The “handwriting brain”: a motor/linguistic network with specialized “writing-specific” areas?
Résumé
Since the first lesion studies from the late nineteenth century until the advent of functional neuroimaging, the uncovering of the cerebral substrates of written production raised the question of the existence of a “writing center” in the brain. A number of neuroimaging studies with healthy subjects have highlighted a broad network of brain areas, some being involved in linguistic processing or in motor control of handwriting, and some playing a “writing-specific” role. Those studies also reported inconsistent results, probably resulting from the diversity of methodologies used. In order to clear up discrepancies, we conducted a meta-analysis of 18 neuroimaging studies involving written language production (using the ALE method). We were thereby able to identify a network of 12 cortical and subcortical areas, mainly on the left hemisphere. Three areas were considered as primarily writing-specific (left SFS/MFG area, left IPS/SPL area, right cerebellum) while the others were more related to non-specific motor (M1/SM1, SMA, thalamus and putamen) or linguistic processes (vPM, PITC) (Planton, Jucla, Roux & Démonet, 2013). We then aim at testing the functional specificity to handwriting of the identified areas with a specific focus on the lateralization of brain response. To do so, we conducted an fMRI experiment in which written naming was compared to drawing and to oral spelling. We extracted BOLD activity within 4 volumes of interests (and their right - or left- counterpart), based on the coordinates from the meta-analysis work (left SFS, left IPS, left vPM, right cerebellum), and compared it across tasks. Results showed that the hypothesis of specificity is questioned when focusing only on the intensity of activation, and that taking into account the inter-hemispheric balance can be an effective way to understand the functional contribution of each of these areas within the whole writing process (Planton, Péran, Longcamp, Démonet & Jucla, in prep).