P 102 Domain-specific effects of transcranial random noise stimulation (tRNS) on auditory feature processing

Abstract

Theoretical background Decoding acoustic speech signals requires to parse the acoustic stream into linguistically meaningful units, and to extract stress and prosody. Accordingly, the temporal and spectral resolution of the auditory cortex is essential in successful speech processing ( Rosen, 1992 ). The reduced acuity in discriminating these features is considered as key factor in dyslexia ( Goswami, 2011 ) but is also a consequence of normal aging ( Walton, 2010 ). Transcranial random noise stimulation (tRNS), a non-invasive brain stimulation technique that applies weak currents at multiple frequencies, is assumed to modulate the excitability of neural assemblies and, in consequence, the reactivity of cortical structures. However, the effect of tRNS on the auditory system is yet unclear. The present study aimed to fill this gap by investigating the impact of tRNS on basic auditory feature processing. Considering the hemispheric specialization of the auditory system in analyzing theses features we hypothesized that a site-specific tRNS application leads to domain-specific changes in auditory features processing. In concrete, tRNS over the left auditory cortex (lAC) was hypothesized to affect the temporal domain while tRNS over the right auditory cortex (rAC) was assumed to modulate the spectral domain. Finally, we investigated whether tRNS over bilateral auditory cortex (bAC) leads to a modulation of both dimensions. Methods On four days 18 healthy, right-handed adults received 30 min. tRNS at an intensity of 1.5 mA either over the lAC, over the rAC, over bAC or sham stimulation while continuous EEG was recorded. The task consisted of the detection of a deviant tone in a sequence of three sine wave tones. While the duration of all tones and the pitch of the standard tones was kept stable, the inter-tone interval (temporal information) and the frequency in that the deviant tone differed from the standard tones (spectral information) was modulated. Results In result, we found that tRNS applied over the lAC and over bAC inferred with task performance. This effect was specifically modulated by the inter-tone interval between the tone triplets but not by the frequency of the deviant tone. On the electrophysiological level, this finding was paralleled by feature-specific brain response patterns in the alpha and the lower gamma band. Discussion Our findings demonstrate the functional relevance of the lAC in processing temporal features. Most importantly, we proved tRNS as a valid technique to modulate basic auditory perception. The current results stress the need for further investigations on the effect of tRNS in the auditory domain. Systematically assessing the most effective stimulation parameters provides the basis for the clinical application of tRNS in the context of speech and language disorders.