300 Hz transcutaneous auricular vagus nerve stimulation (taVNS) impacts pupil size nonlinearly as a function of intensity
Transcutaneous auricular vagus nerve stimulation (taVNS) is a neuromodulatory technique that may have numerous potential health and human performance benefits. However, optimal stimulation parameters for maximizing taVNS efficacy are unknown. Progress is impeded by disagreement on the identification of a biomarker that reliably indexes activation of neuromodulatory systems targeted by taVNS, including the locus coeruleus-norepinephrine (LC-NE) system. Pupil size varies with LC-NE activity and is one potential taVNS biomarker that has shown inconsistent sensitivity to taVNS in prior studies. The present study examined the relationship between pupil size and taVNS using stimulation parameters that have shown promising behavioral effects in prior studies but have received comparatively little attention. Participants received 30-second trains of 50 μs taVNS pulses delivered below perceptual threshold at 300 Hz to the left external acoustic meatus (EAM) while pupil size was recorded during