Authors: Paulesu Eraldo, Danelli Laura, Berlingeri Manuela.
Publication: Frontiers in Human Neuroscience (Frontiers). 8:830 2014 | DOI: 10.3389/fnhum.2014.00830
Developmental dyslexia has been the focus of much functional anatomical research. The main trust of this work is that typical developmental dyslexics have a dysfunction of the phonological and orthography to phonology conversion systems, in which the left occipito-temporal cortex has a crucial role. It remains to be seen whether there is a systematic co-occurrence of dysfunctional patterns of different functional systems perhaps converging on the same brain regions associated with the reading deficit. Such evidence would be relevant for theories like, for example, the magnocellular/attentional or the motor/cerebellar ones, which postulate a more basic and anatomically distributed disorder in dyslexia. We addressed this issue with a meta-analysis of all the imaging literature published until September 2013 using a combination of hierarchical clustering and activation likelihood estimation methods. The clustering analysis on 2360 peaks identified 193 clusters, 92 of which proved spatially significant. Following binomial tests on the clusters, we found left hemispheric network specific for normal controls (i.e., of reduced involvement in dyslexics) including the left inferior frontal, premotor, supramarginal cortices and the left infero-temporal and fusiform regions: these were preferentially associated with reading and the visual-to-phonology processes. There was also a more dorsal left fronto-parietal network: these clusters included peaks from tasks involving phonological manipulation, but also motoric or visuo-spatial perception/attention. No cluster was identified in area V5 for no task, nor cerebellar clusters showed a reduced association with dyslexics. We conclude that the examined literature demonstrates a specific lack of activation of the left occipito-temporal cortex in dyslexia particularly for reading and reading-like behaviors and for visuo-phonological tasks. Additional deficits of motor and attentional systems relevant for reading may be associated with altered functionality of dorsal left fronto-parietal cortex.
This evidence brings new fuel for those believing in the existence of multiple dysfunctional systems in dyslexia without implying the need for focal and highly localized hypo-activations, preferentially associated with single classes of non-reading-like tasks. Rather, this new evidence speaks in favor of a distributed set of local malfunctions in “associative” regions normally involved in more than one behavior/cognitive domain.
Taken together, these results are in line with the present findings, as they support the hypothesis that dyslexia could be the consequence of the co-occurrence of distributed dysfunctional patterns of different functional systems (see also Schurz et al., 2014): our data, however, also suggest a more limited degree of convergence of the multiple systems on high-level regions involved in reading-like as much as in non-reading-like tasks, particularly for the dorsal network identified here.
Taken together our results provide a partial reconciliation of different accounts of dyslexia, those more concerned with the decoding problem of dyslexia, the underlying phonological deficit and the deficit in the conversion from orthography to phonology, and those more focused on motoric and visuo-attentional problems. Interestingly, the more dorsally one moves within the system identified here, the more the contribution of non-reading-like tasks becomes relevant with a mixture of phonological awareness tasks and motoric/attentional tasks.
Paulesu E., Danelli L., Berlingeri M. (2014). Reading the dyslexic brain: multiple dysfunctional routes revealed by a new meta-analysis of PET and fMRI activation studies. Front. Hum. Neurosci. 8:830. 10.3389/fnhum.2014.00830