Mandarin lexical tone learning has repeatedly been identified as a difficult linguistic feature for non-native speakers of tonal languages like English, even for native English learners of Mandarin at high proficiencies (e.g., Pelzl et al., 2019b). Sound perception training has been shown to help native English speakers perceive lexical tone differences, but acquiring lexical tone as a feature still remains difficult, even after as many as 18 training sessions (Bowles et al., 2016; Chandrasekaran et al., 2010; Li & DeKeyser, 2017; 2019; Liu & Chandrasekaran, 2013; Wang et al., 1999; 2003; Wong et al., 2011; Wong & Perrachione, 2007). While much of the tone learning literature has focused on different training interventions to overcome learning plateaus, another type of intervention that could augment learning is non-invasive neurostimulation. Transcutaneous auricular vagus nerve stimulation (taVNS) is a type of safe, non-invasive neurostimulation that delivers electrical current to the ear canal that has been shown to enhance cognition and learning (e.g., Jacobs et al., 2015). This dissertation investigated taVNS and its potential impact as tool to enhance Mandarin tone learning.

Participants in three groups, peristim taVNS, priming taVNS, and a sham taVNS control participated in a double-blind two-day Mandarin phonological and lexical tone training study. Behavioral data including accuracy and reaction time were collected, as was physiological data in the form of pupillometry due to its ties both to cognitive effort and the most well-studied taVNS mechanism of action, the production of norepinephrine. Active taVNS groups received stimulation before or during multiple training and testing tasks across the two days.

This body of work revealed: (1) priming and peristim administrations of taVNS differentially facilitated vocabulary learning of words with Mandarin tone, (2) priming and peristim administrations of taVNS differentially facilitated learning of new phonological tone categories, and (3) the effects of individual differences were substantially and differentially impacted by priming and peristim administrations taVNS, all results compared to a sham control. The evidence herein supports the potential of taVNS as a practical treatment intervention for enhancing language learning and reveals a number of considerations for its use and implementation to be explored in future research.