P007 Comparing the efficacy of transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS) on the auditory temporal resolution in patients with dyslexia

Abstract

The impaired processing of rapidly changing auditory temporal features has been suggested as one key factor in development dyslexia. Voice onset time (VOT), a short period between the release of closure and the start of voicing enables to distinguish categorically between voiced (i.e./da/) and voiceless stop-consonants (i.e./ta/). VOT thus requires the precise perception and categorization of rapidly changing temporal information in the speech signal and can be considered as an important acoustic cue for the encoding of linguistically relevant information. Recently, non-invasive transcranial electrical stimulation has been suggested as a valid tool to modulate VOT-processing in healthy subjects (Heimrath et al., 2015; Rufener et al., 2016). The current study strives to assess the potential of transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS) on VOT-categorization in dyslexic participants. 16 adult dyslexics (age 20–50 years) with normal hearing acuity participated in this study. All participants received on three consecutive sessions either 20 min of tRNS (100–640 Hz), 40 Hz-tACS or sham stimulation over bilateral auditory cortex region while they performed an auditory phoneme categorization task. Subsequently, a passive MMN-paradigm with a standard stimulus (voiced/da/, VOT 20 ms), two within-category deviants (VOT 24 ms, VOT 28 ms) and to between-category deviants (voiceless/ta/, VOT 32 ms, VOT 36 ms) was performed. EEG-data were recorded from three midline electrodes (Fz, Cz, Pz) with the reference electrode at the tip of the nose. We found enhanced MMN-amplitudes at the Fz-electrode for both within-category deviants after 40 Hz-tACS compared to tRNS and sham. This finding indicates that tACS but not tRNS improves the auditory temporal resolution in patients with dyslexia. Our data provides first evidence for a superior effect of tACS compared to tRNS on VOT-processing in dyslexic participants. Of note, we found increased temporal precision of the auditory system after a single session of VOT-categorization accompanied by tACS. Our finding emphasizes the clinical application of tACS in the context of impaired speech processing.