning the performance of several neurological groups on the perception and production of voicing contrasts in speech. Patients with cerebellar damage, Parkinson's disease, specific language impairment, Broca's aphasia, apraxia, and Wernicke's aphasia have been reported to be impaired in the perception and articulation of voicing. The types of deficits manifested by these neurologically impaired groups in creating and discriminating voicing contrasts are discussed and the respective contributions of separate neural areas are identified. A model is presented specifying the level of phonemic processing thought to be impaired for each patient group and critical tests of the model's predictions are identified.

Diminished motor timing control in children referred for diagnosis of learning problems

Disabled readers exhibit motor timing control (MTC) deficits in bimanual coordination relative to average readers. This article evaluates to what extent poor MTC is specific to reading or if it is related to learning problems in general. Children (7 to 11 years of age) referred for learning impairment (LI; n = 100) and same-age children nonlearning impaired (NLI; n = 243) performed a paced finger-tapping task. Greater variability of interresponse intervals was associated with poorer reading, spelling, and arithmetic achievement. The LI group performed more poorly than the NLI group, a difference that persisted even after adjusting for reading skill. Poor MTC is associated with poor reading but may also be a characteristic of children referred for learning problems, possibly signaling increased vulnerability of underlying neural integrative processes relevant to the child's adaptation to academic demands, including reading.

Event-related potentials during auditory language processing in congenitally blind and sighted people

While behavioral studies have documented delayed language acquisition in blind children, other studies have revealed better speech discrimination abilities for blind than sighted adults. Several brain imaging studies have provided evidence for cortical reorganization due to visual deprivation but the cerebral organization of language in blind humans is not known yet. We hypothesized that the increasing specialization of language systems normally observed during development may not take place to the same degree in blind individuals since posterior visual areas do not receive their adequate input. On the other hand, we hypothesized that blind people, due to their greater reliance upon the auditory language signal, may process speech faster than sighted people. To test these assumptions, event-related potentials were recorded while 11 congenitally blind and 11 sighted adults matched in age, gender, handedness and education were engaged in a language task. Participants listened to sentences in order to decide after each sentence if it was meaningful or not. Incongruous sentence-final words elicited an N400 effect in both groups. The N400 effect had a left-lateralized fronto-central scalp distribution in the sighted but a symmetric and broad topography in the blind. Furthermore, the N400 effect started earlier in the blind than in the sighted. Closed class compared to open class sentence middle words elicited a more pronounced late negativity in the blind than in the sighted. These results suggest that blind people process auditory language stimuli faster than sighted people and that some language functions may be reorganized in the blind.

Maturation of white matter in the human brain: a review of magnetic resonance studies

This review focuses on the maturation of brain white-matter, as revealed by magnetic resonance (MR) imaging carried out in healthy subjects. The review begins with a brief description of the nature of the MR signal and its possible biological underpinnings, and proceeds with a description of MR findings obtained in newborns, infants, children and adolescents. On MR images, a significant decrease in water content leads to a decrease of longitudinal relaxation times (T1) and transverse relaxation times (T2) and consequent "adult-like" appearance of T1-weighted and T2-weighted images becomes evident towards the end of the first year of life. Owing to the onset of myelination and the related increase of lipid content, MR images gradually acquire an exquisite grey-white matter contrast in a temporal sequence reflecting the time course of myelination. Albeit less pronounced, age-related changes in white matter continue during childhood and adolescence; white matter increases its overall volume and becomes more myelinated in a region-specific fashion. Detection of more subtle changes during this "late" phase of brain development is greatly aided by computational analyses of MR images. The review also briefly outlines future directions, including the use of novel MR techniques such as diffusion tensor imaging and magnetization transfer, as well as the suggestion for the concurrent use of experimental behavioral test-batteries, with structural MR imaging, to study developmental changes in structure-function relationships.

Age-related improvements in auditory backward and simultaneous masking in 6- to 10-year-old children

This study investigated the development of auditory frequency and temporal resolution using simultaneous and backward masking of a tone by a noise. The participants were 6- to 10-year-old children and adults. On the measure of frequency resolution (the difference in the detection threshold for a tone presented either in a bandpass noise or in a spectrally notched noise), 6-year-old children performed as well as adults. However, for the backward masking task, 6-year-olds had, on average, 34 dB higher thresholds than adults. A negative exponential decay function fitted to the backward masking data for subjects of all ages indicated that adult-like temporal resolution may not be reached until about 11 years of age. These results show that, measured by masking, frequency resolution has reached adult-like performance by 6 years of age, whereas temporal resolution develops beyond 10 years of age. Six-year-old children were also assessed with tests of cognitive ability. Improvements in both frequency and temporal resolution were found with increasing IQ score.

Representation of temporal features of complex sounds by the discharge patterns of neurons in the owl's inferior colliculus

The spiking pattern evoked in cells of the owl's inferior colliculus by repeated presentation of the same broadband noise was found to be highly reproducible and synchronized with the temporal features of the noise stimulus. The pattern remained largely unchanged when the stimulus was presented from spatial loci that evoke similar average firing rates. To better understand this patterning, we computed the pre-event stimulus ensemble (PESE)-the average of the stimuli that preceded each spike. Computing the PESE by averaging the pressure waveforms produced a noisy, featureless trace, suggesting that the patterning was not synchronized to a particular waveform in the fine structure. By contrast, computing the PESE by averaging the stimulus envelope revealed an average envelope waveform, the "PESE envelope," typically having a peak preceded by a trough. Increasing the overall stimulus level produced PESE envelopes with higher amplitudes, suggesting a decrease in the jitter of the cell's response. The effect of carrier frequency on the PESE envelope was investigated by obtaining a cell's response to broadband noise and either estimating the PESE envelope for each spectral band or by computing a spectrogram of the stimulus prior to each spike. Either method yielded the cell's PESE spectrogram, a plot of the average amplitude of each carrier-frequency component at various pre-spike times. PESE spectrograms revealed surfaces with peaks and troughs at certain frequencies and pre-spike times. These features are collectively called the spectrotemporal receptive field (STRF). The shape of the STRF showed that in many cases, the carrier frequency can affect the PESE envelope. The modulation transfer function (MTF), which describes a cell's ability to respond to time-varying amplitudes, was estimated with sinusoidally amplitude-modulated (SAM) noises. Comparison of the PESE envelope with the MTF in the time and frequency domains showed that the two were closely matched, suggesting that a cell's response to SAM stimuli is largely predictable from its response to a noise-modulated carrier. The STRF is considered to be a model of the linear component of a system's response to dynamic stimuli. Using the STRF, we estimated the degree to which we could predict a cell's response to an arbitrary broadband noise by comparing the convolution of the STRF and the envelope of the noise with the cell's post-stimulus time histogram to the same noise. The STRF explained 18-46% of the variance of a cell's response to broadband noise.

Rate of information segregation in developmentally dyslexic children

Slowed processing of sequential perceptual information is related to developmental dyslexia. We investigated this unimodally and crossmodally in developmentally dyslexic children and controls ages 8-12 years. The participants judged whether two spatially separate trains of brief stimuli, presented at various stimulus onset asynchronies (SOA) in one or two senses, were synchronous or not. The stimulus trains consisted of light flashes in vision, clicks in audition, and indentations of the skin in the tactile sense. The dyslexic readers required longer SOAs than controls for successful performance in all six comparisons. The crossmodal spatiotemporal resolution of the groups differed more than unimodal performance. The dyslexic readers' segregation performance was also less differentiated than that of the controls. Our results show that not only sensory but also polysensory nonverbal information processing is temporally impaired in dyslexic children.

Nonlinguistic perceptual deficits associated with reading and language disorders

Recent behavioral evidence supports the idea that some individuals with reading and language disorders are impaired in their perception of nonlinguistic auditory and visual information. More sophisticated measurement paradigms and analysis techniques are leading to a clearer understanding of these deficits and to possibilities for their remediation.

Injury- and use-related plasticity in the adult auditory system

After restricted cochlear lesions in adult animals, the frequency selectivity of neurons in the cortical region deprived of its normal input by the lesion is changed such that the region is occupied by expanded representations of adjacent (perilesion) frequencies. Analogous changes in cortical frequency selectivity and organization are seen as a consequence of behavioral training that enhances the significance of particular acoustic stimuli. The occurrence of such reorganization in a wide range of species (including simian primates) suggests that it would also occur in humans. Direct evidence in support of this suggestion is provided by a small body of functional imaging evidence. Although such reorganization almost certainly does not have a compensatory function, such a profound change in the pattern of cortical activation produced by stimuli exciting perilesion parts of the receptor epithelium would be expected to have perceptual consequences and, perhaps, clinical implications.

Auditory processing parallels reading abilities in adults

A broad battery of psychoacoustic measures and standard measures of reading and spelling were applied to 102 adults. The test group included individuals with a childhood history of reading difficulties and controls with no reported reading difficulties. Reading scores were variable in both groups. Poor auditory processing abilities were recorded in poor readers; particular difficulties were posed by tasks requiring spectral distinctions, the simplest of which was pure tone frequency discrimination. In absolute terms, the greatest deficits were recorded in tasks in which stimuli were presented in brief forms and in rapid succession. Auditory processing abilities accounted for more than 50% of the reading score variance in the control group, but their correlation with reading scores was lower in the group with childhood histories of reading difficulties. The additional variability in the latter group resulted largely from the prevalence of reading-compensated poor psychoacoustic performers, whose short-term word memory was also typically poor. Taken together, these findings support a link between impaired auditory resolution and poor reading. Psychoacoustic difficulties are largely retained through adulthood and may be the source of the retained reading difficulties.

Lexical word formation in children with grammatical SLI: a grammar-specific versus an input-processing deficit?

An ongoing controversy is whether an input-processing deficit or a grammar-specific deficit causes specific language impairment (SLI) in children. Previous studies have focussed on SLI childrens' omission of inflectional morphemes or impaired performance on language tasks, but such data can be accounted for by either theory. To distinguish between these theories we study compound formation in a subgroup of SLI children with 'grammatical (G)-SLI'. An input-processing account (e.g. Leonard, L. (1998). Children with specific language impairment. Cambridge, MA: MIT Press), in which perception and production of inflections requires extra processing resources, would predict that G-SLI children will omit the regular plural -s in compounds (e.g. rat-eater). A grammar-specific deficit account (e.g. Ullman, M. & Gopnik, M. (1994) The production of inflectional morphology in hereditary specific language impairment. The McGill Working Papers in Linguistics, 10, 81-118; van der Lely, H. K. J. & Ullman, M. (1996). The computation and representation of past-tense morphology in normally developing and specifically language impaired children. In A. Stringfellow, D. Cahana-Amitay, E. Hughes & A. Zukowski, Proceedings of the 20th Annual Boston University Conference on Language Development (pp. 816-827). Somerville, MA: Cascadilla Press), in which G-SLI children are impaired in regular inflectional morphology, would predict that G-SLI children will produce regular plural -s forms inside compounds (e.g. *rats-eater). We compared the responses of 16 G-SLI subjects (aged 10 years 4 months to 18 years) with those of 36 normally developing control children (24 matched on language ability and 12 matched on age and cognitive ability). All the groups produced irregular plural nouns in compounds (mice-eater). The normally developing children and teenagers rarely, if ever. produced regular plural nouns inside compounds (*rats-eater), whereas the G-SLI subjects did so often. This pattern of results conflicts with the predictions ofthe input-processing deficit account. The findings support the grammar-specific deficit hypothesis. The data rovide further evidence that specialized grammatical abilities may be differentially impaired within the language system.

Auditory backward-masking performance by children who stutter and its relation to dysfluency rate

The fluency of people who stutter is affected markedly when auditory feedback is altered, suggesting that stuttering may be associated with hearing. Peripheral hearing problems, however, are no more common in people who stutter than in those who do not. Performance was investigated in a task that involves central auditory processing (backward masking). Children who stuttered had deficits in backward masking (indicated by higher thresholds) compared with a group of fluent control children. The backward-masking thresholds were positively correlated with frequency of stuttering.

The effect of marker frequency disparity on the discrimination of gap duration in monkeys

Duration-discrimination thresholds of the silent interval (gap) between two successive tones (markers) were measured in four Japanese monkeys. The task was serial discrimination, and monkeys were required to release the lever when the gap duration decreased from 200 ms. Monkeys successfully acquired the task, and gap thresholds of monkeys were revealed to be larger than previous data with human subjects. Gap thresholds were not affected by marker frequency when the two markers were identical in frequency, though the thresholds increased when large frequency differences existed between markers. The effect of marker frequency disparity on gap thresholds in monkeys is discussed in terms of the difficulty in integrating information from discrete frequency channels.

The neural basis of developmental dyslexia

Until recently, many thought developmental dyslexia was a behavioral disorder that primarily affected reading. In fact, it is a partly heritable condition, the clinical manifestations of which are extremely complex including deficits in reading, working memory, sensorimotor coordination, and early sensory processing. Even though extensive research has characterized these behavioral abnormalities carefully, the biological mechanisms of the clinical manifestations still are poorly understood. Recent research into both the nature of the structural and functional abnormalities in developmental dyslexia and the functional neuroanatomy of reading have rapidly advanced our understanding of the localization of the processes responsible for the signs and symptoms of dyslexia. This paper reviews recent evidence supporting a biological basis for developmental dyslexia.

Auditory temporal processing impairment: neither necessary nor sufficient for causing language impairment in children

Fourteen twin pairs, aged 8 to 10 years, were tested 3 times over 12 months; they included 11 children with language impairment (LI), 11 control children matched on nonverbal ability and age, and 6 co-twins who did not meet criteria for LI or control status. Thresholds were estimated for detecting a brief backward-masked tone (BM), detection of frequency modulation (FM), and pitch discrimination using temporal cues (deltaf0). Both BM and FM thresholds improved with training, and by the 2nd test session, FM thresholds were in the adult range. There were marked individual differences on BM and deltaf0 and, for both tasks, performance correlated with Tallal's Auditory Repetition Task administered 2 years previously. However, no auditory measure gave significant differences between LI and control groups; performance was influenced more by nonverbal than language ability. Some children did have a stable pattern of poor performance on certain auditory tasks, but their good FM detection raised questions about whether processing of auditory temporal information is abnormal. We found no evidence that auditory deficits are a necessary or sufficient cause of language impairments.

Temporal encoding of the voice onset time phonetic parameter by field potentials recorded directly from human auditory cortex

Voice onset time (VOT) is an important parameter of speech that denotes the time interval between consonant onset and the onset of low-frequency periodicity generated by rhythmic vocal cord vibration. Voiced stop consonants (/b/, /g/, and /d/) in syllable initial position are characterized by short VOTs, whereas unvoiced stop consonants (/p/, /k/, and t/) contain prolonged VOTs. As the VOT is increased in incremental steps, perception rapidly changes from a voiced stop consonant to an unvoiced consonant at an interval of 20-40 ms. This abrupt change in consonant identification is an example of categorical speech perception and is a central feature of phonetic discrimination. This study tested the hypothesis that VOT is represented within auditory cortex by transient responses time-locked to consonant and voicing onset. Auditory evoked potentials (AEPs) elicited by stop consonant-vowel (CV) syllables were recorded directly from Heschl's gyrus, the planum temporale, and the superior temporal gyrus in three patients undergoing evaluation for surgical remediation of medically intractable epilepsy. Voiced CV syllables elicited a triphasic sequence of field potentials within Heschl's gyrus. AEPs evoked by unvoiced CV syllables contained additional response components time-locked to voicing onset. Syllables with a VOT of 40, 60, or 80 ms evoked components time-locked to consonant release and voicing onset. In contrast, the syllable with a VOT of 20 ms evoked a markedly diminished response to voicing onset and elicited an AEP very similar in morphology to that evoked by the syllable with a 0-ms VOT. Similar response features were observed in the AEPs evoked by click trains. In this case, there was a marked decrease in amplitude of the transient response to the second click in trains with interpulse intervals of 20-25 ms. Speech-evoked AEPs recorded from the posterior superior temporal gyrus lateral to Heschl's gyrus displayed comparable response features, whereas field potentials recorded from three locations in the planum temporale did not contain components time-locked to voicing onset. This study demonstrates that VOT at least partially is represented in primary and specific secondary auditory cortical fields by synchronized activity time-locked to consonant release and voicing onset. Furthermore, AEPs exhibit features that may facilitate categorical perception of stop consonants, and these response patterns appear to be based on temporal processing limitations within auditory cortex. Demonstrations of similar speech-evoked response patterns in animals support a role for these experimental models in clarifying selected features of speech encoding.

Effects of envelope expansion on speech recognition

This study investigated the effects of expanding the gross time-amplitude variations of 'speech-envelope noise' stimuli on speech recognition. The initial stimuli were VCV logatomes presented in quiet or against a steady white noise with a 0-dB signal-to-noise ratio. Their low-frequency temporal modulations (<500 Hz) were extracted in broad frequency bands, and raised to the power 2. The resulting envelopes were then used to modulate a white noise, and combined to produce the 'speech-envelope noise' stimuli. As a consequence, listeners were forced to identify speech using primarily temporal envelope cues. The results obtained with four normal-hearing listeners show small decrements in recognition performance of 1-15% when expanding the envelope of the speech stimuli presented in quiet. The results also show a small but consistent improvement in performance of 6-14% when expanding the envelope of the speech stimuli presented in noise. These results are consistent with those obtained by Fu and Shannon (J. Acoust. Soc. Am. 104 (1998) 2570-2577) with speech presented in quiet. They also suggest that the reduction in the modulation depth of the speech envelope caused by noise or reverberation could be compensated by expanding low-frequency temporal modulations.

Tactile perception in developmental dyslexia: a psychophysical study using gratings

Multiple sensory abnormalities have been reported in individuals with developmental dyslexia, especially in the visual and auditory systems. We used gratings of alternating ridges and grooves to investigate tactile perception in this disorder using two tasks: spatial acuity-dependent discrimination of grating orientation and discrimination of gratings varying in ridge width. Compared to age-matched normal subjects, dyslexics were significantly impaired on grating orientation discrimination, with mean thresholds that were nearly twice normal. Unlike normal subjects, their performance on this task was slightly but significantly worse on the dominant hand than on the non-dominant hand. Dyslexics also showed a substantial but non-significant trend for impairment on grating ridge width discrimination. A group of subjects with attention deficit disorder did not differ significantly from normal on any of these measures. These findings expand the range of perceptual deficits reported in developmental dyslexia. Possible explanations for the results, including difficulties with temporal processing, abnormal lateral masking or parietal lobe dysfunction are discussed.

Neuronal asymmetries in primary visual cortex of dyslexic and nondyslexic brains

Dyslexic brains exhibit histologic changes in the magnocellular (magno) cells of the lateral geniculate nucleus, and consistent with these changes, dyslexics demonstrate abnormal visually evoked potentials and brain activation to magno-specific stimuli. The current study was aimed at determining whether these findings were associated with changes in the primary visual cortex with the prediction that magno components of this cortex would be affected. We measured cross-sectional neuronal areas in primary visual cortex (area 17) in dyslexic and nondyslexic autopsy specimens. There was a significant interaction between hemispheres and diagnostic category; ie, nondyslexic brains had larger neurons in the left hemisphere, whereas dyslexic brains had no asymmetry. On the other hand, cell layers associated with magno input from the lateral geniculate nucleus did not show consistent changes in dyslexic brains. Thus, there is a neuronal size asymmetry in favor of the left primary visual cortex in nondyslexics that is absent in dyslexic brains. This is yet another example of anomalous expression of cerebral asymmetry in dyslexia similar to that of the planum temporale, which in our view reflects abnormality in circuits involved in reading.

Development of adult-like performance in backward, simultaneous, and forward masking

Some researchers have argued that specific language impairment (SLI) is associated with deficits in processing certain temporal aspects of auditory stimuli. One recent study (Wright et al., 1997) suggests that backward masking in particular poses a problem for children with SLI, as compared to simultaneous or forward masking. Interpretation of this finding is complicated by the fact that very little is known about the development of normal, adult-like performance in these masking paradigms. The study reported here examined performance for children 5-11 years old on forward, simultaneous, and backward masking and compared their performance to that of adults. The data show a trend for improvement in performance with age in all three masking paradigms. There was no evidence for later or more gradual improvement in performance on the backward-masking paradigm. However, backward-masking thresholds were more variable than those in the other conditions and were subject to greater individual differences, even in the adult data set. Manipulation of masker bandwidth yielded no evidence for more adult-like performance in the child data with the wider bandwidth masker. Additional data collected on two naive adult observers show a marked improvement in backward-masking performance over time, suggesting that detection with these stimuli might be particularly subject to practice effects.

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.

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).

Speech sound perception and learning: biologic bases

Historically, auditory research has focused predominantly on how relatively simple acoustic signals are represented in the neuronal responses of the auditory periphery. However, in order to understand the neurophysiology underlying speech perception, the ultimate objective is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. This paper reviews three areas pertaining to the central auditory representation of speech: (1) the differences in neural representation of speech sounds at different levels of the auditory system, (2) the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and (3) the plasticity of speech-sound neural representation and speech perception.

Auditory cortical plasticity: a comparison with other sensory systems

The auditory cortex has a crucial role in higher cognitive functions, including the perception of speech, music and auditory space. Cortical plasticity, as in other sensory systems, is used in the fine tuning of the auditory system for these higher functions. Auditory cortical plasticity can also be demonstrated after lesions of the cochlea and it appears to participate in generating tinnitus. Early musical training leads to an expansion in the representation of complex harmonic sounds in the auditory cortex. Similarly, the early phonetic environment has a strong influence on speech development and, presumably, on the cortical organization of speech. In auditory spatial perception, the spectral cues generated by the head and outer ears vary between individuals and have to be calibrated by learning, which most probably takes place at the cortical level. The neural mechanisms of plasticity are likely to be the same across all cortical regions. It should be useful, therefore, to relate some of the findings and hypotheses about auditory cortical plasticity to previous studies of other sensory systems.

Different origin of auditory and phonological processing problems in children with language impairment: evidence from a twin study

This study investigated the heritability of auditory processing impairment, as assessed by Tallal's Auditory Repetition Test (ART). The sample consisted of 37 same-sex twin pairs who had previously been selected because one or both twins met criteria for language impairment (LI) and 104 same-sex twin pairs in the same age range (7 to 13 years) from the general population. These samples yielded 55 children who met criteria for LI, who were compared with 76 children whose language was normal for their age (LN group). We replicated earlier work showing that group LI is impaired relative to group LN on ART. However, there was no evidence of a heritable influence on ART scores: Correlations between twins and their co-twins were reasonably high for both MZ and DZ twins, suggesting that performance is more influenced by shared environment than genetic factors. Analyses of extreme scores gave a similar picture of nonsignificant group heritability. In contrast, a test of phonological short-term memory, the Children's Nonword Repetition Test (CNRep), gave high estimates of group heritability. In general, CNRep was a better predictor of low language test scores than ART, but ART did make a significant independent contribution in accounting for variance in a test of grammatical understanding.

The effect of presentation rate on serial memory in young children with specific language impairment

The study examined the serial memory ability of a group of preschool children with specific language impairment (SLI) who were compared to age and language control groups. The children were asked to recognize serial patterns under short and long presentation durations. The subjects were presented with images of common objects (that appeared to be easily recoded into a phonological form) and iconic images of scribble drawings and unfamiliar faces (that did not appear to invite recoding). Under long presentation conditions, the performance of children with SLI resembled that of their age-matched peers on all 3 types of tasks. However, under short presentation conditions, children with SLI performed worse than their age-matched peers on all 3 tasks (and similarly to their language-matched peers). The performance of the children with SLI declined dramatically in all conditions when the items were presented for a brief period. If the serial memory deficits of young children with SLI were specific to phonological processing, their performance on recognizing the pattern of common objects should have been impaired, but not their performance with other visual tasks that are less likely to be recoded. Instead, serial memory in children with SLI was affected by presentation duration across tasks. The findings suggest that recognizing serial patterns is dependent, in part, on the speed of processing serial information. The findings are discussed in relation to models of limited capacity processing.

Evidence for a grammar-specific deficit in children

BACKGROUND

Specific language impairment (SLI) is a disorder in which language acquisition is impaired in an otherwise normally developing child. SLI affects around 7% of children. The existence of a purely grammatical form of SLI has become extremely controversial because it points to the existence and innateness of a putative grammatical subsystem in the brain. Some researchers dispute the existence of a purely grammatical form of SLI. They hypothesise that SLI in children is caused by deficits in auditory and/or general cognitive processing, or social factors. There are also claims that the cognitive abilities of people with SLI have not yet been sufficiently characterised to substantiate the existence of SLI in a pure grammatical form.

RESULTS

We present a case study of a boy, known as AZ, with SLI. To investigate the claim for a primary grammatical impairment, we distinguish between grammatical abilities, non-grammatical language abilities and non-verbal cognitive abilities. We investigated AZ's abilities in each of these areas. AZ performed normally on auditory and cognitive tasks, yet exhibited severe grammatical impairments. This is evidence for a developmental grammatical deficit that cannot be explained as a by-product of retardation or auditory difficulties.

CONCLUSIONS

The case of AZ provides evidence supporting the existence of a genetically determined, specialised mechanism that is necessary for the normal development of human language.

Speech modifications algorithms used for training language learning-impaired children

In this paper, the details of processing algorithms used in a training program with language learning-impaired children (LLI's) are described. The training program utilized computer games, speech/language training exercises, books-on-tape and educational CD-ROM's. Speech tracks in these materials were processed using these algorithms. During a four week training period, recognition of both processed and normal speech in these children continually increased to near age-appropriate levels. We conclude that this form of processed speech is subject to profound perceptual learning effects and exhibits widespread generalization to normal speech. This form of learning and generalization contributes to the rehabilitation of temporal processing deficits and language comprehension in this subject population.

Auditory cortical event-related potentials to pitch deviances in rats

We recorded epidural event-related potentials (ERPs) from the auditory cortex in anesthetized rats when pitch-deviant tones were presented in a homogeneous series of standard tones (oddball condition). Additionally, deviant tones were presented without standard tones (deviant-alone condition). ERPs to deviant tones in the oddball condition differed significantly from ERPs to standard tones at the latency range of 63-243 ms. On the other hand, ERPs to deviant tones in the deviant-alone condition did not differ from ERPs to standard tones until 196 ms from stimulus onset. The results suggest that oddball stimuli can be neurophysiologically discriminated in anesthetized rats. Furthermore, as the difference between ERPs to deviant tones and those to standard tones at the 63-196 ms latency range could be detected only when standard tones precede deviant tones it shows concordance with mismatch negativity in humans.

Auditory filter nonlinearity at 2 kHz in normal hearing listeners

Auditory filters broaden with increasing level. Using a recently developed method of fitting filter shapes to notched-noise masking data that explicitly models the nonlinear changes in filter shape across level, results at 2 kHz from 9 listeners over a wide range of levels and notch widths are reported. Families of roex(p,w,t) filter shapes lead to models which account well for the observed data. The primary effect of level is a broadening in the tails of the filter as level increases. In all cases, models with filter parameters depending on probe level fit the data much better than masker-dependent models. Thus auditory filter shapes appear to be controlled by their output, not by their input. Notched-noise tests, if performed at a single level, should use a fixed probe level. Filter shapes derived in this way, and normalized to have equal tail gain, are highly reminiscent of measurements made directly on the basilar membrane, including the degree of compression evidenced in the input-output function.

Central auditory processing disorder in school-aged children: a critical review

The rationale to evaluate for central auditory processing disorder (CAPD) in school-aged children is based on the assumption that an auditory-specific perceptual deficit underlies many learning problems including specific reading and language disabilities. A fundamental issue in this area is whether convincing empirical evidence exists to validate this proposition. Herein, we consider the issue of modality specificity by examining the extent to which reading, language, and attention disorders in school-aged children involve perceptual dysfunctions limited to a single sensory modality. Difficulty in validating CAPD as a diagnostic label is due in large part to use of the unimodal inclusive framework, which has biased the diagnosis to favor sensitivity of test results over documenting the specificity of the deficit. Indeed, empirical research documenting modality-specific auditory-perceptual dysfunction in this population is scarce. Therefore, the existing literature on this topic has not clarified the "true" nature of the problem, and has left many questions about this disorder unanswered. It is argued that demonstrating modality specificity is one way to rule out supramodal disorders as explanations for observed dysfunction. Multimodal perceptual testing is one logical approach to help clarify this area of investigation.

Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10-15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.

Brain morphology in children with specific language impairment

The planum temporale and pars triangularis have been found to be larger in the left hemisphere than the right in individuals with normal language skills. Brain morphology studies of individuals with developmental language disorders report reversed asymmetry or symmetry of the planum, although the bulk of this research has been completed on adults with dyslexia. Pars triangularis has not been studied in the developmental language impaired population. In this study, magnetic resonance imaging (MRI) was used for quantitative comparisons of the planum temporale (Wernicke's area) and pars triangularis (Broca's area) in children with specific language impairment (SLI) and children with normal language skills. The subjects were 11 children with SLI and 19 age- and sex-matched controls between 5.6 and 13.0 years old. Each subject received a neurolinguistic battery of tests and a high resolution volumetric MRI scan. Major results were that (a) pars triangularis was significantly smaller in the left hemisphere of children with SLI, and (b) children with SLI were more likely to have rightward asymmetry of language structures. Furthermore, anomalous morphology in these language areas correlated with depressed language ability. These findings support the hypothesis that language impairment is a consequence of an underlying neurobiological defect in areas of the brain known to subserve language.