Lehongre, K. 1 , Ramus, F. 2 , Schwartz, D. 3 , Pressnitzer, D. 4 & Giraud, A. . 1
1 Inserm U960 - Ecole Normale Supérieure, Paris, France
2 LSCP CNRS UMR 8554, Paris, France,
3 CRICM, CNRS UMR 7225, Inserm UMR-S 975, Paris, France
4 4UMR 8158 CNRS - U. Paris Descartes & DEC, Ecole Normale Supérieure, Paris, France
Whether dyslexia primarily reflects an auditory, phonological or memory deficit has been intensely debated for the past 30 years. We hypothesized that an anomaly in phonemic sampling could account for both phonological and working memory deficits. We used a frequency tagging MEG paradigm and MRI structural imaging to assess cortical entrainment to acoustic modulations ranging from 10 to 80Hz, a property that reflects the cortical ability to sample sensory inputs. We expect dyslexic subjects to exhibit abnormal responses in the 30-40Hz frequency range that carries important phonemic cues. While normal readers exhibited left-dominant auditory steady state responses around 30 Hz, dyslexic subjects only showed enhanced entrainment to modulation frequencies outside the phonemic range up to 80 Hz. The 30 Hz entrainment deficit in the left auditory cortex correlated positively with behavioral measures of phonological output processing, but negatively with those reflecting phonological input. In addition, entrainment to faster rates negatively correlated with verbal working memory capacity. In dyslexics, the left auditory cortex fails to selectively entrain to acoustic modulations conveying phonemic cues, but phase-locks to faster acoustic modulations. While the latter anomaly accounts for verbal working memory deficits in dyslexia, the former one
accounts for distinct facets of the phonological deficit.