Investigating the dyslexic brain – DYSBRAIN

At the cognitive level, our investigations focus on auditory attention and short-term memory, and on how deficits at this level could underlie the difficulties with speech sounds (the phonological deficit) encountered by most dyslexic individuals.
At the brain level, we use magneto-encephalography to investigate to what extent the auditory cortex of dyslexic individuals responds differently to simple sounds, and how this relates to deficits at the cognitive level. We also use magnetic resonance imaging (MRI) to investigate the underlying brain structural differences. In particular, we use high magnetic field MRI and sophisticated acquisition and analysis methods to image the dyslexic brain in far greater details than has been possible before. This allows us to test the hypothesis that there are disruptions of neuronal migration in the brains of dyslexic people.
At the genetic level, we use all the most up-to-date methods of molecular genetics to search the genome of individuals with dyslexia and identify genetic variants that may induce a susceptibility to dyslexia. We search for both common polymorphisms by using DNA chips and rare mutations by sequencing large portions of the genome of 150 participants. Finally, we will also investigate the links between the identified genetic variants and the structural and functional brain differences observed, thereby bridging, gene, brain and behaviour.

At the cognitive level, we have shown that adult dyslexic participants are able to implicitly memorise arbitrary complex noises, as well as a control population. This is one more piece of evidence against theories of dyslexia postulating low-level auditory deficits.
At the brain level, we have shown that the cortical thickness of the brain region supporting visual word forms is lower in dyslexic than in control girls. This result is not observed in boys. This has been replicated in two independent populations, as well as compared to a reading-level matched control group, suggesting that this difference is not entirely due to reduced or altered reading experience, but might be a causal factor in the etiology of dyslexia.
We have also confirmed that dyslexic boys show a diminished or reversed asymmetry of the planum temporale compared to control boys. This result is not observed in girls. This confirms that dyslexic boys’ auditory cortex is organised differently from controls’.
Finally, we have found, in both dyslexic boys and girls, differences in the asymmetry of two white matter tracts that connect cortical regions involved in reading. This result may help understand why certain brain regions seem to communicate less well with each other in dyslexia.

Results obtained so far suggest that the neural basis of dyslexia may partly differ between boys and girls. This suggests that hormonal factors may interact with genetic predispositions and modulate their effects, a hypothesis that would deserve to be tested in future research.

The project « Dysbrain : the dyslexic brain » is a fundamental research project coordinated by Franck Ramus (CNRS), in collaboration with Lucie Hertz-Pannier (CEA) and Thomas Bourgeron (Institut Pasteur). The project started in January 2012 and lasted for 48 months. It received from ANR a grant of 637 000 euros, for a total cost of around 2 million euros.

At the end of the contract, the project has already given rise to 7 articles international scientific journals, including in some of the best journals in neuroscience (J. of Neuroscience, Human Brain Mapping, Cortex). The results have also been divulgated in numerous national and international conferences, as well as in popular science outlets and events.

Developmental dyslexia is a common learning disorder affecting about 3 to 7% of the population, and defined as a specific deficit in reading acquisition that cannot be accounted for by low intelligence, poor educational opportunities, or obvious sensory or neurological damage. Previous research has shown that individuals with dyslexia have specific cognitive deficits, with a brain basis and a genetic contribution. The present project aims to provide important contributions to our understanding of dyslexia at the cognitive, brain and genetic levels, and furthermore to help us understand the mechanisms through which genes influence brain development and structure, in such a way as to produce cognitive deficits that impair reading acquisition.

At the cognitive level, our investigations will focus on auditory attention and short-term memory, and on how deficits at this level could underlie the difficulties with speech sounds (the phonological deficit) encountered by most dyslexic individuals.

At the brain level, we will use magneto-encephalography to investigate to what extent the auditory cortex of dyslexic individuals responds differently to simple sounds, and how this relates to deficits at the cognitive level. We will also use magnetic resonance imaging (MRI) to investigate the underlying brain structural differences. In particular, we will use high magnetic field MRI and sophisticated acquisition and analysis methods to image the dyslexic brain in far greater details than has been possible before. This will allow us to test the hypothesis that there are disruptions of neuronal migration in the brains of dyslexic people.

At the genetic level, we will use all the most up-to-date methods of molecular genetics to search the genome of individuals with dyslexia and identify genetic variants that may induce a susceptibility to dyslexia. We will search for both common polymorphisms by using DNA chips and rare mutations by sequencing large portions of the genome of 150 participants. Finally, we will also investigate the links between the identified genetic variants and the structural and functional brain differences observed, thereby bridging, gene, brain and behaviour.