Combined multi-session transcranial alternating current stimulation (tACS) and language skills training improves individual gamma band activity and literacy skills in developmental dyslexia

Developmental dyslexia is a neurodevelopmental disorder characterized by the pathologically diminished ability to successfully acquire reading and writing skills. Altered cortical activation in the auditory cortex, resulting in inaccurate perception of acoustic information, is thought to provide the neurophysiological basis for impaired phonological processing, which in turn is the bottleneck for successful acquisition of reading and writing skills. Recently, transcranial electrical stimulation has been shown to be an effective method for restoring dysfunctional cortical activation patterns in the disabled brain. In particular, repeated application of electrical stimulation appears to improve neural processes and associated behavior. Here, we aimed to restore dysfunctional activation in the auditory cortex using multi-session gamma-tACS and thereby to enhance the positive effects of a literacy skills training in children and adolescents with developmental dyslexia. Using a double-blind and sham-controlled study design, we investigated the effect of multi-session transcranial electrical stimulation delivered concurrently with an auditory training and in combination with a behavioral training of literacy skills in 29 children and adolescents (13 male, 6 female, aged 8-17 years) with developmental dyslexia. Over a 5-week period, participants received 10 training sessions while 40 Hz-tACS (gamma-tACS) at 1mA intensity was administered for 20 minutes over the bilateral auditory cortex.We found that only participants treated with gamma-tACS showed a shift in the peak frequency of auditory gamma oscillations reflecting a more fine-grained perception of time-critical acoustic information. This amelioration was accompanied by improved phonemic processing skills in the gamma-tACS group. Moreover, 4 months after the intervention, only individuals who received gamma-tACS showed significant improvement in their writing skills. Our results demonstrate that multi-session gamma-tACS can enhance the effects of a behavioral intervention and induce long-term improvement of literacy skills in developmental dyslexia by successfully modulating the gamma peak frequency in the auditory cortex.