Cerebral mechanisms involved in word reading in dyslexic children: a magnetic source imaging approach.
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The purpose of the present investigation was to describe spatiotemporal brain activation profiles during word reading using magnetic source imaging (MSI). Ten right-handed dyslexic children with severe phonological decoding problems and eight age-matched non-impaired readers were tested in two recognition tasks, one involving spoken and the other printed words. Dyslexic children's activation profiles during the printed word recognition task consistently featured activation of the left basal temporal cortices followed by activation of the right temporoparietal areas (including the angular gyrus). Non-impaired readers showed predominant activation of left basal followed by left temporoparietal activation. In addition, we were able to rule out the hypothesis that hypoactivation of left temporoparietal areas in dyslexics was due to a more general cerebral dysfunction in these areas. Rather, it seems likely that reading difficulties in developmental dyslexia are associated with an aberrant pattern of functional connectivity between brain areas normally involved in reading, namely ventral visual association cortex and temporoparietal areas in the left hemisphere. The interindividual consistency of activation profiles characteristic of children with dyslexia underlines the potential utility of this technique for examining neurophysiological changes in response to specific educational intervention approaches. (+info)
The neurological basis of developmental dyslexia: an overview and working hypothesis.
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Five to ten per cent of school-age children fail to learn to read in spite of normal intelligence, adequate environment and educational opportunities. Thus defined, developmental dyslexia (hereafter referred to as dyslexia) is usually considered of constitutional origin, but its actual mechanisms are still mysterious and currently remain the subject of intense research endeavour in various neuroscientific areas and along several theoretical frameworks. This article reviews evidence accumulated to date that favours a dysfunction of neural systems known to participate in the normal acquisition and achievement of reading and other related cognitive functions. Historically, the first arguments for a neurological basis of dyslexia came from neuropathological studies of brains from dyslexic individuals. These early studies, although open to criticism, for the first time drew attention towards a possible abnormality in specific stages of prenatal maturation of the cerebral cortex and suggested a role of atypical development of brain asymmetries. This has prompted a large amount of subsequent work using in vivo imaging methods in the same vein. These latter studies, however, have yielded less clear-cut results than expected, but have globally confirmed some subtle differences in brain anatomy whose exact significance is still under investigation. Neuropsychological studies have provided considerable evidence that the main mechanism leading to these children's learning difficulties is phonological in nature, namely a basic defect in segmenting and manipulating the phoneme constituents of speech. A case has also been made for impairment in brain visual mechanisms of reading as a possible contributing factor. This approach has led to an important conceptual advance with the suggestion of a specific involvement of one subsystem of vision pathways (the so-called magnosystem hypothesis). Both phonological and visual hypotheses have received valuable contribution from modern functional imaging techniques. Results of recent PET and functional MRI studies are reported here in some detail. Finally, one attractive interpretation of available evidence points to dyslexia as a multi-system deficit possibly based on a fundamental incapacity of the brain in performing tasks requiring processing of brief stimuli in rapid temporal succession. It is proposed that this so-called 'temporal processing impairment' theory of dyslexia could also account for at least some of the perceptual, motor and cognitive symptoms very often associated with the learning disorder, a coincidence that has remained unexplained so far. (+info)
Down syndrome and the phonological loop: the evidence for, and importance of, a specific verbal short-term memory deficit.
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Individuals with Down syndrome are thought to perform poorly on tests of verbal short-term memory, such as measures of word span or digit span. This review critically examines the evidence for a specific deficit in verbal short-term memory in Down syndrome, and outlines a range of possible explanations for such a deficit. The potential implications of a verbal short-term memory impairment for broader aspects of development are outlined, in particular with respect to vocabulary development. Possible intervention strategies, which might improve verbal short-term memory performance in Down syndrome are also considered. However, we argue that further research is needed to fully clarify the nature of a verbal short-term memory deficit in Down syndrome, before the merits of these various intervention approaches can be properly evaluated. (+info)
Functional MRI of phonological and semantic processing in temporal lobe epilepsy.
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Phonological and semantic aspects of language were examined in patients with unilateral temporal lobe epilepsy (TLE) and healthy controls using functional MRI. We expected to replicate previous findings in healthy individuals showing relatively greater activation in frontal regions for phonological compared with semantic processing, and greater activation in temporal regions for semantic compared with phonological processing. We hypothesized that differences between patients with left TLE and healthy controls would be found in the pattern of left temporal cortical activation associated specifically with semantic processing. Patients with right TLE were included as a seizure control group. All TLE patients previously showed left hemisphere language dominance on intracarotid sodium amytal studies. Greater blood oxygen level dependent activation was found during phonological processing compared with semantic processing in frontal regions for healthy participants but, contrary to expectation, semantic processing did not lead to increased temporal lobe activity relative to phonological processing. Furthermore, no differences between left temporal patients and controls were found specifically in left temporal cortex. Rather, patients with left temporal seizure foci showed significantly greater left dorsolateral prefrontal activity compared with controls, as well as increased signal change in left inferior frontal and right middle temporal gyrus. Surprisingly, patients with right, but not left, TLE showed poorer performance on the linguistic tasks compared with controls, as well as a decrease in right superior temporal activation. The results converge with studies of dyslexic patients showing increased left frontal activity in the presence of left temporal dysfunction and are suggestive of both inter- and intra-hemispheric functional reorganization of language representation in left TLE. (+info)
Dysarthria as the isolated clinical symptom of borreliosis--a case report.
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This report presents a case of dysarthria due to hypoglossal nerve mono-neuropathy as the only consequence of neuroborreliosis. The 65-year-old man with a seven-months history of articulation disturbances was examined. The speech of the patient was slow and laboured. A slight weakness of the muscles of the tongue (left-side) was observed. The patient suffered from meningitis due to Borrelia burgdorferi infection in 1999 and initially underwent a successful antibiotic treatment. Detailed radiological investigation and psychological tests were performed and co-existing neurological diseases were excluded. To describe profile of speech abnormalities the dysarthria scale was designed based on S. J. Robertson Dysarthria Profile. There were a few disturbances found in self-assessment of speech, intelligibility, articulation, and prosody but especially in the morphology of the articulation muscles, diadochokinesis, the reflexes (in the mouth, larynx and pharynx). Needle EMG examination confirmed the diagnosis of mono-neuropathy of left hypoglossal nerve. The study confirms the fact that neuroborreliosis may evoke chronic consequences. (+info)
Refractory dyslexia: evidence of multiple task-specific phonological output stores.
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We investigated the case of a patient whose reading was characterized by multiple phonemic paraphasic errors. An error analysis of a large corpus of reading responses (758 words, 86 non-words) highlighted the preponderance of phonological errors which did not occur in his naming, repetition or spontaneous speech. His comprehension of the written word was relatively preserved, even for words he was unable to read aloud. We suggest that his impairment lies at the level of the phonological output store. We also demonstrate that his reading performance was facilitated by increasing the response-stimulus delay. The strong influence of temporal factors is shown to be task-specific. Two main points are drawn from our results. First, we argue that our patient can be characterized as having a refractory access type of deficit; to our knowledge, no previous case of a refractory deficit affecting word reading has been reported. Secondly, the task specificity of both the phonological error pattern and the sensitivity to temporal factors is difficult to reconcile with the idea of a unitary phonological output store. Contrary to orthodox neuropsychological models, we propose that there are independent stores specific for reading and spoken output. (+info)
Neuropsychological and phonological evaluation in the Apert's syndrome: study of two cases.
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This study evaluated two cases of Apert's syndrome, through phonological, cognitive, and neuropsychological instruments and correlated the results to complementary exams. In short, this study reveals the necessity of application of neuropsychological, cognitive and phonological evaluation and correlation of the results with complementary testings because significant differences can be present in the Apert's syndrome. (+info)
Amplitude envelope onsets and developmental dyslexia: A new hypothesis.
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A core difficulty in developmental dyslexia is the accurate specification and neural representation of speech. We argue that a likely perceptual cause of this difficulty is a deficit in the perceptual experience of rhythmic timing. Speech rhythm is one of the earliest cues used by infants to discriminate syllables and is determined principally by the acoustic structure of amplitude modulation at relatively low rates in the signal. We show significant differences between dyslexic and normally reading children, and between young early readers and normal developers, in amplitude envelope onset detection. We further show that individual differences in sensitivity to the shape of amplitude modulation account for 25% of the variance in reading and spelling acquisition even after controlling for individual differences in age, nonverbal IQ, and vocabulary. A possible causal explanation dependent on perceptual-center detection and the onset-rime representation of syllables is discussed. (+info)