Abstract
Kognitive Subtypen von Entwicklungsdyslexie können Defizite vor allem in der Phonologie oder der visuell-räumlichen Aufmerksamkeit haben, die mit hirnfunktionellen Unterschieden zu Normallesern einhergehen. Die subtypenspezifische Aktivierungsänderung durch ein visuelles Aufmerksamkeitstraining wurde bisher noch nicht untersucht.
Die vorliegende fMRT-Studie untersucht den Einfluss von defizitspezifischen Trainings vs. Lesetrainings auf die neuronalen Korrelate dyslektischer Subtypen verglichen wiederum mit Normallesern. Dritt- und Viertklässler wurden psychometrischen Tests unterzogen, u. a. zu phonologischen und Aufmerksamkeitsfähigkeiten. Alle Dyslektiker erhielten ein vierwöchiges Training: Dyslektiker mit primär phonologischem Defizit ein rein phonologisches Training, Kinder mit primärem Aufmerksamkeitsdefizit ein Aufmerksamkeitstraining. Ein reines Lesetraining erhielt eine dritte Dyslexiegruppe: Dyslektiker unabhängig von ihrem kognitiven Defizit. Mittels fMRT wurde vor und nach dem Training das Posner Paradigma zur visuell-räumlichen Aufmerksamkeitsausrichtung mit den drei Trainingsgruppen und einer vierten Gruppe, den Normallesern, durchgeführt, um zu untersuchen, wie sich das Training auf die neurofunktionelle Verarbeitung der drei Dyslexiegruppen auswirkt. Die fMRT-Daten ergaben für alle Trainingsgruppen eine signifikante Veränderung im linken inferioren Frontalcortex. Das Maximum dieser Aktivierungsveränderung lag jedoch jeweils an verschiedenen Stellen innerhalb dieser Region: für die Trainingsgruppe Phonologie auf dem GFI in der Broca-Region, für die Trainingsgruppen Aufmerksamkeit und Lesen jeweils im Sulcus frontalis inferior. Die drei Gruppen verarbeiten möglicherweise verschiedene Funktionen in dieser sehr heterogenen Region. Das linkshemisphärische Aktivierungsmuster könnte somit auf unterschiedliche Kompensation nach den verschiedenen Dyslexie-Trainings hinweisen, die nicht in klassischen rechts-hemisphärischen Aufmerksamkeitsarealen, sondern im linkshemispärischen Sprachnetzwerk lokalisiert sind.
Background: Developmental dyslexia is a reading disorder which occurs despite normal intelligence and adequate schooling. Up to 17.5 % of all children are affected. Many causes and theories for dyslexia have been discussed, leading to the notion of distinct cognitive subtypes of dyslexia: Whereas some children have deficits in phonology, others show difficulties in attentional abilities. Furthermore, these two subtypes show brain function differences in comparison to normal readers. The right inferior frontal gyrus (IFG) and the right parieto-occipito-temporal gyrus have a significantly higher activation in dyslexic children. In current literature these two areas are linked to visuospatial processing in normal readers. So far, the subtype specific brain activation of visuospatial attention was not investigated; dyslexics were only considered as one homogenic group. Moreover, there is no evidence as yet on how brain activation changes in these two subtypes after a specific training linked to the primary deficit.
Aims: This fMRI study investigated the effect of different dyslexia trainings on the brain activation concerning the phonological and the visuospatial subtypes of dyslexia. Before scanning, all children completed psychometric tests of phonological and attentional abilities. Next, the dyslexic children underwent a 4-week dyslexic training. Three types of trainings were possible. First, dyslexics with prime phonological deficit received a phonology-based training. Second, dyslexic children whose primary issue was an attentional deficit took part in an attention-based training. Third, children with both kinds of deficits underwent a pure reading training. The aim of the study was to analyse how the various trainings affect brain activations of the three groups while conducting the Posner paradigm.
Methods: A total of 22 dyslexic children and 10 normal readers (mean age: 9;7 years) from German primary schools participated in the study. Visual attention was assessed with the Posner paradigm. Subjects were asked to indicate – by clicking a left or right mouse button as quickly as possible – which of two positions a target stimulus occurred in the left and right periphery of the computer screen. A target stimulus could be preceded by a cue which was either informative (valid) or misleading (invalid). In the case of a valid cue (80 %) the participant could correctly prepare a reaction. Where an invalid cue was given (20 %) attention to the shadowed position had to be changed to the correct position of the target in order to perform the required reaction. The “cue validity effect” which is the reaction time difference between invalid and valid trials is taken to reflect how quickly attention can be shifted to a new location. Smaller effects indicate better performance.
The fMRI experiment was carried out on a 3 T Siemens scanner (Siemens, Erlangen, Germany) at Universitätsklinikum Aachen. 148 echo-planar images (repetition time 2500 ms; field of view 200 mm; flip angle 90°; echo time 30 ms) were acquired from 40 sagittal slices covering the entire brain (slice thickness 3 mm; gap 1 mm). Data analysis was performed using SPM5 running on MATLAB 7.
Results: The behavioural data showed a main effect for the cue validity effect but no differences between the groups or before and after the training. The fMRI data revealed a significant change for all trainings in the left inferior frontal gyrus. Inspecting the maximum of this general change established that different areas within the left inferior frontal gyrus are affected. The phonological training group showed its main activation on the left inferior frontal gyrus in Broca's region (BA 45). The two other training groups exhibited their maximum at the inferior frontal sulcus extending into the inferior frontal junction (IFJ).
Discussion: While the maxima of the three groups lay in different regions within the left IFG, it might be possible that these three groups process visuospatial attention in different areas within this heterogeneous area. BA 45 is normally linked to phonological processing. The inferior frontal sulcus forms the border to inferior frontal junction, which is a functionally distinct region. Abilities such as fixing the location of attention or shifting the attention are located in this area. Recent literature suggests that the right homologue to the left frontal cortex is considered for visuospatial attention. The contralaterally activation existing in this study could be a pattern of compensation following training. Moreover, other studies also displayed left frontal effects after phonological training in dyslexic children. Thus, the left IFG seems to be specific for training effects in dyslexia.
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