Introduction: Research in the past decades suggests a strong impact of the locus coeruleus norepinephrine (LC-NE) arousal system on human experience: when arousal is increased, individuals are generally more alert to sensory stimulation, more motorically active, and exert more cognitive control (e.g., Pfaff, D.W. 2006). Still, the exact contribution of LC-NE to behavioural control is still unclear. Previous human studies have found that pupil size, used as a non-invasive index of arousal, is larger when decisions are made under strict deadlines compared to situations with more time for accuracy, leading to the proposal that LC-NE may generate urgency, speeding up decisions (Murphy, P.R., et al. 2016). However, this contrasts with the perspective that LC-NE serves to enhance information processing (Aston-Jones, G. and J.D. Cohen. 2005), implying that involvement of LC-NE should primarily improve accuracy. In this study, we used transcutaneous vagus nerve stimulation (tVNS) to causally probe the role of LC-NE in regulating speed and accuracy during decision-making.

Methods: we considered decision-making behaviour (speed and accuracy) in 42 healthy human subjects (28 women, mean age 26 ± 4.0 years old) performing the random dot motion discrimination task, with either an active stimulation of LC-NE by means of tVNS or with an electrical stimulation of the earlobe as a sham condition. Subjects performed a total of 8 blocks of 40 trials in a single session. Trains of stimulation were applied for 4 seconds on each trial, with tVNS or sham in separate blocks (counterbalanced). Half of the subjects (Group 1; n=21, 12 women, mean age 26 ± 4.5 years) received stimulation early in the trial, while the other half (Group 2; n=21, 16 women, mean age 25 ± 3.5 years) received stimulation later on during a period covering the decision phase. Pupillometry was used to monitor changes in arousal during the task, as a function of tVNS or sham stimulation.

Results: In both groups, tVNS led to significantly larger pupil sizes than sham stimulation (both t > 1.8, both p < 0.05), indicating effective activation of LC-NE during decision-making under tVNS condition. Interestingly, such an increase in arousal was paralleled by a higher decision accuracy with tVNS compared to sham (76 ± 8.3% versus 74 ± 8.5% on average in both groups), though this effect was only significant in Group 1 (t (20) = 2.68, p = 0.01) but not in Group 2 (t (20) = 0.43, p = 0.67). Yet, when considering all subjects from both groups together, there was a positive correlation between effects on pupil size and accuracy: the more tVNS increased pupil size during decisions, the more accuracy was enhanced (r = 0.48, p < 0.01).  We did not find any significant effect of tVNS on decision speed.

Conclusion: Our results are inconsistent with the view that LC-NE generates urgency as we did not observe any effect of tVNS on decision speed. Rather, the enhanced accuracy with tVNS suggests that LC-NE facilitates information processing to optimize the performance of the decision-making process.