Poster B15, Thursday, August 16, 3:05 – 4:50 pm, Room 2000AB

Transcutaneous vagus nerve stimulation enhances non-native speech categorization

Fernando Llanos1, Jacie R. McHaney1, Matthew K. Leonard2,3, William L. Schuerman2, Han G. Yi2, Bharath Chandrasekaran1,4,5,6,7;1Department of Communication Sciences and Disorders, The University of Texas at Austin, 2Neurological Surgery, University of California, San Francisco, 3Weill Institute for Neurosciences, University of California, San Francisco, 4Institute for Mental Health Research, The University of Texas at Austin, 5Department of Psychology, The University of Texas at Austin, 6Department of Linguistics, The University of Texas at Austin, 7Institute for Neuroscience, The University of Texas at Austin

Learning to categorize non-native speech is extremely challenging for adults. This challenge is due to a loss of sensitivity to acoustic features that are not linguistically relevant in the native context. In rodent models, pairing vagal nerve stimulation (VNS) with sounds enhances cortical auditory plasticity that is specific to stimulus spectrotemporal properties (Engineer et al., 2011, 2015). VNS-induced cortical plasticity is similar to cortical plasticity induced by nucleus basalis stimulation (Engineer et al., 2011), suggesting that effects arise from upregulation of the cholinergic system. Based on these findings, it may be possible to use VNS to facilitate non-native speech category learning in humans. Here, we examine the extent to which pairing auricular transcutaneous vagus nerve stimulation (tVNS) with auditory training stimuli facilitates learning of Mandarin Chinese tones. Mandarin Chinese has four phonologically distinctive tone contours: high-level (T1), low-rising (T2), low-dipping (T3), and high-falling (T4). These linguistically relevant tones are perceptually cued by at least two dimensions: pitch height and pitch direction. English learners of Mandarin tones are perceptually more sensitive to pitch height cues than to pitch directional cues (Gandour, 1981). Native English speakers naïve to tonal languages were trained with feedback (correct/incorrect) to categorize Mandarin tones across six training blocks of 40 speech stimuli with high phonetic variability (four tones, five syllables, four talkers). Training was followed by one generalization block of 40 novel stimuli with no feedback A 500-ms train of 15 biphasic electric pulses (pulse width = 100 µs) was transcutaneously delivered at 25 Hz through the ear concha, targeting the auricular branch of the vagus nerve (Frangos et al, 2015). Pulses started 300 ms before the target stimulus onset. Pulse amplitude was set to 0.2 mA below the sensory threshold using a two-step staircase procedure. To examine the specificity of tVNS in tone learning, we paired VNS with different tone sets. In a pitch direction condition (N=10), tVNS was paired with tones from the two tone categories contrasting in pitch direction (T2, T4). In the pitch height condition, tVNS was paired with tones from the categories contrasting in pitch height (T1, T3). We compared the proportion of correct responses across blocks between the two tVNS conditions and a control condition of twenty native English speakers receiving Mandarin tone training with no tVNS (Reetzke et al., 2018). Linear mixed-effects analyses with block, control, and tVNS conditions as fixed effects, and subject as random effects, revealed an enhancement of accuracy-by-block under tVNS, relative to the control database (b=2, SE=0.73, t=2.72, p=0.01). This result provides evidence for tVNS-induced enhancement in Mandarin tone learning. Linear-mixed effect analyses also revealed an enhancement of accuracy-by-block in the pitch-height condition, relative to the pitch-direction condition (b=2.45, SE=1.08, t=2.26, p=0.02), and the control group (b=5.85, SE=1.75, t=3.33, p<0.001) suggesting that VNS-induced plasticity can be modulated by stimulus parameters. These results indicate that tVNS can enhance sensitivity to linguistically-relevant acoustic dimensions, thus setting a preliminary basis for potential translational applications in individuals with speech perception deficits.

Topic Area: Perception: Auditory