Transcutaneous Auricular Vagus Nerve Stimulation: Motor Learning & Mechanisms

Recovery of hand function is a top priority for survivors of stroke and spinal cord injury, yet most have unresolved impairments that disrupt their ability to function independently in everyday life. Rates of recovery associated with traditional rehabilitation are low, emphasizing the need for adjunct therapies that improve functions impaired by injury. Pairing stimulation of the vagus nerve (VNS) with task repetition has been shown to enhance outcomes, but VNS requires implantation procedures that are invasive and costly. There is evidence to support that vagal nerve fibers innervate the skin of the external ear, providing a potential noninvasive, affordable alternative to implantable VNS. Whether transcutaneous auricular “vagus nerve” stimulation (taVNS) enhances motor learning is not well understood. Moreover, where and how stimulation should be administered are recognized barriers to clinical translation. The objectives of this proposal are to: 1) Identify the pupil-indexed site on the external ear for taVNS, and 2) Evaluate placebo-controlled effects of taVNS timing and intensity on motor learning. To accomplish the first objective, pupil diameter will be recorded while brief pulse trains of electrical current are applied to putative sites of vagal innervation on the external ear. Pupil dilation is an autonomic response reflecting physiological brain states that support learning. The site on the external ear where stim-evoked pupil dilations are elicited most readily will be targeted during training on a motor task. To accomplish the second objective, individuals will be randomized into one of five groups in which stimulation amplitudes below or above perceptual threshold are administered at the same time or after performance criteria are met during individual repetitions on the trained motor task. Stimulating electrodes will be placed on the ear in a control/sham group, but no stimulation will be administered. Learning will be inferred from changes in performance relative to baseline one week and one month following the final training session. Knowledge gained from this project will be used to guide future therapeutic trials that examine effects of taVNS to improve functions impaired by neurological injury. Results will also inform the use of taVNS to treat symptoms associated with other clinical disorders prevalent in U.S. Veterans and for applications that seek to enhance human learning and performance.