Less developed corpus callosum in dyslexic subjects—a structural MRI study
Introduction
Recent genetic, brain morphology and functional imaging studies (see [1] for review) have shed light on the neurodevelopmental origin of dyslexia. A biological basis is suggested with subsequently affected neurocognitive processes. Due to the important role of interhemispheric transmission of visual and auditory information in reading, several studies have hinted towards the role of the corpus callosum (CC) and interhemispheric transfer of information in dyslexia [2], [3], see also [4] for a review. Case studies have moreover indicated that patients with callosal agenesis are more impaired in phonological reading [5]. Finally, Summerfield and Michie [6] reported that interhemispheric transfer and sequential processing of dyslexic 9-year-old children was comparable to or worse than the performance of 7-year-old control children.
The slow anterior to posterior myelination of fibres in the CC during childhood suggests an ongoing developmental process to establish efficient communication between the hemispheres [7], [8]. This maturation is consistent with the increment of complex neuropsychological abilities and maturation of the corresponding cortical areas. Light-microscopic examination of the CC has revealed a consistent pattern of regional differentiation of the fibre types in the CC, with large diameter fibres showing a peak of density in the posterior midbody [9]. Fibres in this area connect the primary and secondary auditory cortical areas [10]. Thompson et al. [11] investigated growth patterns in the developing brain by warping algorithms, and followed the CC development of the same individuals from 7 to 11 years of age. They found a striking regional growth throughout the CC with peak values occurring in this area (isthmus).
Several studies have looked for structural CC abnormalities in dyslexic subjects, however with inconsistent results. A look at these studies indicates wide variations in the subjects’ age, sex, handedness, and with respect to study setting (for review see [4], [12]). Most of the studies also only investigated size differences of the CC. For example, Larsen et al. [13] failed to detect structural differences in the CC of adolescent dyslexics and controls. Similarly, Pennington et al. [14] analysed the brain morphology of 75 dyslexic adolescent twins and controls matched for gender, handedness and IQ. No abnormality, either for overall CC size or for subregions of the CC was found in the dyslexic group.
Njiokiktjien et al. [15], on the other hand, compared children with developmental dysphasia and dyslexia to a heterogeneous clinical group of children with learning disabilities. They found that dyslexic subjects with a hereditary disposition had an abnormally large CC compared to non-familial cases. Hynd et al. [16], however, found a smaller genu region in the dyslexic children compared with the control subjects. Finally, Duara et al. [17] and Rumsey et al. [18] reported enlarged posterior part of the CC in adult dyslexics.
To our knowledge, only Robichon and Habib [19] compared the size and explicitly also the shape of the CC in a group of adult dyslexics and a control group. An interesting finding was that the two groups differed with respect to the shape of the CC, with the dyslexic subjects exhibiting a more circular and rounded callosal shape than the control subjects. See also Robichon et al. [20] who reanalysed the previous data with a new angulation technique that permits the analysis of CC position in the brain, and found a lower situated CC in dyslexics.
Studies also have differed with regard to the applied MR technique as well as analysis method (e.g. slice thickness and determination of the midsagittal slice for CC measurement) [21]. No anatomical landmarks show where fibres from the corresponding cortical areas cross in the midsagittal view of the CC. Therefore, methods for subdivision of the CC have varied between studies.
Due to the inconsistent findings in previous studies, we wanted to compare both shape and size in a population-based sample of 20 right handed dyslexic boys and 20 controls from the same school classes as the dyslexic subjects, with a CC manual contour tracing technique and a new shape analysis method. Thus, the groups were comparable with respect to age, gender, handedness, factors known to influence CC development [22], [23], [24]. Since dyslexia may be considered a phonological decoding disorder [25], we hypothesised that the region of the CC containing the auditory fibres, namely the posterior midbody/isthmus region, would be of particular interest. Fibers from the language areas in the superior temporal gyrus (Wernickes’ area, planum temporale) pass through the isthmus area [26].
In order to explore differences in CC fibre growth patterns we compared the representative (average) CC shapes of the dyslexic and the control group, respectively. The shape prototypes were aligned with respect to the similarity group of planar transformations (scaling, rotations and translations) in a Procrustes analysis. Such an approach has been used in other CC shape comparison studies [27], [28]. Constructing of shape prototypes has attracted considerable interest [29], [30], but practical methods for computing such prototypes are somewhat scarce. This is mainly due to the fact that the statistical shape theory [29], [30] can only be applied to sets of points of equal cardinality between which point correspondences have been established. However, in most instances, the data consist of a set of contours with different point counts and no known point correspondences. We used a new method for automatic shape alignment and prototype computation that has been developed by Duta et al. [31], [32] which overcomes the need for manual point correspondences. This allows to draw conclusions regarding latent morphological features of the CC in a sample of CC contours in shape space [33].
Section snippets
Subjects
The children were recruited from the 12 largest elementary schools in the city of Bergen1 [34], [35]. Informed consent was obtained from children and parents for each successive stage of the screening procedure according to the declaration of Helsinki and the regional ethical committee had approved this study before its start. All children were in the
Area measurements
Inter-rater reliability for the CC perimeter measurement was 0.92. The intra-rater reliability was 0.93 and 0.94.
There were no statistically significant differences between the groups with respect to overall CC area, areas of the seven subregions, thirds, or midsagittal cortical brain areas (see Table 2).
The outcome of the multivariate nearest neighbour classification study confirmed the previous result of no CC area differences between the dyslexic and the control group. No combination of
Discussion
Using new methods for statistic shape analysis we found reliable differences in the length of the CC in shape space, with a shorter posterior midbody in the dyslexic group. This shape abnormality was confirmed by an automatic classification procedure with an accuracy of 78% over the entire sample. Although this accuracy may not be considered high enough for an “automatic screening tool”, we believe that it provides evidence that the CC midbody shape of dyslexic children actually is shorter.
Acknowledgements
This study was financially supported by the Regional Competence Centre for Child and Adolescent Psychiatry (K.v.P.), the Norwegian Research Council (A.L., E.H., K.H.), the Nyquist legacy (E.H., K.H.), the Innovest Foundation Haukeland University Hospital (A.L., A.I.S., L.E., K.H.) and Siemens Corporate Research, Princeton, USA (N.D.). We thank the Department of Radiology, Haukeland University Hospital, for providing access to the MR scanner, and especially Roger Barndon and Anne Marie Kira for
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