Right frontotemporal activation by tonal memory in dyslexia, an O15 PET Study

Word frequency effects in the left IFG in dyslexic and normally reading children during picture naming and reading

Word frequency effects have been reported in numerous neuroimaging studies with typically reading adults, emphasising the role of the left inferior frontal gyrus (LIFG). Within LIFG, different cytoarchitectonic modules (areas 44 and 45) have been related to phonological vs. lexico-semantic processing, respectively. This fMRI study investigated the differential impact of word frequency on LIFG activation in reading and picture naming in primary school children with and without developmental dyslexia. All children showed the typical LIFG frequency effect in both tasks. The effect was comparable in a fronto-orbital region anterior-inferior adjacent to area 45. During reading but not picture naming, a second effect was observed in area 44. Here, the fMRI effect for lexical frequency was stronger for the dyslexic than the normal readers. These findings demonstrate the neural underpinnings of a selective deficit in dyslexic children in the graphemic input lexicon, whereas abstract lexical representations appear to be processed equally well in dyslexic and normally reading children. To conclude, the present fMRI study demonstrated differential impact of word frequency on LIFG activation in primary school children during reading but not picture naming. Apart from extending previous knowledge from studies with adults to childhood, the study sheds further light on a potential neural mechanism for deficient grapheme-to-phoneme conversion in dyslexic children.

Neurokognitive Korrelate der Dyslexie

Die Überblicksarbeit widmet sich kognitiven und neuronalen Grundlagen der Dyslexie. Ausgehend von einer Darstellung der wichtigsten kognitiven und neurobiologischen Theorien der Entstehung von Dyslexie werden Ergebnisse zu spezifischen Störungen des neuronalen Lesenetzwerks bei Menschen mit Dyslexie aus Postmortem-Untersuchungen und strukturellen sowie funktionellen Bildgebungsstudien berichtet. Die Befunde legen nahe, dass Dyslexie ein multidimensionales Problem darstellt, das mit verschiedenen kognitiven, sensorischen und motorischen Defiziten und spezifischen Störungen auf neuronaler Ebene einhergeht. Zukünftige Forschung sollte sich daher verstärkt individuellen Profilen der Störung auf kognitiver wie neuronaler Ebene widmen.

Atypical brain development: a conceptual framework for understanding developmental learning disabilities

This article presents ideas that are, in part, a response to the ambiguity in the neurological research on learning disorders, the growing awareness that developmental disabilities are typically nonspecific and heterogeneous, and the growing scientific literature showing that comorbidity of symptoms and syndromes is the rule rather than the exception. This article proposes the term atypical brain development (ABD) as a unifying concept to assist researchers and educators trying to come to terms with these dilemmas. ABD is meant to serve as an integrative concept of etiology, the expression of which is variable within and across individuals. ABD does not itself represent a specific disorder or disease. It is a term that can be used to address the full range of developmental disorders that are found to be overlapping much of the time in any sample of children. Although similar in spirit to the older term of minimal brain dysfunction (MBD), in that it closely links neurology with behavioral difficulties, ABD as proposed here differs in several ways. In support of the ABD conceptual framework, first, we consider the ABD concept in terms of its superiority to the older notion of MBD. Second, we provide a brief review of the burgeoning literature on the overlap of the various developmental disabilities. Third, we review some of the scientific literature that supports the ABD concept. Our sole purpose in proposing this concept is to initiate dialogue and debate on several critical issues across a wide variety disciplines. Hence, this article is not intended to be a definitive statement of a rigid perspective. It reflects neither a nonmalleable philosophical position, nor any type of condemnation of other perspectives. It does, however, reflect a data-based and philosophical trend visible in the field of learning disabilities, as well as the broader area of childhood developmental disorders.

Phonological processing in dyslexic children: a study combining functional imaging and event related potentials

Difficulties in phonological processing are currently considered one of the major causes for dyslexia. Nine dyslexic children and eight control children were investigated using functional magnetic resonance imaging (fMRI) during non-oral reading of German words. All subjects silently read words and pronounceable non-words in an event related potentials (ERP) investigation, as well. The fMRI showed a significant difference in the activation in the left inferior frontal gyrus between the dyslexic and control groups, resulting from a hyperactivation in the dyslexics. The ERP scalp distribution showed a significant distinction between the two groups concerning the topographic difference for left frontal electrodes in a time window 250-600 ms after stimulus onset for non-word reading. Both the fMRI and the ERP results support differences in phonological processing between dyslexic and normal-reading children.

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.

Abnormal functional activation during a simple word repetition task: A PET study of adult dyslexics

Eight dyslexic subjects, impaired on a range of tasks requiring phonological processing, were matched for age and general ability with six control subjects. Participants were scanned using positron emission tomography (PET) during three conditions: repeating real words, repeating pseudowords, and rest. In both groups, speech repetition relative to rest elicited widespread bilateral activation in areas associated with auditory processing of speech; there were no significant differences between words and pseudowords. However, irrespective of word type, the dyslexic group showed less activation than the control group in the right superior temporal and right post-central gyri and also in the left cerebellum. Notably, the right anterior superior temporal cortex (Brodmann's area 22 [BA 22]) was less activated in each of the eight dyslexic subjects, compared to each of the six control subjects. This deficit appears to be specific to auditory repetition as it was not detected in a previous study of reading which used the same sets of stimuli (Brunswick, N., McCrory, E., Price, C., Frith, C.D., & Frith, U. [1999]. Explicit and implicit processing of words and pseudowords by adult developmental dyslexics: A search for Wernicke's Wortschatz? Brain, 122, 1901-1917). This implies that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural). Other studies of normal subjects indicate that attending to the phonetic structure of speech leads to a decrease in right-hemisphere processing. Lower right hemisphere activation in the dyslexic group may therefore indicate less processing of non-phonetic aspects of speech, allowing greater salience to be accorded to phonological aspects of attended speech.

Morphological alteration of temporal lobe gray matter in dyslexia: an MRI study

Functional imaging studies of developmental dyslexia have reported reduced task-related neural activity in the temporal and inferior parietal cortices. To examine the possible contribution of subtle anatomic deviations to these reductions, volumes were measured for the major lobes of the brain, the subcortical nuclei, cerebellum, and lateral ventricles on magnetic resonance imaging (MRI) scans from 16 right-handed dyslexic men, ages 18 to 40, and 14 matched controls, most of whom had previously undergone PET imaging. A specific decrease in tissue volume was localized to the temporal lobes and was particularly prominent on the left (p < .01). An analysis of tissue composition revealed that this reduction was primarily attributable to decreased gray matter within the left temporal lobe (p < .002). Further segmentation of the temporal lobe showed that this reduction was not confined to the superior temporal gyrus, the primary location of primary auditory cortex. Reductions of temporal lobe gray matter may reflect a regional decrease in neuronal number or neuropil, which in turn may result in reading impairment.

Explicit and implicit processing of words and pseudowords by adult developmental dyslexics: A search for Wernicke's Wortschatz?

Two groups of male university students who had been diagnosed as dyslexic when younger, and two groups of control subjects of similar age and IQ to the dyslexics, were scanned whilst reading aloud and during a task where reading was implicit. The dyslexics performed less well than their peers on a range of literacy tasks and were strikingly impaired on phonological tasks. In the reading aloud experiment, simple words and pseudowords were presented at a slow pace so that reading accuracy was equal for dyslexics and controls. Relative to rest, both normal and dyslexic groups activated the same peri- and extra-sylvian regions of the left hemisphere that are known to be involved in reading. However, the dyslexic readers showed less activation than controls in the left posterior inferior temporal cortex [Brodmann area (BA) 37, or Wernicke's Wortschatz], left cerebellum, left thalamus and medial extrastriate cortex. In the implicit reading experiment, word and pseudoword processing was contrasted to visually matched false fonts while subjects performed a feature detection paradigm. The dyslexic readers showed reduced activation in BA 37 relative to normals suggesting that this group difference, seen in both experiments, resides in highly automated aspects of the reading process. Since BA 37 has been implicated previously in modality-independent naming, the reduced activation may indicate a specific impairment in lexical retrieval. Interestingly, during the reading aloud experiment only, there was increased activation for the dyslexics relative to the controls in a pre-motor region of Broca's area (BA 6/44). We attribute this result to the enforced use of an effortful compensatory strategy involving sublexical assembly of articulatory routines. The results confirm previous findings that dyslexic readers process written stimuli atypically, based on abnormal functioning of the left hemisphere reading system. More specifically, we localize this deficit to the neural system underlying lexical retrieval.

Dyslexia, gender, and brain imaging

Future brain imaging studies of dyslexia should have a sufficient number of males and females to detect possible gender differences in the neurological underpinning of this disorder. Detailed knowledge about such differences may clarify our understanding of the structural and functional impairments which lead to the phonological deficits that characterize dyslexia. Functional brain imaging studies have shown that males and females exhibit different patterns of brain activation during phonological processing. Further differences between the brains of males and females have been suggested by studies of normal brain development, morphology, and functional activation during reading. Animal studies have shown that lesions, similar to those seen in postmortem studies of dyslexia, affect rapid auditory processing in males, but not in females. The large body of research on gender differences in brain development, functional organization, and activation during reading tasks urges separation of males and females in dyslexia research in order to minimize variance and to detect subtle, but functionally-relevant, differences. Well-controlled studies, with large numbers of male and female dyslexics, may produce more sensitive and accurate identification of the neurological substrates of dyslexia.

The planum temporale: a systematic, quantitative review of its structural, functional and clinical significance

The planum temporale (PT) is a triangular area situated on the superior temporal gyrus (STG), which has enjoyed a resurgence of interest across several disciplines, including neurology, psychiatry and psychology. Traditionally, the planum is thought to be larger on the left side of the brain in the majority of normal subjects [N. Geschwind, W. Levitsky, Human brain: left-right asymmetries in temporal speech regions, Science 161 (1968) 186-87.]. It coincides with part of Wernicke's area and it is believed to consist cytoarchitectonically of secondary auditory cortex. Consequently, it has long been thought to be intimately involved in language function. The PT is, therefore, of relevance to disorders where language function is impaired, such as schizophrenia and dyslexia. The gross anatomical boundaries remain in dispute, and only recently has its cytoarchitecture begun to be studied again after 60 years silence, and finally its functional significance is only now being explored. In the first part of this review the structural aspects and anatomical boundaries of the PT in the normal brain from post mortem and magnetic resonance imaging (MRI) and methods of measurement are discussed. In the second part, studies of the functional significance of the PT in the normal brain are reviewed critically. Finally a meta-analysis of MRI measurements of the distribution of planum anatomy in normal subjects is presented. Comparison is made with clinical populations, including schizophrenia and dyslexia, and the influence of handedness and gender on such measurements is quantified. Although there are many ways of defining and measuring the PT with a wide variety of results, overall there is a significant leftward asymmetry in normals, which is reduced in left handers and females. The leftward asymmetry is much reduced in patients with schizophrenia due to a relatively larger right PT than normal controls. The review is intended to guide future researchers in this area.

Corpus callosum morphology, as measured with MRI, in dyslexic men

To test the hypothesis of anomalous anatomy in posterior brain regions associated with language and reading, the corpus callosum was imaged in the midsagittal plane with magnetic resonance. The areas of the anterior, middle, and posterior segments were measured in 21 dyslexic men (mean age 27 yrs, SD 6) and in 19 matched controls. As predicted, the area of the posterior third of the corpus callosum, roughly equivalent to the isthmus and splenium, was larger in dyslexic men than in controls. No differences were seen in the anterior or middle corpus callosum. The increased area of the posterior corpus callosum may reflect anatomical variation associated with deficient lateralization of function in posterior language regions of the cortex and their right-sided homologues, hypothesized to differ in patients with dyslexia.