Dyslexia: a neuroscientific puzzle.

The initial assumptions as to the existence of neurodevelopmental disorders have been confirmed by almost a century of studies focused on the nature and causes of reading disability. Dyslexia is an umbrella term for the various manifestations of reading disabilities, each of which is related to its own complex of neuropsychological dysfunctions, sometimes accompanied by neurological symptoms that can be indicative of the specific location involved. Electro-physiology and neuro-imaging in relation to metabolic activity have also confirmed that, to some extent, there is dysfunction in various cerebral areas, more frequently in the left than in the right hemisphere, less often between the hemispheres (corpus callosum) and outside the classical language areas. The rather rare subtype referred to as visual dyslexia and the inter-modal disconnection type resembling alexia are eclipsed by dyslexias related to spoken language: the dysphonemic (auditory-phonological) subtype and the subtypes that are concerned with the production of speech (fluency, word-finding, syntax, sequences), semantics (language comprehension) and linguistic memory. Although it is considered that there are basic brain dysfunctions in dyslexics such as deficits in sequential information processing, there is no "grand unified theory" of dyslexia. There are some new insights as regards the aetiology. In addition to indications of such possible causes as perinatal brain damage, genetic or chromosomal anomalies that manifest themselves in the substrate in an unclear manner, there is also the possibility that the cortical language areas can be disturbed in their development (migration disorders and abnormal asymmetry) due to genetic or epigenetic hormonal and/or immunological factors, so that the normal left hemispheric dominance does not develop. The diagnosis and treatment of dyslexia should begin at an early age and should be acknowledged as being of great social importance.