Reading impairment in childhood: overview of the electrophysiological correlates of developmental dyslexia

Electrophysiological measurements enabled us to greatly increase our knowledge on the most spread learning disability in childhood represented by the Developmental Dyslexia (DD). The present chapter reviews the most relevant studies, which used either Event Related Potentials (ERPs) or Electroencephalographic (EEG) bands, to investigate reading disabilities in developmental age. Several studies are here described, which succeeded in showing processing abnormalities not only in dyslexics, but also in children genetically at risk of dyslexia, through the analysis of both the "classical" electrophysiological components (i.e., MMN, P300, N400) and earlier evoked potentials. The electrophysiological markers of neuronal dysfunctions found in these children helped scientists to uncover the psychophysiological mechanisms chiefly involved in this language disorder. These are: deficits in speech sound processing and impairments in manipulating the phonological features of grapheme strings. A considerable advance in the field has been recently reached by using EEG bands, traditionally used for detecting group differences in resting state, but currently extended to the measure of cognitive activation obtained through different experimental tasks. Such studies revealed functional differences in both fast and slow EEG rhythms between dyslexics and controls. The use of two functionally distinct EEG rhythms, theta and beta bands, in line with several ERP results, supports the view that dyslexics' reading difficulties are related to a linguistic impairment which is represented essentially at phonological level. Similar conclusions were further supported by the analysis of delta rhythm - a functional marker of cortical inhibition - during the performance of different linguistic tasks. Group differences in delta activation point to a delay in dyslexics' brain maturation. The review of recent literature on electrophysiological correlates of DD, on one hand, supports the view that ERPs, with their excellent time resolution, can represent an optimal tool for investigating language disorders mainly along time domain. On the other hand, EEG bands are able to show both functional and maturational aspects of dyslexics' brains, thus representing a distinct measurement with respect to ERPs. These two electrophysiological methods provide complementary information for a better understanding of the neural and cognitive mechanisms involved in DD. In conclusion, electrophysiological measures could represent an effective tool for making early diagnoses of dyslexia or for predictions of future reading problems in infants at risk, but they could also be valuable for testing the efficacy of rehabilitative trainings.