Transcutaneous vagus nerve stimulation during semantic verbal fluency tasks: a high-density diffuse optical tomography study

Introduction: Transcutaneous auricular vagus nerve stimulation (taVNS) is a promising adjunct for post-stroke language recovery. There are no established biomarkers for the neuromodulatory effects of taVNS. High-density diffuse optical tomography (HD-DOT) offers a non-invasive marker of task-related cortical activation. We explore HD-DOT in determining the effects of taVNS during a semantic verbal fluency task (VFT).

Methods: Twenty-four healthy adult participants performed a semantic VFT in a block design, whilst HD-DOT was recorded across the bilateral prefrontal cortex. Participants received sham (earlobe) taVNS followed by active (tragus) taVNS. Statistical analysis was performed at a group level within condition (task–baseline) and between conditions (active taVNS vs. sham taVNS) using node-level and parcellation approaches.

Results: Twenty-one participants (87.5%) met the signal quality threshold. At the uncorrected level, significant increases in task-related activity were seen in the triangular part of the left inferior frontal gyrus, during both the active and sham taVNS. Correcting for multiple comparisons, significant increases were seen in the left lateral orbitofrontal cortex during sham stimulation. No significant differences in oxygenated haemoglobin concentration increase were seen between active and sham taVNS.

Discussion: Simultaneous recording of brain activity to taVNS during a semantic VFT was feasible and well tolerated. Task-related activation was seen in brain regions associated with language production and decision-making. Through this exploratory work, healthy adults do not demonstrate any significant enhancement of frontal cortical activation from taVNS. Future studies should combine whole-brain HD-DOT and taVNS in clinical cohorts.