“…Using pupil size as a putative marker of noradrenergic activity in humans (Gilzenrat et al, 2010;Murphy et al, 2014), taVNS applied at the standard clinical duty protocol of 30 s on/30 s off typically results in an absence of a tonic change in pupil size (Borges et al, 2021;Burger et al, 2020;D'Agostini et al, 2021D'Agostini et al, , 2022Keute et al, 2019;Warren et al, 2019; see Table 1), although contrary results have been reported both for taVNS (Capone et al, 2021) and human iVNS (Desbeaumes Jodoin et al, 2015). In contrast, in more recent studies with short, phasic-like (600ms -2 min) stimulation pulses, taVNS has been found to elicit a rapid pupil dilation, peaking and returning to baseline within a few seconds from stimulation onset (D'Agostini et al, 2023;Lloyd et al, 2023;Sharon et al, 2021;Urbin et al, 2021). A comparable effect has been found in rodents with iVNS (Collins et al, 2021;Hulsey et al, 2017;Mridha et al, 2021).…”