According to the definition adopted unanimously by the entire scientific community, dyslexia is a neurobiological language disorder of genetic origin, and it manifests itself through an inability of varying acuteness to learn reading and writing, despite normal intelligence and normal auditory and visual perception, a favourable social environment, as well as an absence of significant psycho-emotional disorders.

It affects 5 to 10 % of the population.
 
For Tomatis, dyslexia cannot be summarised as a simple reading disorder.
Dyslexia is a syndrome, that is to say a set of clinical signs belonging to the same clinical entity.
Indeed, it is well known that dyslexic children also very often suffer from oral language disorders and dysorthography, but also, to varying degrees, from a whole set of motor disorders: balance and coordination disorders, dyspraxia, dysgraphia, as well as difficulties organising themselves temporally.
Moreover, disturbances concerning calculation and the abilities of arithmetic reasoning can also be observed.
 
Thus, dyslexia presents itself as a sensory-motor disorder which must be treated as a whole.
This particular approach must be directly related to the Tomatis�s specific conception of the workings of the ear, the latter being considered at the same time as a sensorial and a motor organ, given the close interrelation existing between the vestibule and the cochlea.
 
Depending on their respective deregulation, but also on how harmoniously these two parts of the ear interact, dyslexia will present different aspects, with the motor disorders at times manifesting themselves very strongly or on the contrary, very discretely.

As a result of the fundamental relation existing between the ear and the voice, a relation which constitutes one of the founding principles of the Tomatis system, the crucial, determining factor of dyslexia is that of a lack of articulatory awareness of the sounds of language, causing a disorder in verbal short term memory and a lack of phonological awareness, indispensable conditions for the implementation of a normal acquisition of reading.

The lack of articulatory awareness is itself rooted in a listening deficiency (and not simply a hearing deficiency) concerning the sounds of language, which consequently cannot be reproduced with full precision and the necessary subtlety.
 
Thus, in the treatment of dyslexia, the Tomatis method seeks at the same time to take into account the expressive side and the receptive side of written language.