Word Reading Processing In Children: A Functional MRI Study At 1.5 T

摘要

Functional MRI is a tool for investigating brain function non-invasively. Our study addresses the question whether functional MRI can be used in children to differentiate activated brain areas in phonological tasks. A set of hierarchically structured reading tasks was used in 17 healthy right-handed children (median 14y 1m). FMRI was acquired during perception of letter strings, silent reading of non-words, silent reading of words, and during performance of a task with phonological transformation using a T2* weighted GRE sequence with three slices (TR/TE/α 68ms/50ms/10°). Postprocessing was done using SPM99. Activation was observed only in Broca's area while silent reading of real words. Phonological strategies (non-word reading and transformation task) resulted in activation in the frontal and temporal regions, as well as bilaterally in Broca`s area in the inferior frontal gyrus. Conclusion: The observed activation patterns in children are different to reported areas in adults. This could be caused by the stronger use of the grapheme-to-phoneme mapping as a serial process in children in contrast to adults. Introduction Until recently, studies of brain function in children have been limited to indirect techniques like electroencephalography (EEG) and event-related potentials, which provide information regarding the timing of sensory and cognitive events. Other brain imaging techniques, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) are available, but these techniques require exposure to radiation. Consequently, these neuroimaging methods are limited in their use in pediatrics. In recent years, functional magnetic resonance imaging (fMRI) has been introduced to developmental research. The principle of this noninvasive technique is based on the so called BOLD-effect (blood oxygenation level dependent effect) which is caused by the different behavior of oxygenated and desoxygenated hemoglobin (HbO2 and deHbO2) in a magnetic field [1]. Neuronal activation causes an increase in regional blood flow and blood volume which in turn overcompensates for the increase in oxygen use. The result is a locally decreased concentration of paramagnetic deHbO2 at the capillary venous level and, therefore, a smaller susceptibility difference between venous blood and brain parenchyma, which leads to a local signal intensity increase on T2*-weighted sequences (gradient echo conventional sequences or echo planar imaging)[2]. Depending on the field of view, an in-plane resolution of up to 1mm is possible, which overcomes the restricted spatial resolution of SPECT or PET. Noninvasiveness, independence from any exogenous tracer, relatively high spatial and temporal resolution, and the possibility to acquire simultaneously activation and topographic data are the main advantages of functional MRI in comparison to other neuroimaging methods. Functional MRI may contribute to a better understanding of functional anatomy of reading in the intact human brain and its development. To date, only few pediatric functional MRI studies have been published [3].Former functional imaging studies in adults using PET and fMRI have demonstrated that different brain areas are involved in reading processes [4,5,6,7,8,9,10,11,12,13,14]. These studies have also improved the knowledge about the functional specialisation of cortical regions that subserve different aspects of word processing in normal reading adults. With respect to pediatrics the functional mapping of language prior to neurosurgery was studied [15]. In a comparative fMRI study of dyslexic children and adolescents restricted activation of brain regions during various language tasks could be shown compared to normal reading adults [16]. Significantly less activity in the inferior frontal gyrus and the supramarginal gyrus in the dyslexic group could be observed in a comparison of dyslexic adolescents and age-matched controls using a phoneme discrimination task [17].