Humans have devised endless strategies to stave off tiredness in our busy modern world. Be it industrial-strength coffee, energy drinks of unearthly hues or complex sleep routines, everyone has their own go-tos for staying alert. Now, researchers claim, a new fatigue-fighting technique may have emerged in the form of non-invasive electrical brain stimulation.
The research, published by scientists at the Air Force Research Laboratory and Department of Defense provider Infoscitex, Inc., used a form of nervous system stimulation called cervical vagus nerve stimulation (VNS).
Whilst previous studies have shown that transcranial direct current stimulation (tDCS), an electrical stimulation technique that involves passing electrical current through the scalp, could improve cognitive abilities under fatigue, other brain stimulation techniques have yet to be explored.
The team, including lead author Lindsey K. McIntire, hypothesized that VNS could be even more effective than tDCS. This is because both techniques target an area of the brainstem called the locus coeruleus (LC) that secretes the wakefulness hormone norepinephrine.
While tDCS signals must travel through the skin, skull and the rest of the brain to reach the LC, VNS techniques stimulate the vagus, the central nerve of the parasympathetic nervous system. The device used in this study, the gammaCore Sapphire, stimulates the nerve by placing it against the skin of the neck. The authors hoped that this could be a quicker route to activating the LC. Additionally, VNS can be self-administered and only takes six minutes, rather than the half-hour required for tDCS, making it a more attractive option for fatigue relief.
To test their theory, the team used 40 volunteers, all personnel at Wright-Patterson Air Force Base near Dayton, OH. The participants stayed awake for 34 hours in total. After 12 hours had passed, they were either given rounds of VNS or fake stimulation using an identifical device that vibrated and clicked like the real thing, without any electrical impulses being passed through the skin. The group was then given four cognitive task sets to assess their alertness.
The results were mixed. In two of the tasks, no significant effects of VNS were noted. One was a test of sustained attention called the Mackworth clock test and the other a test of memory called the N-back test.
On the other two tasks, VNS-stimulated participants performed significantly better at two time points roughly 12 and 15 hours after stimulation. The tasks were the psychomotor vigilance task (PVT) and the Air Force–Multi-Attribute Task Battery (AF-MATB). The performance improvement in the AF-MATB was driven by tasks where close visual attentiveness was required. The PVT is also a highly visual task, and the authors suggest that the apparent benefits of the VNS device arose from improvements to executive control, which they describe in their paper as “the ability to regulate perceptual and motor processes in order to respond in an adaptive way to novel or changing task demands.”
While the results from the behavioral tasks were mixed, subjective reports from volunteers were more robust. Significant differences were noted from 21 hours to 31 hours after stimulation, as the VNS group reported much smaller increases in fatigue.
Will VNS devices be used to fight off fatigue outside of labs any time soon? The authors point out that their data will need to be confirmed by other labs using study groups that are more diverse in terms of sex and age. Additionally, they note that cervical VNS, the specific type of VNS technology used in the study, has only been tested in clinical populations – such devices are medically approved for the treatment of clusters headaches. Until further research is conducted, keep that coffee grinder close to hand.
McIntire, LK, McKinley, RA, Goodyear, C et al. Cervical transcutaneous vagal nerve stimulation (ctVNS) improves human cognitive performance under sleep deprivation stress. Communications Biology. 2021;4(634). doi: 10.1038/s42003-021-02145-7