Evaluation of the combined effect of reading training and tACS on reading-related cognitive abilities in developmental dyslexia: an investigation by neuropsychological assessment

Neural oscillations in dyslexia have been the subject of numerous studies aimed at better understanding the neurobiological basis of this condition. Numerous studies have shown that people with dyslexia exhibit abnormal neural oscillations, especially in the frequency bands of alpha (8-12 Hz) and gamma (25-80 Hz) (Hanslmayr, S. et al., 2007). Alpha activity inhibits signal transmission between brain areas and is crucial in filtering irrelevant information during language processing. Gamma activity, on the other hand, refers to the synchronization of local neural activities and the formation of functional networks between different brain regions involved in reading. (Lizarazu Ugalde, M. 2021).

Various studies, also using brain imaging techniques such as magnetoencephalography (MEG) and EEG, have shown reduced synchronization of neural oscillations in people with dyslexia during reading and text comprehension (Molinaro et al., 2016). It has been observed, for example, that children with dyslexia show reduced alpha activity in the occipital region of the brain during tasks involving in the visual perception of words. This reduced alpha activity could make it more difficult to inhibit irrelevant visual information and interfere with accurate reading (Specht et al., 2009).

In addition, gamma activity is impaired in people with dyslexia, with reduced synchronization between the different brain regions involved in reading. This could cause a lack of coherence and stability in neural networks involved in reading, impairing text comprehension and reading fluency (Marchesotti et al., 2020).

We know how neural oscillations in dyslexia play a crucial role in the brain dysfunction underlying this condition (Lallier et al., 2017). Abnormalities in alpha and gamma frequencies, which affect selective inhibition and synchronization between brain regions involved in reading, can impair language processing and text comprehension (Vidyasagar, 2013). A better understanding of neural oscillations in dyslexia could contribute to developing new therapies and targeted interventions to improve reading and comprehension skills in individuals with this condition.

In addition, it has been shown that electrophysiological activity can explain individual differences in cognitive abilities and provide measures that support behavioral data in different tasks in people without neurological disabilities (Donoghue et al., 2022; García-Ponsoda et al., 2023). Thus, it appears to be a promising measure for assessing reading ability in individuals with developmental dyslexia.

For example, studies such as the one conducted by Hoeft et al. (2007) showed that dyslexic individuals may exhibit reduced activity in the gamma waves during reading- related tasks. Hoeft et al. hypothesized that gamma waves, which represent a frequency of neural activity between 25 and 100 Hz, may play a critical role in reading and that dysfunction in their training may contribute to the reading difficulties found in people with dyslexia (Hoeft et al., 2007). The authors recruited a group of individuals with dyslexia (average age: about 14) and a control group of equal age and reading ability. Reading stimuli, such as words and sentences, were used, and brain activity was recorded using MEG (Hoeft et al., 2007). During the reading tasks, the authors observed significant differences in gamma-wave activities between the control group and the dyslexic group (Hoeft et al., 2007; Marzbani et al., 2016).

In the control group, gamma-wave activities were mainly detected in brain regions associated with reading, such as the occipital cortex and left temporal cortex. These activities were consistent and synchronized during the reading of words and sentences. In the dyslexic group, however, gamma-wave activities were reduced and less coherent. In particular, alterations were observed in brain regions involved in visual word recognition and phonological processing, such as the occipital cortex and left temporal cortex.

These alterations indicate a dysfunction in processing reading-related information in people with dyslexia. This finding supports the authors' hypothesis that gamma waves play a crucial role in reading and that alterations in their activity may contribute to the reading difficulties found in people with dyslexia. The authors suggest that such dysfunction could result from altered neural connectivity in the brains of people with dyslexia. In conclusion, the study by Hoeft et al. (2007) showed that dyslexic individuals exhibit significant differences in gamma wave activity during reading tasks compared to controls. [...]