Clinical Study: VNS Reduces State-Anxiety in Golfers with Performance Anxiety

Stimulating the Auricular Vagus Nerve on Both Sides of the Head Is Safe and Significantly Reduces State-Anxiety

Abstract

Transcutaneous electrical nerve stimulation is a promising drug-free technology that could treat a variety of conditions, including performance anxiety in golfers. The vagus nerve is a key component in the parasympathetic nervous system, and the anxiolytic effects of vagus nerve stimulation in humans has been well documented. The auricular branch of the vagus nerve (ABVN) is found in and around the ear area and has connected fibers with the great auricular nerve. The great auricular nerve (GAN) is part of the cervical plexus and has similar connections in the brainstem as the vagus nerve. In the last few decades, the great auricular nerve has been a nerve target for treating pain, but little is known about the physiological effects of GAN stimulation. This study aimed to quantify the effects of ABVN + GAN stimulation on physiological activity and golf hitting performance of healthy golfers. 10 minutes of bilateral ABVN + GAN stimulation at the tympanomastoid fissure on the side of the neck was compared with sham stimulation (0mA) on 18 golfers during a hitting task. Results revealed a significant increase in quality of feel of each shot (p=0.05, effect size: 0.83; 9% increase in active group, 0% change in placebo) and a significant decrease in state anxiety (p=0.005, effect size: 0.42). There were no significant changes in other performance or physiological metrics. While not statistically significant, there was a 31% increase in HRV in the active group and 0% change in the sham group. While ABVN + GAN stimulation enhanced feel of the golf swing but did not affect hitting performance of healthy golfers, it may be an effective drug-free treatment for performance anxiety and may help golfers and other high stress professionals keep calm during high-anxious situations.

Authors and Affiliations:

Dr. Nicholas Hool^a,b

^a WearTech, 3110 N Central Ave UNIT 153, Phoenix, AZ 85012, USA

^b Hoolest Performance Technologies, Inc., 2398 East Camelback Road, Suite 1020, Phoenix, AZ 85016, USA

1. INTRODUCTION

Anxiety in Golf

Anxiety in golf and other sports can be described as an unpleasant motivational state that consists of a cognitive side (worrying thoughts) and a somatic side (physical symptoms). Many golfers perform their best when they are anxious because they have a heightened focus and can keep their physical symptoms under control. However, uncontrolled negative thoughts and anxiety symptoms often lead to uncharacteristically poor performance. Typical scenarios that lead to performance anxiety in golfers include the first tee shot (“first tee jitters”), playing in front of a crowd, playing with a lead, competing against a particular individual, and having to make short breaking putts (Smith, 2003). Symptoms of anxiety include tense muscles, elevated heart rate, sweaty palms, jerky and mechanical movements in the swing, and yips in the putting stroke, chipping or full swing. Anxiety is often considered part of the mental game of golf, and the best golfers are capable of keeping their anxiety under control. However, many golfers struggle with anxiety and end up losing their interest in the game because they are unable to overcome the problem. Neuromodulation technology is a promising drug-free method that may help golfers, athletes, and everyday people keep their anxiety symptoms under control and improve their quality of life.

Neuromodulation in Golf

Studies looking at effects of non-invasive electric neuromodulation on human performance mostly consist of transcutaneous direct current stimulation (tDCS) applied to the left dorsolateral prefrontal cortex. One study found that tDCS may enhance flow state activity in video gaming and other sports (Gold, 2019). Another study found that active tDCS resulted in greater putting accuracy compared to sham tDCS in novice golfers (Zhu, 2015). Studies have shown that expert athletes show lower amounts of activity in brain regions like the posterior cingulate, the amygdala-forebrain complex, and the basal ganglia compared to novice athletes (Milton, 2007). In order to truly enhance human performance, stimulation methods should aim to modulate brain activity associated with reduced anxiety and increased flow state (mental state of peak performance). Peripheral nerve stimulation may be a more suitable alternative to tDCS due to easier access to peripheral nerves and their communications with brain regions involving fear, anxiety, and flow state activity.

The GAN has been used as a target in Auricular Acupuncture for hundreds of years to treat a variety of ailments such as epilepsy and pain (Shu, 2004. Usichenko, 2016 & 2017). Shu et. al. found decreases in glutamine and increases in GABA in the hippocampus of a rat having an experimentally induced seizure after stimulating the GAN in the ear. Nerve tracing studies have shown that the GAN has connections in the nucleus of the solitary tract (NTS), the solitary nucleus (SN) of the medulla oblongata, the spinal trigeminal nucleus and communicates (thin nerve fibers connect to the two nerves) with the auricular branch of the vagus nerve (ABVN) (Liu, 1988. Ginsberg, 2000). fMRI studies have shown that stimulation of the GAN at the earlobe caused deactivation in the hippocampus, posterior cingulate gyrus, parahippocampal gyrus, and the amygdala, regions associated with the fear and anxiety response (Yakunina, 2017). Because the GAN targets and synapses in similar brain structures as the ABVN, it may be an effective stimulation target for the treatment of anxiety. There are a handful of devices known as Cranial Electrotherapy Stimulation (CES) devices that have been approved by the FDA to treat insomnia, anxiety, depression, and pain. One of these devices is called Alpha Stim, a device that uses ear-clip electrodes to bilaterally stimulate branches of the GAN on the ear lobe. Studies have demonstrated that this device may be an effective treatment for anxiety disorders like Post Traumatic Stress Disorder and General Anxiety Disorder (Barclay, 2014). One recent study found that 10 minutes of bilateral ABVN + GAN stimulation in 9 Olympic archers caused a significant decrease in resting heart rate of 10.6 bpm after stimulation (Pre: 77.03 ± 14.52 bpm, Post: 66.43 ± 8.53 bpm) (Rodriguez, 2020). However, no studies have analyzed how ABVN + GAN stimulation might affect performance anxiety in golfers. This present study aims to quantify the effects of ABVN + GAN stimulation on physiological activity, performance anxiety and hitting performance of healthy golfers.

Anatomical locations within the ear for transcutaneous vagus nerve stimulation (tVNS) and GAN stimulation require electrode designs that are not ideal for a variety of potential patient populations, such as athletes with performance anxiety. Dry electrode designs would cause too much skin irritation for an athlete to consider using when trying to relax, and wet electrode designs are too complicated of a user experience, as the athlete would need to carry a bottle of gel or saline with them at all times in order to use the electrode. Therefore, we propose to stimulate over a location called the tympanomastoid fissure on the side of the neck and below the ear, a region behind the earlobe and anterior to the mastoid process that contains the ABVN and the GAN (Kiyokawa, 2014). This location is easier to access than locations within the ear and does not require custom electrodes for each patient to achieve maximum comfort during stimulation. We will use the same dry-hydrogel electrode design that was used in the experiment discussed in Chapter 2.

Hypothesis

We hypothesize that non-invasive electrical nerve stimulation of the ABVN + GAN will cause a statistically significant change in performance outcomes in a golf hitting task.

We hypothesize that non-invasive electrical nerve stimulation of the ABVN + GAN will cause a statistically significant change in physiological activity (heart rate, heart rate variability, brain activity, muscle tension, skin conductance, skin temperature, respiration rate) and psychological activity (STAI score).

We hypothesize that non-invasive electrical nerve stimulation of the ABVN + GAN will cause a statistically significant change in the motion of the golf swing.

2. METHODS

Volunteer golfers reported to TopGolf in Scottsdale to complete a hitting task over a period of one hour or less. Throughout the study, we measured brain activity, heart rate, heart rate variability, skin conductance, skin temperature, muscle tension, and blood pressure of each participant. We also measured golf performance metrics that consisted of swing speed, tempo, feel, and accuracy of each shot.

Participants were healthy golfers aged 18-75 years of age with a handicap of 20 or lower. We excluded minors, adults who are unable to consent, and prisoners from participating in this study.

Golfers wore the Muse brain sensing headband (with Opti Brain software) and had their clubs fitted with the Blast motion sensor to capture swing metrics. Once fitted with sensors, golfers were allowed to hit 5 warm-up shots at the 150-yard target to get comfortable with the setup. After the warmup, the first hitting baseline trial began. Golfers hit 10 shots at the 150-yard target, with the goal of hitting each shot as close to the center as possible. Each shot was recorded for accuracy, swing speed, swing tempo, swing time, and feel of the shot on a scale of 1-10, with 10 being the best shot they have ever hit. Brain activity in the one second before swing initiation was recorded for each shot as well. For each shot the golfer made in the target, they were awarded $5. After the 10 shots were hit, the golfer hit an 11^th shot. The 11^th shot had to land in the target if the golfer wanted to keep any of the money they just made. If they missed the 11^th shot, they lost all their earned money. Golfers then rated on a scale of 1-10 how anxious they felt over the 11^th shot, with 10 being the most anxious they have ever felt playing golf.

After the first baseline hitting task, golfers sat down and were fitted with the remaining sensors while they completed the State Trait Anxiety Inventory (STAI) survey. Then 5 minutes of resting biometrics was recorded. After the 5-minute rest period, we gave the golfer 10 minutes of auricular nerve stimulation (Figure 1, Figure 2, Table 1). Participants were instructed to increase the stimulation intensity until a comfortable setting, or the maximum output had been reached.

After the stimulation, golfers hit 10 more golf shots at the 150-yard target, with the same conditions as the first hitting baseline period. For each shot made in the target, they earned $5. They then hit an 11^th shot again and made the shot in the target in order to keep the money they have made throughout the entire study session. Golfers rated on a scale of 1-10 how anxious they felt over the 11^th shot.

After the 11^th shot, golfers were fitted with the biosensors as they completed the STAI once more and then we recorded 5 minutes of resting biometrics. After the 5 minutes, participants completed a questionnaire asking about their experience.

Figure 1: ABVN + GAN target for stimulation

Figure 2: Location of biphasic electrical stimulation of the ABVN + GAN over the tympanomastoid fissure

Data Analysis Approach

The Muse headset along with Opti Brain software was used to collect EEG at the FP1 and FP2 locations. EEG activity was only collected during the hitting task. During each shot, EEG was captured in the one second before the start of the swing. EEG activity was separated into frequency components (theta 4-7Hz, alpha 8-12Hz, beta1 13-20Hz, beta2 21-30Hz) using a Fast Fourier transformation, and the pre-stimulation values were compared with the post-stimulation values using a two-tailed independent t-test for all participants in the active and sham groups.

The Nexus system was used to record heart rate and HRV (2048Hz sampling rate) using electrodes placed on the chest, galvanic skin response (32Hz sampling rate) and skin temperature (32Hz sampling rate) using electrodes placed on left hand fingers, and EMG (2048Hz sampling rate) activity using electrodes placed on the upper left and right trapezius muscles. Blood pressure was monitored using a Life Source blood pressure cuff. Physiological activity was averaged in each of the 5-minute resting periods, and pre-stimulation and post-stimulation values was compared using a two-tailed independent t-test for all participants in the active and sham groups.

The Blast Motion sensor was used to collect swing tempo, swing speed, and swing time. Shot accuracy was measured by taking the score reported by the TopGolf scoring software. Each shot was given a score between 5 and 10, with 5 being in the outside circle of the target and 10 being in the center circle of the target. A score of 0 was used if the target was missed completely. Participants rated the feel of each shot on a scale of 1-10, with 10 being the best feeling shot ever and 1 being the worst shot ever. Both score and feel were averaged for all 10 shots in each hitting task, and pre-stim and post-stim values were compared using a two-tailed independent t-test for all participants in the active and sham groups. A chi-squared test was used to compare the shot made outcome in the pressure shot scenario in both groups.

The State-Trait Anxiety Inventory survey was used to measure anxiety throughout the study. The S-Anxiety scale (STAI Form Y-1, Spielberger 2010) “consists of twenty statements that evaluate how respondents feel ‘right now, at this moment.’ Scores on the S-Anxiety scale increase in response to physical danger and psychological stress and decrease as a result of relaxation training. The S-Anxiety scale has been found to be a sensitive indicator of changes in transitory anxiety experienced by clients and patients in counseling, psychotherapy, and behavior-modification programs. The scale has also been used extensively to assess the level of S-Anxiety induced by stressful experimental procedures and by unavoidable real-life stressors such as imminent surgery, dental treatment, job interviews, or important school tests”. Pre-stim and post-stim STAI values were averaged in both active and sham groups and were compared using a two-tailed independent t-test.

3. RESULTS

Performance Metrics

The only significant change in motion or performance measures was in the feel of the golf swing. The golfers rated the feel closer to 10 in the VNS condition compared to the sham condition (Table 2). There were no significant changes in any of the other performance metrics during both active and sham groups.

The average baseline score, swing speed, and shots made were higher in the active group, but there was no significant change in score, swing speed, and shots made post stimulation in either group. A two-tailed independent t-test (pre stim, post stim) revealed that feel of each shot was significantly improved in the active group only (p = 0.05).

Pressure Shot Performance

Of the 10 golfers in the active group, 2 golfers did not make any shots and therefore did not have a pressure shot to hit. Of the 8 golfers in the sham group, three did not make any shots and therefore did not have a pressure shot to hit. Two-tailed independent t-tests revealed there were no significant changes in feel, score, swing tempo, swing time, swing speed, and anxiety over the shot between baseline and post stimulation in both active and sham groups. A chi-squared test revealed no significant change in shots made in the pressure shot scenario in both groups.

Physiological Metrics

In the active group, a two-tailed independent t-test revealed a significant reduction (36% lower) in STAI scores between baseline and post stimulation timepoints (p<0.05). There were no other significant changes in physiological activity between baseline and post stimulation timepoints in the active group.

In the sham group, two-tailed independent t-tests revealed a significant reduction in Systolic and Diastolic blood pressure values (p<0.05). There were no other significant changes in physiological activity between baseline and post stimulation timepoints in the sham group. 

EEG Activity – FP1 and FP2

EEG activity was measured for each shot at locations FP1 and FP2 and only the data during the 1 second before the start of the swing was used for data analysis. A Fast Fourier transform using the Opti Brain application was used to separate the EEG signal into its frequency components. Two-tailed independent t-tests did not reveal any significant changes in activity over FP1 and FP2 locations between baseline and post stimulation timepoints in either active or sham groups. One participant in the sham group had poor signal quality during each hitting task and therefore did not contribute to the data analysis.

EEG Synchrony – FP1 and FP2  

EEG synchrony activity was measured over locations FP1 and FP2 and only the data during the 1 second before the start of the swing was used for data analysis. Synchrony activity was quantified using synergy, or coherence across theta, alpha, and beta frequencies over each electrode. Two-tailed independent t-tests did not reveal any significant changes in synchrony activity over FP1 and FP2 locations between baseline and post stimulation timepoints in either active or sham groups. One participant in the sham group had poor signal quality during each hitting task and therefore did not contribute to the data analysis.

4. DISCUSSION

Neuromodulation is a promising technology that could provide effective drug-free anxiety relief and performance enhancement for many types of athletes and other people in general. This study aimed to quantify a novel method of peripheral nerve stimulation on physiological activity and hitting performance of healthy golfers. The data did not support the hypotheses that ABVN + GAN stimulation would cause a significant change in hitting performance or physiological activity, but the stimulation did cause a significant increase in feel and a significant decrease in anxiety levels.

Effect on Performance

The data does not support the hypothesis that ABVN + GAN stimulation would cause a significant change in performance metrics, except for feel, compared to a sham. In the sham group, there were no significant changes in any of the performance metrics, and there were no apparent trends in the data. In the active group, there was no noticeable change in score, swing tempo, swing time, and the number of shots made. Swing speed in the active group was reduced by 2mph, suggesting there may have been a relaxation effect after stimulation, but the change was not significant.

While not a direct indicator of performance, feel was significantly increased after ABVN + GAN stimulation in the active group only (Pre: 6.83, Post: 7.43, p=0.05, effect size=0.83). Feel of the shot is not dependent on the outcome of the shot, but rather how the shot literally felt. Higher feel shots typically result in better outcomes, as golfers can learn to associate good shots with the right feel, but it is not always the case. Past studies looking at how neurofeedback training affects putting performance have demonstrated significant improvements in performance, but non-significant improvements in feel (Crews, 2016. Gook, 2018). Considering how the golfers in this current study in the active group had higher baseline performance scores than in the sham group, it was unlikely there would be a significant increase in performance. However, the increase in feel in the active group only is indicative that ABVN + GAN stimulation may be an effective treatment for golfers who have decreased feel due to performance anxiety, and therefore may improve performance during moments of high anxiety. One would expect that as feel increases, state anxiety would decrease. Data from this study revealed a correlation coefficient of -0.6 between post stimulation feel and STAI scores in the active group, which suggests that if ABVN + GAN stimulation increases feel, it would decrease state anxiety.

There were no significant changes in pressure shot performance metrics in either active or sham groups. In the active group, there was an increase in feel (pre: 6.88, post: 7.13) and a decrease in subjective anxiety levels (pre: 5.75, post: 5), but these changes were not significant. However, there were a number of participants that did not complete the pressure shot. In the active group, there were only 8 golfers that performed well enough to hit a pressure shot, and in the sham group, there were only 5 golfers. It is likely that the length of time between receiving the treatment and hitting the pressure shot was the reason there was no significant change in pressure shot outcomes. The acute anxiolytic effects of the stimulation may have faded by the time the golfers had to hit the second pressure shot. There was a decrease in pressure shot anxiety in the active group (Pre: 5.75 ± 1.58, Post: 5.00 ± 1.51) and an increase in pressure shot anxiety in the sham group (Pre: 3.00 ± 2.55, Post: 3.20 ± 2.17), but the changes were not significant. ABVN + GAN stimulation may have the greatest effect on performance immediately after treatment is finished. However, it is difficult to make any reasonable conclusions about the effect of ABVN + GAN stimulation on performance under pressure as there were very few participants that completed the pressure shot.

Effect on Psychological Activity

The data suggests that ABVN + GAN stimulation has a significant effect on psychological activity by reducing state-anxiety levels. In the active group, there was a significant change in STAI score (Pre: 41.10 ± 14.65, Post: 26.10 ± 4.09, p<0.05). According to the State-Trait Anxiety Inventory for Adults – Manual, scores between 20-37 are considered “Mild”, scores between 38-44 are considered “Moderate”, and scores between 45-80 are considered “Severe”. Patients with “Severe” levels of anxiety would be considered clinically anxious, and patients with “Mild” levels of anxiety would be considered healthy. Therefore, clinically meaningful reductions in anxiety should reduce STAI scores from “Severe” or high “Moderate” to low “Moderate” or “Mild” levels. STAI scores were reduced in the sham group, but the change was not significant. The STAI score reduction may have been greater in the active group because the baseline STAI scores were 6.35 points higher than the baseline scores in the sham group (41.10 ± 14.65 in the active group compared to 34.75 ± 13.58 in the sham group); however, the difference in baseline scores between the two groups was not significant.

Past research using the STAI to assess acute changes in anxiety levels indicate treatments are more likely to be effective the higher the baseline state anxiety is. One study observing the effects of a 20-minute mindfulness technique in 20 individuals reported a change in STAI from 49.6 points to 29.5 points in the active group, compared to 47.9 points to 47.3 points in the sham group (Pawlow, 2003). Another study testing a 30-minute mindfulness technique in 24 individuals reported significant changes in STAI scores from 39.3 points to 29.2 points in the active group and 37.8 points to 30.4 points in the sham group (Knowlton, 2006). No studies have looked at the effects of ABVN + GAN stimulation on changes in STAI scores, but one study looking at the effects of earlobe stimulation for 60 minutes in 17 individuals reported a change in STAI scores from 34.9 points to 38.9 points in the active group and 31.5 points to 34.6 points in the sham group (Hill, 2015). Past research has shown that high-anxious individuals show bigger reductions in anxiety levels compared to low-anxious individuals. This suggests that ABVN + GAN stimulation may be more effective at reducing state-anxiety the higher a person’s state-anxiety score. If a person is only feeling mildly anxious, there likely will not be any clinically meaningful anxiolytic effect from ABVN + GAN stimulation.

Effect on Physiological Activity

There were no significant changes in heart rate between baseline and post stimulation in either group. This may not necessarily mean that ABVN + GAN stimulation has no effects on heart rate because heart rate was likely higher to account for the physical motion of hitting the golf shots. It was also a relatively hot day and the study was conducted outside, so it is likely that heart rate would tend to stay higher to account for the heat of the day and the physical nature of the hitting task. As none of the participants reported severe levels of anxiety, it would be interesting to observe the effects of ABVN + GAN stimulation on severely anxious golfers with higher resting heart rates. The data does, however, provide evidence that bilateral ABVN + GAN stimulation does not have adverse effects on the heart such as causing bradycardia.

Heart Rate Variability is a common method of analyzing cardiac activity that involves measuring the time differences between successive heartbeats (RR intervals). Changes in HRV activity are often associated with changes in human performance, and therefore, HRV is a common metric to be measured in sport studies. This study analyzed the HRV time component root of the mean squared differences of successive RR intervals (RMSSD) and the frequency components very low frequency (VLF, <0.04Hz), low frequency (LF, 0.04Hz – 0.15Hz), high frequency (HF, 0.15Hz – 0.4Hz) and the low frequency-high frequency ratio (LF/HF). RMSSD is associated with short-term rapid changes in heart rate and would be expected in increase after successful vagus nerve stimulation (DeGiorgio, 2010). The HF component of HRV is associated with parasympathetic activity, and the LF is a measure of both sympathetic and parasympathetic activity. The ratio LF/HF has been proposed as an index of sympathetic to parasympathetic balance of heart rate fluctuation (De Couck, 2017. DeGiorgio, 2010). Successful stimulation of the vagus nerve should both increase the HF and decrease the LF components on HRV (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996).

There were no significant changes in any of the HRV metrics after ABVN + GAN stimulation. It was expected that ABVN + GAN stimulation would have similar effects as tVNS on HRV due to similar projections in the brain stem as the vagus nerve. It is known that the vagus nerve is the main nerve that controls HRV, but even stimulating the vagus nerve does not consistently produce a change in HRV as one would expect. De Couck et. al. 2017 found that stimulating the ABVN in the concha for 10 minutes did not produce any significant change in HRV metrics, and other studies looking at invasive VNS found that the method does not increase HRV (Setty, 1998. Jansen, 2011). Inconsistent effects on HRV may be due to indirect effects on the heart, as the GAN and ABVN are afferent branches that travel towards the brainstem, rather than directly to the heart.

Effect on Brain Activity

While there were no significant changes in EEG activity after treatment in either active or sham groups, there were some interesting observations. In the active group, power values in each frequency band hardly changed after stimulation at both FP1 and FP2 locations, whereas in the sham group, there appeared to be a trend of decreased activity over FP2 and increased activity over FP1. While not statistically significant, it is clear that EEG power over FP1 was higher in the sham group compared to the active group, and the sham group had a lower average hitting accuracy during both hitting tasks. Past sports studies have demonstrated that higher activity in the left hemisphere 1 second before the putt is associated with poor performance (Crews, 1993. Salazar, 1990), and while the ABVN + GAN stimulation did not cause an improvement in hitting accuracy, it also did not cause left hemispheric EEG activity to increase, which means it may have actually had an effect on hitting performance. However, the data does not support any claims that ABVN + GAN stimulation had any effect on EEG activity. Further research is needed to understand the effects of ABVN + GAN stimulation on EEG activity over FP1 and FP2 locations.

Synergy is another measurement of brain activity that describes synchrony of brain activity across theta, alpha, and beta frequency bands and it can be calculated from a single electrode or from multiple electrodes at once to quantify whole brain synchrony. Past research has demonstrated that high-performing business people have higher synergy levels in frontal brain structures (Harung, 2012), and EEG neurofeedback protocols that train to increase synergy in golfers have led to significantly increased synergy levels and improved putting performance (Crews, 2016 & 2018; Gook, 2018). This suggests that methods and devices that affect synergy levels may affect performance. However, data from this study did not show a significant effect on synergy levels. Gook et. al. showed a significant increase in synergy levels after neurofeedback training from 67.94 ± 3.94 percent to 72.67 ± 4.18 percent, which was correlated with improved performance, whereas baseline synergy values in this study were 65.41 ± 3.62 percent over FP1 and 64.74 ± 4.84 percent over FP2, with no significant change over either location. More research is needed to understand optimal synergy levels for peak performance.

Limitations of the Study

The main limitation of this study was the small sample size of golfers with a wide range of skill level. The study did not analyze the effect of ABVN + GAN stimulation on golfers of separate skill levels. There were no significant changes in brain activity or performance metrics other than feel in either active or sham groups, but there may have been significant changes in these metrics if observed strictly in an expert population or a novice population. Gold et. al. found that 20 minutes of tDCS did not improve video game performance of trained players but did improve performance of untrained players (Gold, 2019). Novice and untrained athletes have more room to improve than elite athletes, and therefore, it is more likely that a treatment would improve the physical performance in a novice population and have minimal effect on an elite population. However, feel is a major component of performance, and if ABVN + GAN stimulation improves feel, then it will likely have effects on performance of elite athletes, especially during high-pressure, real-world situations where anxiety is likely to negatively affect feel and performance.

Novice golfers have been shown to have higher levels of brain activity than expert golfers just before hitting a shot, especially in the posterior cingulate area, as they must process more sensory information than an expert to successfully execute a shot (Milton, 2007). Other studies have shown that lower motor skills in athletes correlate to higher activity in the posterior cingulate gyrus (Janke, 2000; Puttemans, 2005). A common cause of poor performance in expert golfers is overthinking and over-processing information before hitting a shot. Since ABVN + GAN stimulation has been shown to decrease activity in the posterior cingulate gyrus (Yakunina, 2017), it may produce a more noticeable effect on performance and brain activity in an expert population alone, as it may help them stop overthinking and return to an automatic state of performance.

It has been hypothesized that novices have greater activity in limbic areas in the brain compared to experts because they must exert more energy to process the golf swing (Milton, 2007). High activity in these areas may be essential if a novice wants to efficiently learn the golf swing, so ABVN + GAN stimulation may reduce the novice’s ability to learn the golf swing. However, it could also be argued that because ABVN + GAN stimulation has shown to decrease activity in the amygdala (Yakunina, 2017) and significantly reduces state anxiety, it could be an effective treatment for golfers of any level who experience anxiety before or during a round.

Regarding the nerve target, stimulation was applied on the side of the neck behind the earlobe and anterior to the mastoid process, and over a location called the tympanomastoid fissure. This location is known to contain branches of the GAN and the ABVN (Kiyokawa, 2014), but ABVN branches at this location are not as superficial as GAN branches, and may not truly be activated, hence the non-significant effects on cardiac-related metrics. fMRI studies similar to past protocols like Kraus et. al. (2007) should be performed to observe brain activity in regions known to have vagal projections to verify if stimulating at this location truly does activate the ABVN or just the GAN alone.

5. CONCLUSION

The results of this study demonstrate that ABVN + GAN stimulation at the tympanomastoid fissure did not have significant effects on hitting performance or physiological activity of healthy golfers, but it did significantly improve quality of feel for each shot and reduce state anxiety after a golf hitting task. Future research should assess the effects of ABVN + GAN stimulation in a novice or elite athlete populations alone and should include only severely anxious athletes. More research is needed to verify the anxiolytic effects of ABVN + GAN stimulation in athletes, but ABVN + GAN stimulation is a promising technology that could help all types of people reduce anxiety during stressful situations.

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