The visual deficit theory of developmental dyslexia

Brain imaging in neurobehavioral disorders

Neuroimaging studies of neurobehavioral disorders are using new imaging modalities. In dyslexia, anatomic imaging studies demonstrate an abnormal symmetry of the planum temporale. Functional imaging supports the hypothesis that developmental dyslexia is frequently the result of deficits in phonologic processing and that normal reading requires a patent network organization of a number of anterior and posterior brain areas. In autism, anatomic imaging studies are conflicting. Functional imaging demonstrates temporal lobe abnormalities and abnormal interaction between frontal and parietal brain areas. In attention-deficit-hyperactivity disorder, imaging studies suggest an abnormality in the prefrontal and striatal regions. Neuroimaging studies are often contradictory, but trends, especially with functional imaging analysis, are evolving. Because neurobehavioral disorders seem to be a result of a dysfunction in brain circuits, no one region will be abnormal in all patients studied. Further studies with well-defined patient populations and appropriate activation paradigms will better elucidate the pathophysiology of these conditions.

Developmental dyslexia: the cerebellar deficit hypothesis

Surprisingly, the problems faced by many dyslexic children are by no means confined to reading and spelling. There appears to be a general impairment in the ability to perform skills automatically, an ability thought to be dependent upon the cerebellum. Specific behavioural and neuroimaging tests reviewed here indicate that dyslexia is indeed associated with cerebellar impairment in about 80% of cases. We propose that disorders of cerebellar development can in fact cause the impairments in reading and writing characteristic of dyslexia, a view consistent with the recently appreciated role of the cerebellum in language-related skills. This proposal has implications for early remedial treatment.

Aktuelle Forschung auf der Suche nach neurobiologischen Korrelaten der Lese-Rechtschreib-Störung

Zusammenfassung: Fragestellung: Im vorliegenden Übersichtsartikel werden Schwerpunkte der aktuellen Forschung zu neurobiologischen Korrelaten der Lese-Rechtschreibstörung (LRS) aufgezeigt. Methodik: Es wurden etwa 120 Artikel ausgewertet, die überwiegend in den letzten sechs Jahren erschienen sind. Ergebnis: Es lassen sich drei Schwerpunkte unterscheiden. Der erste Forschungsschwerpunkt thematisiert ungewöhnliche Asymmetrien der Plana temporalia und Auffälligkeiten des Corpus callosum. Die Vertreter des zweiten Ansatzes analysieren abnorme Aktivierungsmuster des linkshemisphärischen Frontal- und Temporallappens. In diesem Zusammenhang wird eine Dysfunktion des linkshemisphärischen Gyrus angularis diskutiert. Die Vertreter der dritten Störung untersuchen Auffälligkeiten im Bereich der Wahrnehmung. Sie vermuten ein globales Defizit der Verarbeitung schneller Reize und finden Korrelate der LRS in Defiziten subkortikaler Systeme der akustischen und visuellen Informationsverarbeitung. Schlussfolgerungen: Aufgrund wiederholter Negativbefunde erscheint der erste Ansatz als zu unspezifisch. Forschungsbemühungen konzentrieren sich zunehmend auf den zweiten und dritten Schwerpunkt.

Temporal processing deficits in remediation-resistant reading-impaired children

There is considerable interest in whether a deficit in temporal processing underlies specific learning and language disabilities in school-aged children. This view is particularly controversial in the area of developmental reading problems. The temporal-processing hypothesis was tested in a sample of normal children, 9-11 years of age, and in a sample of age-matched children with reading impairments, by assessing temporal-order discrimination. Five different binary temporal-order tasks were evaluated in the auditory and visual sensory modalities. Other basic discrimination abilities for single auditory stimuli were also assessed, including just noticeable differences (JNDs) for frequency and intensity and a simple threshold detection task. In these tasks, the temporal dimension was the duration of the individual stimuli (20 and 200 ms). All data were obtained using forced- choice psychophysical methods, either in a single-track adaptive format or using psychometric functions. The results from these experiments showed that children with reading impairments had deficits in temporal-order discrimination, but these effects were not modality specific. These same children also had significantly elevated frequency and intensity JNDs and their performance on these tasks were not dependent on stimulus duration. No group differences were observed on the threshold detection task, and the derived measurements of temporal integration (i.e. the threshold difference between the 20- and 200-ms stimuli) were considered normal, averaging 11.7 dB. As a whole, discrimination deficits observed in the reading-impaired group only occurred with suprathreshold stimuli. The deficits were neither modality specific nor temporal (duration) specific.

Microstructure of temporo-parietal white matter as a basis for reading ability: evidence from diffusion tensor magnetic resonance imaging

Diffusion tensor magnetic resonance imaging (MRI) was used to study the microstructural integrity of white matter in adults with poor or normal reading ability. Subjects with reading difficulty exhibited decreased diffusion anisotropy bilaterally in temporoparietal white matter. Axons in these regions were predominantly anterior-posterior in direction. No differences in T1-weighted MRI signal were found between poor readers and control subjects, demonstrating specificity of the group difference to the microstructural characteristics measured by diffusion tensor imaging (DTI). White matter diffusion anisotropy in the temporo-parietal region of the left hemisphere was significantly correlated with reading scores within the reading-impaired adults and within the control group. The anisotropy reflects microstructure of white matter tracts, which may contribute to reading ability by determining the strength of communication between cortical areas involved in visual, auditory, and language processing.

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.