Cortical sources of resting state EEG rhythms are abnormal in dyslexic children

Previous studies have been inconclusive whether dominant resting state alpha rhythms differ in amplitude in dyslexic subjects when compared to control subjects, being these rhythms considered as a reflection of effective cortical neural synchronization and cognition. Here we used a validated EEG source estimation to test the hypothesis that resting state alpha rhythms are abnormal in dyslexic subjects and are related to reading deficits.

Eyes-closed resting state electroencephalographic (EEG) data were recorded in 26 dyslexics (12 males, mean age of 11 years ± 0.5 standard error of mean, SEM) and 11 age-matched normal control subjects (8 males, mean age of 11 years ± 0.7 SEM). EEG rhythms of interest, based on individual alpha frequency peak, were the following: about 2-4 Hz (delta), 4-6 Hz (theta), 6-8 Hz (alpha 1), 8-10 Hz (alpha 2), and 10-12 Hz (alpha 3). For the higher frequencies, we selected beta 1 (13-20 Hz), beta 2 (20-30 Hz), and gamma (30-40 Hz). Cortical EEG sources were estimated by low resolution electromagnetic tomography (LORETA). LORETA solutions were normalized across all voxels and frequencies.

Compared to the control children, the dyslexics showed lower amplitude of parietal, occipital, and temporal alpha 2 and alpha 3 sources. In the dyslexics, some of these sources were correlated to reading time of pseudo-words (parietal alpha 2, r=-0.56, p=0.02; parietal alpha 3, r=-0.58, p=0.02; temporal alpha 3, r=-0.57, p=0.02); the higher the alpha power, the shorter the reading time.

Dyslexic children are characterized by limited abnormalities of resting state EEG rhythms as to topography (posterior regions) and frequency (alpha), which were related to phonological encoding (pseudo-words reading).

Dyslexia may be associated to some functional impairment of cortical neuronal synchronization mechanisms involved in the resting state condition.