Morphological processing as a top-downcompensatory process in dyslexic adults.Evidence from MEGEddy CavalliLaboratory of Cognitive Psychology, Aix-MarseilleUniversity and CNRSeddy.cavalli@univ-amu.frCoauthors: Pascale Colé (Laboratory of CognitivePsychology, UMR 7290, Aix-Marseille Universityand CNRS), Chotiga Pattamadilok (Laboratory ofspeech and language, Aix-Marseille Universityand CNRS), and Johannes Ziegler (Laboratory ofCognitive Psychology, UMR 7290, Aix-MarseilleUniversity and CNRS)Developmental dyslexia is a neurologicaldisorder mainly characterized by severe andpersistent deficits in reading and phonologicaldecoding skills. According to the compensatoryhypothesis that dyslexics may be prone to relyon semantic units, morphological processing isproposed in this study as a plausible candidateto compensate for reading difficulties. This studyaims to investigate both the time course andlocalization of morphological, orthographic andsemantic processing using MEG associated witha primed-lexical decision task in French dyslexicuniversity students. We used 48 sets of fourprime-target word pairs with the same targetacross the four experimental conditions. Wordpairswere morphologically related floral – FLEUR[floral-flower], orthographically related fleuve –FLEUR [river-flower], semantically related tulipe– FLEUR [tulip-flower] or unrelated cantine –FLEUR [canteen-flower].Continuous MEG of cerebral activity wasrecorded using a 248-channel biomagnetometersystem. We focused our MEG analyses on theleft inferior fronto-occipital cortex includingthe FFG and posterior part of LITG, the LIFG(BA45/47) and the left orbitofrontal gyrus Results showed a dissociated pattern ofactivation both spatially and temporallybetween groups. While skilled readers exhibiteda bottom-up process with early orthographiceffect (M170) in posterior LITG and late morphosemanticeffect (M350) in LIFG BA47, dyslexicreaders exhibited a top-down process withearly morpho-semantic effect [100-200]ms inLIFG BA45 and late morpho-orthographic andsemantic effects (M350) in FFG. This findingsuggests a functional reorganization of the brainareas in dyslexic adults that might explain howthey managed to compensate for their readingdeficits.
