A brain abnormality is thought to be the cause of dyslexia.

A brain abnormality is thought to be the cause of dyslexia

According to research conducted by Inserm researchers and CNR, a single anomaly in the auditory cortex of the brain could be at the root of the three main difficulties encountered by dyslexics.

Linking a letter to a sound is not easy for everyone. And it is this difficulty in matching graphemes (letters or groups of letters) to phonemes (sounds of speech) that characterizes dyslexia. This includes a learning disability that can be very disabling.

According to research carried out by Inserm and CNRS researchers and published in the journal Neuron of December 21st, a single anomaly in the auditory cortex of the brain could be at the origin of the three main difficulties encountered by dyslexics. Namely, succeed in mentally manipulating speech sounds, short-term memory difficulties (such as the ability to repeat a list of words) and a slowing down of the ability to quickly name sets of images.

Reduced sensitivity of the left cortex

For a long time, the majority hypothesis of scientists has been that dyslexia is due to an anomaly in the development of brain areas involved in the representation and processing of speech sounds (phonology).

To study this, researchers at Inserm and CNRS recorded the brain activity of 44 adults, including 23 dyslexics, in response to a noise whose rhythm ranged between 10 and 80 Hz. Result: dyslexics showed sensitivity reduced from the left auditory cortex to modulated sounds around 30 Hz.

However, the response of the left cortex to these frequencies would be necessary to the division of speech into linguistic units that can be associated with letters. Hence the difficulty of quickly naming a series of images.

Anne-Lise Giraud, co-author of the study, explains to France Soir that dyslexics "cut up sounds so finely that they can not be associated with graphemes".

On the other hand, the dyslexics show an increased sensibility to the amplitude modulations of the sounds located beyond 40 Hz. This would explain the deficit of memory in the short term. Indeed, neurobiologist Anne-Lise Giraud continues in the daily, "if they cut more finely, they make more small packets and it clogs their memory in the short term."


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