Transcutaneous auricular vagus nerve stimulation during a 3-back task: a high-density functional near-infrared spectroscopy study

Working memory issues are an often life-impairing deficit seen in several neurological disorders. Transcutaneous vagus nerve stimulation (tVNS) is a promising neuromodulatory adjunct to cognitive training and may improve working memory. High-density functional near-infrared spectroscopy (HD-fNIRS) is a modern neuroimaging modality that can assess cerebral blood flow in cortical areas. In the current study, we used HD-fNIRS to determine the effects of tVNS during a working memory task. Twenty-two healthy adult participants (12 female, 10 male) performed a 3-back task in a block design whilst high-density fNIRS was recorded across the bilateral prefrontal cortex using 36 sources and 48 detectors (1728 channels). Sham (earlobe) tVNS was delivered during the first task trial and active (tragus) tVNS was delivered during the second task trial. Statistical analysis was performed at a group level within trial (task - baseline) and between trials (active tVNS vs. sham tVNS) using node-level and regional approaches. Task-related increases in HbO were seen in the right dorsolateral middle frontal gyri, left inferior frontal gyrus and right lateral orbitofrontal cortex under both active and sham tVNS. Decreases were observed in the bilateral superior medial frontal gyri and medial orbital frontal cortices. No significant differences were seen between sham and active tVNS. Simultaneous recording of HD-fNIRS during a 3-back task and concurrent tVNS was feasible and well-tolerated. Robust task-related activation was seen in lateral frontal areas. No significant active-versus-sham differences in cortical activity or behavioural performance were observed under the stimulation parameters used here; however, this finding should be interpreted cautiously because the fixed-order design may have introduced fatigue or habituation effects. HD-fNIRS could be used in future studies of working memory and neuromodulation in clinical cohorts.