A review on motion sickness of autonomous driving vehicles

2.1. Mechanism of motion sickness

Motion sickness was first documented over 2,000 years ago by the Greek physician Hippocrates, who observed that “motion disturbs the body when sailing at sea” [14]. This condition can manifest in various environments such as land, sea, air, space, and simulators. Common symptoms of motion sickness encompass nausea, vomiting, sweating, fatigue, disorientation, dizziness, and loss of coordination (Table 1). Humans rely on multiple sensory systems for spatial orientation [15], including proprioceptive input from limbs and torso movements; vestibular input from the perception of balance through the vestibular system; and visual input for establishing a visual framework [16]. These three interdependent sensory systems coordinate to form a spatial reference system. Disruption of this system can lead to discomfort. Currently regarded as a natural response to an unnatural environment, motion sickness is commonly explained by theories such as sensory conflict theory [17], evolutionary theory [18], and postural instability theory [19].

Sensory conflict theory: According to the sensory conflict theory, motion sickness arises from a discrepancy between the internal sensory system and the external visual system. When there is an inconsistency between bodily sensations of movement and visual perception, it can trigger symptoms such as nausea, dizziness, and vomiting. For instance, in a boat or car, motion sickness occurs when the internal sensory system detects body movement while the external visual system fails to recognize this movement, resulting in a conflict between these two systems.

Evolutionary theory: The evolutionary theory posits that motion sickness is a natural response developed through human evolution. This theory suggests that symptoms of motion sickness served as an adaptive genetic pattern for our ancestors to avoid ingesting potentially harmful substances. The human inner ear and balance system are sensitive to detecting and responding to potential toxins, leading to symptoms of motion sickness that compel individuals to steer clear of hazardous situations.

Postural instability theory: Postural instability theory states that prolonged movements cause postural instability which leads to motion sickness. For example, after extended periods of travel by car, boat, or plane, the body's balance system gradually adapts itself to continuous movement; however, when this movement ceases abruptly, the balance system remains in an adapted state causing symptoms of motion sickness. This theory explains motion sickness as maladjustment and imbalance within the postural system.

Motion sickness is a significant concern in the field of vehicle engineering due to its substantial interindividual variability and potential impact on the comfort and safety of vehicle occupants. Sensitivity to motion sickness varies considerably across different age groups, with infants generally exhibiting lower sensitivity, while susceptibility begins in children aged 6-7 years and peaks at age 9 years [20]. the likelihood of experiencing motion sickness also varies significantly between genders, with women having a notably higher incidence compared to men [21]. It has been estimated that women are at least three times more likely than men to report being affected [22]. Speculations have arisen regarding the potential relationship between motion sickness occurrence and the female menstrual cycle [23]. Studies have indicated that hormone fluctuations may be associated with symptoms of motion sickness, suggesting that factors influencing this condition could potentially be linked to hormonal changes during a woman’s menstrual cycle [24]. Various factors influence motion sickness (such as age, gender, eating habits, and low-frequency vibration), some of which exhibit high individuality as depicted in Fig. 1.

Fig. 2Factors affecting motion sickness

The close correlation between low-frequency vibration and motion sickness has been widely acknowledged by scholars. Low frequency vibration has a significant impact on the degree of motion sickness [25], especially low frequency vibration below 1 Hz [26], especially low frequency vibration below 0.5 Hz [27], which is more likely to cause motion sickness. Griffin et al. [28], pioneers in the field of motion sickness research derived a mathematical model for motion sickness through vibration experiments and summarized the relationship between vibration direction, frequency, amplitude, and motion sickness. Furthermore, Lawther introduced the frequency domain weighted representation method [29] to establish a link between the influence of motion sickness and sensitivity to low-frequency vibrations, thereby laying an important foundation for quantitative evaluation and prediction of motion sickness. These findings have been standardized in ISO 2631-1(1997) [30] and ISO 6841(1987) [31], which also consider the impact of human posture. Building upon this knowledge base, single-degree-of-freedom [32] and multi-degree-of-freedom [33] mathematical models related to the local activities of drivers and passengers have been further investigated.

2.2. Sickness in autonomous vehicles

Autonomous vehicles bring about changes in the driver’s role to passengers, especially driving behaviors such as rapid acceleration, rapid deceleration, and sharp turns, making passengers experience unaccustomed motion stimuli [34]. Large vibration acceleration not only reduces riding comfort but also easily causes motion sickness symptoms [35]. in terms of motion direction, the sensitivity of the human body to horizontal motion is slightly greater than that of vertical and pitching motion [36], that is, in complex motion with different motion directions but the same other elements (including multiple motion directions), the contribution rate of motion sickness is mainly due to horizontal vibration [37].

In the journey of autonomous vehicles, motion sickness is not only affected by vehicle acceleration and low-frequency vibration. As passengers have more options for free movement, this can also lead to visual stimulation and head tilt, producing VIMS and OKMS, which are also important factors in motion sickness [38]. The passengers of autonomous vehicles have more freedom of choice, which can exacerbate head movements, which increases the likelihood of motion sickness [39]. The main causes of dizziness are related to vestibular organ dysfunction or lack of coordination with other sensory systems [40]. The vestibular organs are a group of sensory organs located in the inner ear that detect the position, movement, and acceleration of the head. Vertigo may occur when the perception of motion received by the vestibular organs conflicts with the perception of motion received by other sensory systems, which is shown in Fig. 3.

Fig. 3Diagram of the vestibular system principles

Passengers experience different types of motion sickness caused by visual motion scenes or simulated movements in a virtual environment. In autonomous vehicles, passengers have more time for their own activities, especially for entertainment or work. When passengers perform these activities during the journey, the head tilt causes Optical dynamic motion sickness (OKMS); and when the vision is stimulated, it produces Visually Induced Motion Sickness (VIMS) [41], which is considered a precursor to motion sickness. in autonomous vehicles, these two types of motion sickness are the main types in autonomous vehicles due to the increased autonomous activities of passengers. VIMS usually occurs when there is little or no physical movement. The eye tracker system is associated with continuous low-frequency head movements, which can maintain the image stability of the retina [42]. Nystagmus is the involuntary, rapid, repetitive movement of the eye, and the movement of large visual scenes induces visually-induced nystagmus, leading to convection and VIMS as shown in Fig. 3. in self-driving cars, VIMS is unlikely to occur because of the actual physical motion. However, VIMS should be considered when using virtual reality (VR) as an entertainment option for self-driving cars or to alleviate motion sickness. in order to mitigate motion sickness using an immersive approach, the movement of the virtual environment must be accurately synchronized with the actual movement of the vehicle, thus minimizing sensory conflict. Any inconsistency between sensory inputs increases the risk of motion sickness. a major challenge for autonomous vehicles is the limited availability of external visual stimuli, so eliminating sensory conflict in virtual and real environments is difficult. Optical dynamic motion sickness (OKMS) occurs when the head is rotated relative to the visual scene under the influence of the pseudo-Coriolis effect (PCE) [43]. The Coriolis effect (PCE) refers to a type of motion sickness that occurs when the head is tilted during body rotation [44]. The perceived rotation is caused by the rotation of the visual scene around a vertical axis, while the subject remains relatively still with respect to the scene [45]. Thus, perceived self-motion and head tilt can lead to PCE, resulting in motion sickness in virtual environments [46]. in self-driving cars, OKMS do not exist because passengers rarely participate in the spinning motion [47]. However, when passengers read or use electronic devices, they can encounter PCE due to a downward tilt of the head [48].

Fig. 4VIMS schematic diagram

3.1. Subjective evaluation method

Subjective evaluation methods mainly include questionnaire survey and symptom diagnosis.

In 1975, Golding proposed a Motion Sickness Susceptibility Questionnaire (MSSQ). This questionnaire assesses an individual's susceptibility to motion sickness based on whether the individual has experienced motion sickness in real life and the frequency of its occurrence [49]. in 2006, the MSQ was simplified, and the accuracy of the simplified MMSQ was proved to be no less than that of the original, and the simplified MSSQ is now mainly used [50]. MSQ is easy to operate, economical easy to implement, and has a certain accuracy. in response to motion sickness caused by VIMS, Kennedy modified the MSSQ survey form and established the SSQ survey form to enhance the ability to identify “problem” simulators and improve diagnostic capabilities [51].

However, the two questionnaire methods for appeals are based on empirical approaches following the onset of motion sickness, lacking a real-time evaluation method. To address this gap, Keshavarz et al introduced the Rapid Motion Sickness Scale (FMS) [52] to enable real-time measurement and rapid assessment of motion sickness [53]. Participants were asked to verbally rate the severity of their motion sickness on a 20-point scale at regular intervals. Nevertheless, FMS does not play a pivotal role in assessing motion sickness and has certain limitations [54].

The diagnosis of symptoms primarily relies on the observation of clinical manifestations and signs in patients to determine the severity of motion sickness. The evaluation criteria mainly encompass vomiting, vertigo, and postural stability [55]. Currently, a prevalent method for symptom diagnosis involves comprehensive assessment based on diverse symptoms and signs associated with human motion sickness.

The subjective evaluation method possesses the advantages of convenience, cost-effectiveness, and widespread adoption, making it a continuously evolving approach. However, its ability to accurately predict the nonlinear changes in motion sickness severity over time remains limited.

3.2. Physiological signals

The objective measurement method aims to assess the severity of motion sickness by quantifying individual physiological indicators.

Electrogastrogram (EGG) is utilized for detecting alterations in stomach activity during the onset of motion sickness [56]. EGG changes are highly sensitive when motion sickness occurs (EEG activity increases significantly), thus it can be employed to assess the severity of MS symptoms [57]. However, EGG alone cannot serve as an indicator of MS severity due to its susceptibility to various factors. More accurate outcomes can be achieved by integrating other measurement techniques.

Heart rate variability (HRV) refers to the extent of fluctuation in intervals between consecutive heartbeats, which is assessed by analyzing the temporal gap between adjacent heartbeats. This physiological parameter serves as an indicator of the autonomic nervous system's functional equilibrium. Motion sickness induced by vestibule-visual conflict has been associated with an elevation in HRV [58], wherein increased sympathetic nerve activity and decreased vagus nerve activity are observed with escalating severity of motion sickness [59].

Electroencephalogram (EEG) is the electrical signal generated by the activity of neurons in the brain. with the occurrence of motion sickness, the central motor area, sensory area, and visual area showed a trend of increasing $\theta$ power spectral density centroid frequency in EEG signals [60], while $\alpha$ and $\gamma$ power in the motor area, parietal lobe and occipital lobe also increased significantly, and the increase was positively correlated with the degree of individual discomfort [61]. Therefore, EEG provides an objective reference for evaluating the severity of motion sickness.

Postural stability assessment is often used to measure an individual's ability to control and balance their body in different postures, which is crucial to understanding the mechanism of motion sickness. When visual and sensory feedback were excluded, postural stability predicted the severity of motion sickness. After the journey, the more severe the motion sickness, the worse the postural stability [62]. Patients with visually induced motion sickness, due to excessive visual input, are more prone to symptoms of motion sickness [63], which may include vertigo, dizziness, nausea, and other symptoms, because excessive visual information may conflict with the input of other sensory systems, aggravating the performance of motion sickness [64]. It is worth noting that there are significant individual differences in the assessment of postural stability.

The Vestibulo-Ocular Reflex (VOR) is a physiological mechanism that regulates eye movement to maintain visual stability by perceiving and compensating for head movements [65]. Motion sickness often arises from dysfunction in the vestibular system, making VOR testing crucial for assessing vestibular function and evaluating the adequacy of eye responses during head movements [66]. in patients presenting with motion sickness symptoms, alterations in the vestibular eye movement reflex may manifest as follows: 1. Abnormal eye tracking: Eye tracking may become erratic or inaccurate when the patient moves their head, resulting in unstable visual images due to ineffective eyeball following [67]. 2. Delayed or inadequate response: Normally, VOR promptly adjusts eye position to counteract visual instability caused by head movement [65]; however, during motion sickness onset, this response may be delayed or insufficient [68].

Objective evaluation methods typically involve the comprehensive measurement of human physiological indicators; however, relying solely on these indicators may not accurately depict motion sickness due to individual differences that influence changes in physiological signals, leading to a lack of statistical significance in measurement results. Therefore, integrating these metrics with other approaches is a prudent choice. As of now, there is no established “gold standard” for diagnostic test methods and measurement standards for motion sickness. Hence, it is imperative to employ diverse methodologies and consider multiple outcomes in the comprehensive assessment of motion sickness.

4.1. Passenger-centered relief motion sickness

By providing passengers with advanced vehicle and road information, it aids in mitigating the occurrence of motion sickness. This approach aligns with the sensory conflict hypothesis and encompasses the acquisition of visual, auditory, and tactile cues.

Tal D, Gonen A, and Wiener G observed a significant increase in motion sickness when the visual field was blocked. to effectively reduce the incidence of motion sickness [69], researchers suggest providing passengers with an unobstructed visual field environment. Turner M. and Griffin M. J. conducted numerous long-distance travel experiments and concluded that enhancing passengers’ forward visual field can greatly decrease the occurrence of motion sickness [71]. C. R. Gordon and O. Spitzer’s research on seasickness also reached a similar conclusion [70]. Therefore, Diels and Bos proposed placing displays near passenger windows to allow them to perceive their surroundings, reducing sensory conflict while obtaining entertainment information [72]. Bos and Kinnon propose displaying real-time road views to passengers through screens [73], enabling access to road information for passengers. Tal D. suggests informing passengers about their surroundings as a means to reduce sensory conflict [74]. When using displays to provide a comfortable visual field and monitor movement information, it is important to select appropriate size and positioning options [75]. Utilizing virtual reality devices to create synchronized sight between passengers’ vision and vehicle movement scenes through visual input [76], ensuring consistency between their visual cues, vestibular system, and sensory input can alleviate motion sickness symptoms [77]; this method holds promise. Shielding visual input can also significantly alleviate motion sickness; however, it is not a complete solution [78].

Traditional car drivers typically do not experience motion sickness due to their ability to anticipate the vehicle's driving path and state in advance, actively adjusting their heads and upper bodies towards the direction of rotation [79]. However, when transitioning from a driver to a passenger role in autonomous driving [80], this predictive capability diminishes, thereby increasing the risk of motion sickness [81]. Consequently, enhancing passengers’ trajectory prediction abilities and reducing the likelihood of motion sickness in autonomous driving can be achieved by providing advanced indications about future vehicle movement directions through internal audio systems for guiding responses [82], or utilizing wearable devices employing various vibration methods to alert passengers about upcoming operations [83], [84]. Bjka and Bnmya suggest that expanding window areas and using elevated seats can offer a broader and clearer field of vision, ensuring clear visibility of roads with visual cues for vehicle trajectories, thus minimizing the occurrence of motion sickness [85].

4.2. Vehicle-centered motion sickness relief

This paper analyzes the influence of vehicle factors on motion sickness, as shown in Fig. 7.

Fig. 6The influence of vehicle factors on motion sickness

The occurrence of motion sickness in autonomous vehicles can be reduced by optimizing the motion planning algorithm and selecting routes with good traffic conditions [86], as speed, vertical vibration, lateral acceleration, and horizontal acceleration have been found to impact motion sickness. Waymo’s research team suggests that avoiding traffic congestion can minimize rapid vehicle accelerations and decelerations, thereby reducing the incidence of motion sickness [88]. Simulating the smooth driving style of an experienced driver can also help alleviate motion sickness [89]. Additionally, many scholars have utilized suspension systems to mitigate vehicle vibrations and subsequently reduce motion sickness [90]. by employing control optimization techniques such as fuzzy PID control algorithm [91] or LQG control [92], low-frequency vibrations within the range of 0.1-0.5 Hz can be avoided while improving ride comfort and minimizing car-induced motion sickness occurrences [93]. Furthermore, active suspension control optimization has shown the potential to reduce the likelihood of experiencing motion sickness by achieving a more stable driving condition through controlling vertical acceleration [94], pitch acceleration [95], and roll Angle acceleration [96].

In terms of vehicle materials, piezoelectric metamaterials show a unique ability to effectively control the amplitude and direction of wave propagation. Taking this property into account, the use of piezoelectric metamaterials to improve the material of vehicle chassis has become an interesting option. By applying piezoelectric metamaterials to the chassis, the phenomenon of low-frequency vibration transmission to the passenger area can be effectively reduced. This method is not only expected to improve the comfort of passengers but also to improve the overall performance and driving experience of the vehicle [97]. The special properties of piezoelectric metamaterials make them have broad application prospects in vehicle engineering and provide new possibilities for reducing vibration transmission and improving ride quality.

By optimizing the structural design of vehicle chassis, the transmission of low-frequency vibration from road excitation to passengers through suspension can be effectively mitigated, thereby reducing the incidence of motion sickness [98].

By integrating sensors that record passengers’ physiological signals and combining them with vehicle movement data, Jaguar Land Rover researchers have provided personalized autonomous driving capabilities. Give each passenger a health score and establish a basic set of driving parameters based on matching Settings, eliminating the problem of different passengers having to adjust different data. When passengers are working or relaxing in the car, this method can reduce the incidence of motion sickness by 60 % [99]. Adapting self-driving cars to passenger needs to achieve personalization and adapt to different driver and passenger seat positions can also reduce motion sickness. The occurrence of motion sickness is also affected by the direction of the seat [100]. Changing the direction of the seat to make it parallel to the direction of the front and back movement of the body and the direction of stomach peristalsis can effectively prevent the occurrence of motion sickness [101].