From ear to eye? No effect of transcutaneous vagus nerve stimulation on human pupil dilation: A report of three studies

Transcutaneous stimulation of the auricular branch of the vagus nerve (tVNS) has been proposed as a treatment for a spectrum of physical and psychological disorders. One of the proposed working mechanisms of tVNS is a modulatory effect on the locus coeruleus - noradrenaline (LC-NA) network. We tested this hypothesis in humans in a series of three studies: one focusing on high trait worriers, and two in healthy populations. In all three studies, we tested whether tVNS increases resting pupil diameter - as an index of LC-NA network activity. Additionally, we tested whether tVNS affects task performance and task-related pupil dilation during an Attentional Blink task. We found no evidence that tVNS increases pupil diameter or task-related pupil dilation in any of the tasks. No consistent effects of tVNS on performance on the attentional blink task were found. Overall, the results of these studies indicate that tVNS does not affect these behavioral and physiological indices of noradrenergic activity.