Motion sickness (MS) arises from a perceptual discordance between expected and actual motion, giving rise to symptoms such as fatigue, cold sweats, pallor, dizziness, nausea, disorientation, vomiting, etc. (Keshavarz and Golding, 2022). In contrast, Simulator Adaptation Syndrome (SAS) is a variant of MS encountered within virtual simulators. Although the reported occurrence rates and severity level can vary significantly, it is important to note that premature termination rates during virtual reality sessions due to SAS can reach as high as 60% (Saredakis et al., 2020). Different theories have emerged to elucidate the origins of MS and SAS, such as postural instability theory (Riccio and Stoffregen, 1991) or the sensory conflict theory (Reason and Brand, 1975), where the etiology of SAS is described in terms of a conflict between the motion imposed by the simulator and the body’s natural balance (postural instability theory), or because a discrepancy between the individual’s prior experiences and the simulator’s visual, vestibular, or somatosensory feedback (sensory conflict theory).

Different studies have described countermeasures to reduce MS and SAS symptoms (see Keshavarz and Golding, 2022 for a current review of MS countermeasures) such as biofeedback and cognitive methods (Bos, 2015; Yen Pik Sang et al., 2003) or drugs (Lucot, 1998). Recently, using two countermeasures conjointly to reduce MS and/or SAS has proven highly effective. For example, Bos (2015) found that the use of head vibration and mental distraction reduced MS symptoms by 39% (although the research was not specific to SAS). In addition, it was found that vibration in isolation only reduced MS symptoms by 25% and mental distraction by 19%. Gálvez-García et al. (2015) studied the combination of Galvanic cutaneous stimulation (GCS) and auditory stimulation for SAS, finding an additive effect between both techniques with a 73% reduction of symptoms about the baseline condition, compared to lower effectiveness when both techniques were applied separately (50% for GCS and 48% for auditory stimulation). The authors concluded that both techniques additively improved body balance (i.e., measured through head movements) with a direct correlation with SAS symptoms. Moreover, the driving performance variables were not impaired, which is crucial to recommend these techniques to prevent SAS. Similarly, Gálvez-García et al. (2020a,b) studied the combination of GCS and tactile stimulation for SAS. They concluded that both techniques simultaneously decreased SAS more effectively (78%) than individual administration (i.e., 49 and 48% for GCS and tactile stimulation). The authors stated that GCS and tactile stimulation were additive to reducing SAS symptoms but did not find an additive effect on body balance. Whereas GCS improved body balance, tactile stimulation did not, concluding that the reduction in SAS for this technique was due to the distraction from its symptoms.

The above paragraph supports the notion of the advantage of using two countermeasures to reduce SAS. Moreover, the effectiveness of GCS in mitigating SAS has been widely tested. However, recent research (Espinoza-Palavicino et al., 2023) stated that Transcutaneous Vagal Nerve Stimulation (tVNS) showed better results at decreasing SAS than GCS (along with a better improvement in body control and driving performance variables).

The features of these techniques are outlined below. On the one hand, GCS stimulates (below the motor threshold) the cutaneous nerve fibers of the sternocleidomastoids by placing the electrodes 3–4 cm below the mastoid process. This area contains a high concentration of sensitive subcutaneous fibers (Lazorthes, 1981) that target the temporoparietal junction (Pérennou et al., 2001). The temporoparietal junction is involved in the processing of vestibular and visual information (Donaldson et al., 2015; Ventre-Dominey, 2014). This area is comparable to the parietal-insular-vestibular area identified in monkeys (Grüsser et al., 1990; Sugiuchi et al., 2005), which contains neurons that respond to somesthetic stimulation, particularly from the neck, in addition to visual and vestibular input. Thus, GCS would target these neurons at the temporoparietal junction, providing the central nervous system with information about the head and trunk position in space (Grüsser et al., 1990). In this line, GCS has been shown to improve balance in patients with neglect (Guariglia et al., 2000) and in fixed simulators where vestibular information is absent (e.g., Gálvez-García et al., 2020a,b). In addition, the impact of anodal transcranial direct current stimulation on the temporoparietal junction in virtual reality sickness has been directly tested (Takeuchi et al., 2018), where improvements in body balance and sickness were found. On the other hand, tVNS is an electrical stimulation technique that enables the direct stimulation of the vagus nerve via the acoustic meatus of the outer ear (Farmer et al., 2021). This ultimately results in the stimulation of the nucleus tractus solitarus and locus coeruleus, which leads to a significant release of gamma-aminobutyric acid (GABA) and norepinephrine (Warren et al., 2018). An increase in the levels of these neurotransmitters has been observed to result in enhanced cortical activity in several different structures, including the cerebellum and frontal lobes (Badran et al., 2018). In contrast to GCS, no direct relationship has been demonstrated between tVNS and temporoparietal junction activation in improving balance. Without ruling out this relationship (see, e.g., Fang et al., 2016, where tVNS modulates temporoparietal function), other possible mechanisms may influence the improvement of body balance by tVNS, such as activation in the nucleus tractus solitarus and locus coeruleus, which transmit afferent sensory information through connections with the thalamus, orbitofrontal cortex and medulla (George et al., 2000, Sigurdsson et al., 2021). In this vein, various studies have found that tVNS improves body balance (i.e., gait parameters) in people with Parkinson’s disease (Marano et al., 2022; Zhang et al., 2023), which is consistent with Espinoza Palavicino’s research finding improved body balance (along with reduced SAS symptomatology). Furthermore, tVNS can enhance concentration and cognitive flexibility (Borges et al., 2020; Colzato et al., 2018). This may mitigate the symptoms of anxiety and stress associated with SAS by focusing the participant’s attention on the task at hand.

In this context, we want to determine the effectiveness of tVNS and GCS in conjunction by decreasing SAS, having regard to the following as a base: (a) the effectiveness demonstrated by tVNS individually (e.g., Espinoza-Palavicino et al., 2023), and (b) the effectiveness of GCS individually (e.g., Gálvez-García et al., 2015) and in conjunction with other techniques (e.g., Gálvez-García et al., 2020a,b). Furthermore, body balance will be measured in both GCS and tVNS conditions through head balance. Finally, driving performance variables will be measured to test the impact of the combination of GCS and tVNS on driving behavior. Importantly, we will also measure the effectiveness of tVNS and GCS in isolation as a baseline to establish the hypothesized improvement of both techniques in conjunction. Indirectly, this will also provide further evidence for the previous results of Espinoza-Palavicino et al. (2023) about the effectiveness of GCS and especially tVNS in isolation. In short, we hypothesize that using both interventions in conjunction will positively affect the body balance, reducing SAS and increasing driving performance compared to administering both interventions in isolation.

Espinoza-Palavicino et al. (2023) stated the benefit of tVNS and GCS to decrease SAS in a driving task when both techniques were applied individually, with better results in the case of tVNS (i.e., less SAS symptomatology with a better body balance leading to better driving performance). Following this study and previous evidence that supports the use of different techniques together to alleviate SAS (e.g., Bos, 2015; Gálvez-García et al., 2020b), the main aim of this study was to test the effectiveness of tVNS and GCS in conjunction at mitigating SAS in a driving task. These two techniques were administered concurrently and individually in active and sham conditions and evaluated in contrast to a control situation where no stimulation was given.

The data pattern confirms that active GCS + tVNS condition decreased SAS more effectively when both techniques were administrated conjointly (93%) than in isolation (69% for active GCS and 83% for active tVNS). In this context, the importance of SSQ scores should be carefully weighed. Stanney et al. (1997) stated that SSQ scores may be linked with negligible (<5), minimal (5–10), significant (10–15), and concerning (15–20) symptoms. Any value >20 means a critical symptomatic factor. This means that participants in the no stimulation condition (i.e., baseline) show high SAS symptomatology (67.05 SSQ scores). However, this is not surprising since previous research where the same driving task was used also showed high SSQ scores in baseline conditions (e.g., Espinoza-Palavicino et al., 2023; Gálvez-García et al., 2020b). This is due to a demanding driving task because participants had to maintain high speeds in the curves, which would facilitate SAS symptoms. As previously stated, this manipulation was fundamental to homogenizing driving variables among participants (since varying speeds, particularly on curves, could interfere with the sickness ratings). In contrast, the isolated administration of active GCS and tVNS reflect SSQ total scores of 21 and 12, respectively. This reflects that although active GCS is a good technique for reducing SAS, it still states severe symptoms. However, active tVNS applied in isolation reflect scores below 15, which means significant symptoms, although not as severe as in active GCS. Importantly, this data pattern also provides further evidence for the previous results of Espinoza-Palavicino et al. (2023) about the remarkable effectiveness of active tVNS in isolation. Thus, our results confirm the administration of tVNS as an effective method to reduce SAS with better results than GCS.

More importantly, active GCS + tVNS condition result in SSQ scores of 4.27. This reflects negligible SAS symptoms, showing a better SSQ reduction than active tVNS and GCS applied in isolation. This confirms our central hypothesis about a better reduction in SAS symptoms when GCS and tVNS were applied concurrently. Moreover, using two techniques to decrease SAS confirms previous literature where different techniques were combined (e.g., Bos, 2015; Gálvez-García et al., 2020a). In this sense, it is essential to highlight that applying active GCS and tVNS together has obtained a 93% reduction of SAS symptoms compared to baseline, which shows greater effectiveness than previous research. As previously stated in the Introduction, the combination of GCS and auditory stimulation reached a 73% reduction in symptoms (Gálvez-García et al., 2015). In a similar vein, the combination of GCS and tactile stimulation showed a 78% reduction (Gálvez-García et al., 2020a). However, although effective, the reported reduction in SAS symptoms was still significant (i.e., scoring above 10 on SSQ total scores). Thus, it can be suggested that the additive use of active GCS and tVNS with negligible symptoms (i.e., scoring under 5 on SSQ total scores) in comparison to GCS and auditory stimulation (Gálvez-García et al., 2015) or GCS and tactile stimulation (Gálvez-García et al., 2020b). Finally, it should be noted that previous research shows large individual differences in susceptibility to simulator sickness. Therefore, SSQ scores often reported larger variations than our results (e.g., Keshavarz et al., 2018). The small variation in SSQ scores in the present study might have been because participants with scores above 65 (75th percentile) or 0 on the Motion Sickness Susceptibility Questionnaire were excluded from the study due to their heightened susceptibility or disinclination to experiencing SAS in the same order. This may have homogenized the differences in susceptibility to simulator sickness. In addition, it may explain the small variation in SSQ scores in previous research with the same inclusion criteria (e.g., Espinoza-Palavicino et al., 2023).

Although we did not address SAS theories in this research, some considerations must be highlighted. Improvements in SAS symptoms due to active conditions in GCS and tVNS are linked with enhanced postural control, as indicated by reduced head movement in both the X-and Y-axes. In this way, the improvement of SAS due to an improvement in postural adjustment could support the postural instability theory (Riccio and Stoffregen, 1991). However, the reduction of symptoms in sham conditions of GCS and tVNS was not based on improving balance ability (i.e., no differences in head sway between sham conditions and baseline). This supports previous studies claiming that the impairment of balance ability is not the only mechanism underlying SAS (Bos, 2015; Keshavarz et al., 2015). More importantly, it supports different approaches to the mechanisms and mitigation of SAS, such as the distraction from symptoms awareness (Reason and Brand, 1975), in line with previous research where different distractors are effective in mitigating SAS (e.g., D’Amour et al., 2017; Gálvez-García et al., 2017). In any case, it should be noted that although effective, the reduction of SAS symptoms in sham conditions still reflected severe symptoms, with active conditions (where there was a positive impact on body balance) being much more effective in reducing SAS symptoms. Moreover, our findings reinforce the idea that SAS is a complex phenomenon influencing body balance and other elements like attention.

Some additional considerations regarding the processes by which active tVNS and GCS reduced SAS should be explained. As noted in the Introduction, the critical and main factor behind the enhancement in SAS may be attributed to the improved body balance from tVNS. However, active tVNS and GCS might also have distracted the participants from the symptoms in a similar way to sham conditions. Moreover, as stated in the Introduction, tVNS could improve different attention-related processes, such as concentration and cognitive flexibility, in line with previous research (Borges et al., 2020; Colzato et al., 2018). This could decrease SAS symptoms by focusing the participant on the task to be performed (i.e., driving) and distracting from the symptoms.

All sham and active conditions for GCS and tVNS improved driving performance in the demanding driving task (i.e., speed constraints), as evidenced by faster speeds accompanied by increased steering movements consistent with prior research with tVNS and GCS (e.g., Espinoza-Palavicino et al., 2023; Gálvez-García et al., 2020b). This improvement is reflected in increased speed and more steering movements, where this driving pattern is optimum for the task. Overall, the results in driving performance confirm the central hypothesis of this study; among the tested conditions, active GCS + tVNS delivery is the most effective technique to improve SAS symptoms and, by extension, driving performance, which is fundamental for its recommendation for future interventions to reduce SAS.

The improvement in driving performance variables negatively correlates with SAS symptoms, reflecting that more conservative driving arises from SAS (Helland et al., 2016). Thus, the increase in driving performance variables could be merely interpreted as a reduction in SAS symptoms. However, other cognitive factors positively influenced by tVNS could have an impact on the motor improvement of driving performance, such as motor inhibition (Jongkees et al., 2018), alertness (Raedt et al., 2011), associative memory (Roosevelt et al., 2006), concentration and cognitive flexibility (Borges et al., 2020; Colzato et al., 2018; both also highlighted as mechanisms that decrease SAS in this discussion), and multitasking (Sommer et al., 2023).

Although the results of our research are promising, future research is needed to generalize our results and to overcome some of the limitations of this research. First, active GCS + tVNS has been proven suitable for decreasing SAS in a demanding driving task where participants must maintain high speeds in the curves, leading to increased SAS symptoms. Thus, exploring different demanding driving tasks and speed constraints should be recommended to extend the applicability of our findings. Second, future research with special populations prone to develop SAS (e.g., elderly drivers; Keshavarz et al., 2018) would help to generalize our results and the overall impact of both techniques in SAS. Third, subsequent research could also employ real-time measurements such as skin conductance (Gavgani et al., 2017; Mwange et al., 2022). Fourth, active tVNS and GCS have been tested as suitable countermeasures to alleviate SAS and improve driving performance. However, other studies focused on the efficacy of tVNS in cognitive processing in driving simulators could obtain results biased by the efficacy of tVNS in SAS. Thus, this factor should be considered an explanatory element (or at least to control it). Fifth, it has been corroborated that active tVNS produces less head movement, decreasing SAS and thus leading to an improvement in driving performance variables. However, the corroboration of the mechanism of SAS reduction by tVNS (i.e., the mentioned relationships) should be further explored by examining mediation and causality (i.e., directional effect). For this purpose, we recommend using structural equation modeling with a large sample size. Sixthly, given the reduction in head movements demonstrated by tVNS and GCS, their utilization may alleviate other forms of motion sickness. This is because individuals prone to motion sickness have been observed to exhibit a greater range of head movements, particularly when traveling in vehicles such as ships, aeroplanes, or cars (Iskander et al., 2019). This is exemplified by carsickness, which is caused by varying lateral accelerations (Kuiper et al., 2018). In this regard, it is recommended that further research be conducted to confirm the benefits of tVNS and GCS in addressing carsickness. Especially in the context of autonomous vehicles, where users may experience variations in acceleration accompanied by the performance of different activities that result in greater head movements in various directions relative to vehicle motion, potentially leading to an increased incidence of motion sickness. Finally, we want to stress that future research lines could test combining more than two techniques to alleviate SAS. For example, tVNS and GCS with auditory stimulation (e.g., Gálvez-García, 2015), habituation and gradual exposure to the simulator which are highly effective (e.g., Heutink et al., 2019), and pleasant olfactory stimulation (Keshavarz et al., 2015) among other countermeasures.

In summary, our results support that tVNS and GCS, when applied jointly, are particularly effective in reducing SAS in a fixed-base simulator, improving driving performance variables. We conclude that SAS symptom reduction is due to an improvement in neuromodulation of motor control that impacts body balance and the driving task itself. All this, along with a distraction of the SAS symptoms and a possible enhancement in other cognitive mechanisms such as concentration, cognitive flexibility, and multitasking. Overall, an effective ergonomic tool for minimizing SAS is proposed. Therefore, we recommend using tVNS and GCS in conjunction to alleviate SAS in future interventions.