Exploring parameters of gamma transcranial alternating current stimulation (tACS) and full-spectrum transcranial random noise stimulation (tRNS) on human pharyngeal cortical excitability.

Abstract

Transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS) have been shown to have physiological and functional effects on brain excitability and motor behavior. Yet, little is known about their effects in the swallowing system. To examine the effects and optimal stimulation parameters of tACS and tRNS for modulating excitability of human pharyngeal motor cortex. 10Hz (alpha), 20Hz (beta), 70Hz (gamma) tACS, 0.1-640Hz (full-spectrum) tRNS, and sham were applied over pharyngeal motor cortices at 1.5mA current intensity for 10min in 15 healthy participants. Pharyngeal motor-evoked and thenar motor-evoked potentials (PMEPs and TMEPs) were assessed before and up to 2h after stimulation with single-pulse transcranial magnetic stimulation. Averaged MEP amplitude and latency changes were analyzed using repeated measures ANOVA (rmANOVA). Two-way rmANOVA across all active interventions demonstrated a significant MEP interaction both in the stimulated pharyngeal cortex (F (4, 56)=1.731, p=0.038) and in the ipsilateral thenar cortex (F (4, 56)=1.506, p=0.048). Compared to sham, subsequent post hoc tests showed site-specific and sustained (60-120min) increases in PMEPs with gamma tACS and tRNS (p=0.005, p=0.027, respectively) and for TMEPs with beta tACS (p=0.006). Our findings suggest that the effects of tACS and tRNS are frequency-dependent and cortical (representation) site-specific with both gamma tACS and full-spectrum tRNS enhancing human pharyngeal cortical excitability. These techniques hold promise as potential treatments for neurological dysphagia.