Transcutaneous Vagus Nerve Stimulation During Motor Activity in Healthy Volunteers: A High-Density Diffuse Optical Tomography Study

Background: Stroke is a leading cause of long-term disability worldwide. Non-invasive or transcutaneous auricular vagus nerve stimulation (taVNS) shows promise in promoting neuroplasticity and supporting motor recovery. There are currently no validated biomarkers of taVNS. High-density diffuse optical tomography (HD-DOT) is a portable neuroimaging technology that uses near-infrared light to map cortical activity via the quantification of changes in blood oxygenation. The aim of this study was to determine whether HD-DOT could detect motor task-related activity with concurrent taVNS. Methods: Thirty-one healthy participants completed right and left finger tapping tasks with concurrent sham (earlobe) and then active (tragus) taVNS in a within-subject block design. HD-DOT was recorded across the bilateral sensorimotor cortex using 36 sources and 48 detectors (1728 channels). Cortical reconstructions were parcellated and block-averaged task-related oxygenated and deoxygenated haemoglobin changes were compared between sham and active taVNS conditions. Results: In a group-level analysis, appropriate lateralised task-related haemodynamic responses were seen in the contralateral sensorimotor regions, demonstrating the validity of HD-DOT. Between-group comparisons showed no significant change in task-related activation during right finger tapping tasks under active vs. sham taVNS conditions. A non-significant redistribution of task-related activity to the right motor cortex was seen with left finger tapping under active taVNS compared to sham taVNS. Conclusions: Simultaneous recording of neural responses to taVNS during motor activity was feasible and well tolerated. Reliable task-related activation was recordable. Future studies of whole brain HD-DOT in people with stroke will help evaluate its potential as a biomarker in taVNS.