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CYBERSICKNESS:
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DEFINITION OF CYBERSICKNESS:

Cybersickness is similar to motion sickness and typically occurs during or after immersion in a virtual environment. Cybersickness is believed to occur primarily as a result of conflicts between three sensory systems: visual, vestibular and proprioceptive. Accordingly, the eyes perceive a movement that is out of sync by a few milliseconds with what is perceived by the vestibular system, whereas the remainder of the body remains almost motionless (Stanney, Kennedy, & Kingdon, 2002). Cybersickness can also be caused by factors related to the use of virtual reality equipment (e.g. heaviness of the helmet, closeness of screen to the eyes). Lawson, Graeber, Mead and Muth (2002) note the added possibility that these side effects are also connected to Sopite Syndrome (fatigue due to the movements).

According to Kennedy, Lane, Berbaum and Lilienthal (1993), the temporary side effects associated to cybersickness can be divided into three classes of symptoms related to the sensory conflicts and to the use of virtual reality equipment: (1) visual symptoms (eyestrains, blurred vision, headaches), (2) disorientation (vertigo, imbalance) and (3) nausea (vomiting, dizziness). Visual symptoms typically occur as a result of closeness of the screen and are limited primarily to the use of a virtual helmet. The nausea and disorientation experienced are temporary, comparable to reading in a moving vehicle and are caused primarily as a result of sensory conflict.

Other symptoms that can be experienced during or after a virtual immersion include: general discomfort, difficulties in the ability to focus, increased salivation, excessive sweating, feelings of heaviness in the head region, stomach awareness and burps. Lawson et al. (2002) also add to this a fourth class of symptoms forming what is called Sopite Syndrome. Sopite Syndrome consists of motion sickness presenting solely through symptoms of fatigue (concentration difficulties, apathy, chronic fatigue, weakness, heaviness, etc.,). This syndrome is believed to result from imbalances in the vestibular system. Lawson et al. (2002) cite a study conducted by Graybiel and colleagues in the 1960’s that showed this possibility given that subjects were successful in intentionally self-recording their head movements even after experiencing nausea.

Moreover, the healthy participants were exposed during the course of two days to a revolving room and reported feeling symptoms of fatigue and apathy, even at a weak rotation level (1.71 to 3.82 turns per minute). In contrast, the participants in the control group, having lost their vestibular function, did not feel the same symptoms under identical conditions.

Cybersickness do not represent a disease but rather a normal physiological response to a non-habitual or an unusual stimulus. On a sensory level, the cybersickness are actually common among individuals engaged in virtual reality: 50% to 100% of them experience dizzy spells and 20% to 60% of individuals experience unspecific abdominal symptoms (Lawson et al., 2002). Although additional symptoms and their frequency have been documented, research shows that oculo-motor problems typically predominate the human response to a moving virtual environment (Lawson et al., 2002). Moreover, the intensity of cybersickness varies significantly from person to person.

At least 60% of virtual reality environment users report having felt symptoms of cybersickness during a first session. The proportion of individual that feel more severe and long-term secondary effects is comparable to the proportion of individuals who suffer from a sensitivity to motion sickness. Approximately 5% of users actually feel no side effects of any kind as a result of being immersed in the virtual reality environment.
SENSORY SYSTEMS:

Structure of the inner ear:

The inner ear is also called the labyrinth, given that it has a complicated formation (Marieb & Laurendeau, 1993). The inner ear is composed of two major divisions : the bony labyrinth and the membranous labyrinth:

1. The bony labyrinth:

The bony labyrinth (Figure 1) is a system of canals composed of three regions: the vestibule, the cochlea, and the semi-circular canals.

a) The vestibule :

The vestibule is an egg-shaped cavity situated at the center of the bony labyrinth (between the cochlea and the semi-circular canals). This small sensory organ allows for the detection movement, rotation and movements of the head and generates a corresponding signal. The vestibule works in conjunction with the otholiths and the semi-circular canals to allow for the maintenance of equilibrium.
Figure 1


b) The cochlea :

The cochlea is a spiraled bony cavity that emerges from the anterior of the vestibule. Its structure plays a central role in the perception of sound.

c) The semi-circular canals :

The semi-circular canals emerge from the posterior of the vestibule and are divided into three sections: anterior, posterior and lateral. Each canal contains a semi-circular membranous conduit. These conduits each carry a bulbous end called a bulb that reacts to the angular movement of the head.

2. The membranous labyrinth :

The membranous labyrinth (attached image) is a network of vesicles and membranous conduits logged in the bony labyrinth. Two otholiths organs, the saccule and the utricle (vesicles) shelter the balance receptors, that react to gravity and encode changes in the position of the head (Marieb & Laurendeau, 1993).
Figure 2
The vestibular system and immersion in virtual reality:

The following side effects can be felt following exposure to virtual reality: fatigue discomfort, nausea, headaches, eyestrains, etc. The vestibular system is thought to be implicated in a number of these side effects. Stimulation of the vestibular system can affect a variety of behaviors (Stoffrengen, Draper, Kennedy, & Compton, 2002) :

a) Gaze stabilization. In order to maintain proper vision, the eyes have to be stabilized in relation to the object being viewed. The stabilization of the eyes in connection with the illuminated environment requires information on the position of the head versus the environment; the vestibular system offers this information. One speaks often here of the vestibulo-occular reflex. When the head begins to move in a particular direction, the vestibular system feels the movement and sends the information directly to the oculo-motor visual system.

When the head is moving during an immersion in a virtual reality environment, a slight time delay can occur between the displacements perceived by the vestibular system and the corresponding visual information transmitted by the computer. This delay has been known to cause nausea and a feeling of disorientation seeing as it translates (incorrectly) a message indicating that the stabilizing visual mechanism has malfunctioned.

b) Balance and control. The vestibule is renown for influencing control, posture and balance. At the same time, posture and balance are very much influenced by the stimulation of other sensory systems such as vision, hearing, and touch, these are in turn not necessarily influenced by the vestibular system. During a virtual reality session, the vestibule and the other sensory systems are directly stimulated by the virtual environment. This interaction between the vestibule and the other sensory systems does not perfectly agree with the virtual environment (e.g. quality of the image may be compromised, the delay between the image and head movements) and this can cause symptoms of cybersickness.

c) Hand-eye co-ordination. There is often a tendency for persons to point at objects that are out of their reach. In these situations, the pointing behaviour is not verified by the sense of touch, but instead must be checked or verified by other types of stimulations (e.g. by the sense of vision). The precision of guided vision fluctuates in function of the changes occurring in the sensory systems and in the vestibular system as well as in the interactions between them. In the virtual environment, the changes in vestibular adaptation often produce an error at this point and cause the user to experience side effects.

d) Other influences. The vestibular system may also directly or indirectly influence other autonomous functions (e.g. vasomotor, cardiac, gastro-intestinal and respiratory reflexes) thereby creating diverse physiological sensations. Vestibular problems could thus play a role in the development of panic disorders.

The visual system:

Humans have binocular vision that can be defined as a simultaneous formation of two images of one object on the retinas of both eyes. Given that the eyes are placed at the front of the skull oriented more or less in the same direction, their visual fields overlap considerably, but capture images from a slightly different angle.

Binocular vision provides a reduced visual field, however, it does allow for stereoscopic vision, which in turn allows the individual to evaluate distances and to situate objects in space when they are near. Stereoscopic vision requires both the coordination of the two eyes and the precise convergence of the eyes onto the viewed object. The accommodation of the eye which in turn focalizes the eye onto the object as well as the convergence of the two eyes to view the object (when the object is situated less than three meters from the viewer) allow the perception of the distance of the object from the viewer. When the object is situated more than three meters away from the viewer, the phenomenon of convergence is no longer required because the two eyes are directed forward. (May & Badcock, 2002).

Visual system and immersion in virtual reality:

During immersion in virtual reality, ocular problems result, in part, from the fact that the person is wearing a helmet and that the screen is located in close proximity to the eyes. These are temporary effects and are comparable to viewing television from a close distance. On the other hand, the eye tends to adapt rapidly and virtual reality helmets have been developed to minimize this phenomenon. It is nevertheless recommended that the individual limit his/her exposure to virtual reality from 20 to 30 minutes and then take a break before continuing the immersion. Cybersickness can also be caused by the delay occurring between the image projected in the helmet and movement of the person undergoing the virtual reality immersion. This delay affects the coordination of the eyes, which must undergo a period of rehabilitation. Side effects are also likely to occur when there is a conflict between the accommodation and the convergence of the eyes.

In virtual reality, the helmet can be equipped with a monoscopic system (bi-ocular helmet) or a stereoscopic system (binocular helmet) Wann & Mon-Williams, 2002) :

a) The monoscopic system : the monoscopic system implies that the two eyes perceive the same image, which gives the viewer an impression of perceiving images in 2D. The image of the virtual environment projected onto the screen may appear less realistic but is unlikely to cause visual stress. It is possible for a binocular visual reality helmet to bring about changes in the visual system but this requires a poor quality optical system and poorly performing position encoders.

b) The stereoscopic system : the stereoscopic system allows for the projection of a different image for each eye, which gives the viewer an impression of perceiving images in 3D. The likelihood of experiencing cybersickness is more likely with the use of a stereoscopic binocular helmet because the eyes have to put in even more effort for the coordination and adaptation to occur. The phenomena of convergence and accommodation for evaluating distance are similar inside a virtual environment. These efforts carried out by the eyes may cause dizzy spells, nausea, headaches etc., particularly when one is moving within a virtual environment and when there is a delay occurring between the movements. The use of a stereoscopic screen can provoke visual stress because even miniscule changes in the position of the helmet on the head can create a significant change at the level of the gaze angle.

Another area of the visual system in a virtual reality environment that has undergone much study is the relationship between cybersickness, vection, postural problems and other perceptuo-motor side effects. Vection is defined as the illusion of displacement induced by moving images. During vection, as with the perception of moving stimuli, the movement perceived by the visual fields can cause an individual to experience a significant degree of discomfort, particularly if the person is viewing from a stationary position (e.g. when viewing IMAX films).

Circular vection has been shown to induce symptoms of motion or transportation sickness in approximately 60% of exposed healthy individuals (Hettinger, 2002). This type of sickness has traditionally been attributed to the occurrence of a conflict between the various sensory systems connected with orientation and with self-perception of an individual’s movements.

The proprioceptive system:

The proprioceptive system is directly related to the motor system of the human body. Internal signals correspond to the control of movements contributing to proprioceptive and spatial vigilance.

Proprioceptive system and immersion in virtual reality:

An individual’s perceptions of control and of movement depend a great deal on the proprioceptive system (linking the desire to make a certain movement and the movement itself). The virtual environment will cause the individual to make perceptual and motor errors until the time at which the individual has adapted internally to the new environment. This adaptation can take from up to 30 to 60 seconds (Stanney & al. 2002) and may still continue after immersion. When the user has adapted to the virtual environment and has returned to making the same movements in the real environment, the adaptation gained in the virtual reality environment may still be present and result in the experiencing of « after-effects ». These secondary effects may include : deviated execution of a limb and of the general movement of one's body, proprioceptive errors, an erroneous estimation of the external forces experienced by the individual and an impoverished sense of visual and auditory localization (DiZio & Lackner, 2002).

Many virtual environments can introduce in the human being a sensimotor rearrangement that results in cybersickness, followed by a proprioceptive adaptation to this environment. When the individual returns into the real environment, the systems must undergo a period of rehabilitation after termination of immersion (Welch, 2002).

There are two important facets of the priorioceptive system that can contribute to adaptation : (1) an intuitive sensation of the position and of the orientation of one's body (2) the sensation of force or effort. This system is comprised of an interaction of efferent and afferent signals with respect to: the position of a limb, the position of the body and of a movement, the internal representations of corporal schemas, one's spatial orientation and finally with respect to the representation of environmental constraints. All of this can provoke different types of side effects. The principles that govern the specificity of one's adaptation are related to the types of side effects that will be felt during the users return into the real world in order to carry out the tasks of daily living.
THEORIES ABOUT CYBERSICKNESS:

In order to explain the appearance of negative symptoms related to displacement and to cybersickness, three primary explanatory theories have been suggested: sensory conflict (conflict occurring among the sensory systems), the theory of intoxication and ecological theory (Harm, 2002).

1. The theory of sensory conflict :

The theory of sensory conflict remains the most prevalent explanation for the appearance symptoms caused by displacement. This theory holds that the orientation of the human body in a three-dimensional space requires a minimum of four points of entry of sensory information into the central nervous system: (1) the otoliths organs offer information concerning linear acceleration, velocity and incline; (2) the information on angular acceleration is provided by the semi-circular canals; (3) the visual system provides information concerning the body's orientation with respect to the visual scene; (4) the systems of touch, and of kinaesthetic pressure, provide information with respect to the limbs and the body's position. When the environment is altered in a way so as to produce a misalignment or discord between the sensory systems of the body that are already in place, symptoms of cybersickness can appear (Harm, 2002). This theory is generally adequate but it fails to properly explain (in enough detail) certain particular situations, for example, when the user experiences sensory conflict but fails to experience symptoms of cybersickness. In addition, this theory makes it difficult to describe the possibility of quantifying the conflict or of explicating its underlying mechanisms. Nor does it include the occurrence of sensory-motor conflict or of adaptation without conflict in its explanation.

Disorientation and nausea, for example, seem to be caused by a conflict between the information received by the brain from diverse sensorial systems. Body movements are perceived by the visual, vestibular and proprioceptive mechanisms. In the absence of visual elements (e.g. blackness, fog), orientation and movement of the head are detected by the vestibular system. In normal situations, the information coming from these systems agree perfectly. But in virtual reality, the eyes indicate that the person has moved while the vestibular and proprioceptive systems indicate that the person has not moved. The inverse situation occurs when one is reading while in a moving vehicle. Thus, during one's experience of sensorial conflicts, the person often experiences symptoms of cybersickness.

Nevertheless, certain problems have been raised over the years including the absence of adequate explanations as to why only certain types of conflicts are related to nausea, why the same conflict does not present in all individuals (or the same individual in difference circumstances) and how to quantify the degree of sensory conflict present in a given situation when linked with the frequency and severity of motion sickness. Sroffregen & Riccio (1991) proposed a promising alternative explanation, suggesting that motion sickness, whether or not it occurs in a real or virtual environment, results from an interruption of normal activities of postural control. However, the theory of sensory conflict seems to have value in terms of its ability to explain several of the situations resulting from a prolonged postural disorder and/or faintness due to displacement. It is possible that the two theories are correct and that this type of sickness, including when induced by vection, ultimately results from the in habitual interruption of the normal cycle and of the perception of action.

2. Intoxication theory:

Intoxication theory, according to Treisman (1977), holds that the mechanisms provoking the sicknesses resulting from movement are a result of a corporal response functioning to withdraw toxins from the stomach (Harm, 2002). Treisman has proposed that the movement is simply an artificial stimulus that activates certain mechanisms, or more simply put, movements act on the mechanisms created and developed to respond to the physiological disturbances resulting from absorbed toxins. The neurological activity allowing for coordination of the entry of information from all of the sensory systems in order to control the limbs and ocular movements is disturbed by the effects of neurotoxins. Consequently, the disturbance of this activity by unnatural movements is interpreted as being an indication of toxin absorption that activates a mechanism producing an emetic response.

Money & al. (1996) have proposed that the sicknesses resulting from movement represent an intoxication, initiated by the vestibular system, comprised of two major phenomena : the response of the stomach that is emptying (parasympathetic control) and the stress response (sympathetic control). The vestibular system is thought to be implicated in the regulation of the autonomous nervous system, although the procedure remains dubious.

A criticism concerning the theory of intoxication relates to the time period necessary for a toxin to affect the vestibular mechanisms (Harm, 2002). It would appear that the duration would be too long to allow the efficacy of vomiting as a means of eliminating the toxins in the gastro-intestinal region. It follows that vomiting would be one of the last options of defence against intoxication.

3. Ecological theory :

The ecological theory holds that motion sickness resulting from transportation is caused by prolonged periods of postural instability, an idea that contradicts the theory of sensory conflict. This approach is based on a perception of action, were the interactions between the human and his/her environment are at the centre of the problem. Riccio & Stoffregen (1991) have recognized the existence of changes in sensory stimulation occurring during induced situations, but suggest that they are determined by changes in the way in which the environment manages postural control. The duration and the intensity of the instability predict the intensity of the symptoms.

There are some limits to this approach. Firstly, this theory does not entirely allow for an explanation as to why individuals with labyrinth problems (inner ear) do not feel sickness due to displacement. Secondly, this approach does not offer a clear explanation as to the implied mechanisms and as to their operation. Finally, this approach does not offer a clear explication of the reasons as to why postural instability causes sickness due to displacement.

In conclusion, the three theories, as well as their limitations in terms of explaining motion sickness demonstrate well the complexity of this phenomenon. Both the sensory conflict and the ecological theories, for their part, explain the conditions underlying to this type of sickness but fail to offer reasons as to the appearance of symptoms. The theory of intoxication, on the other hand, attempts to explain evolutionary causes of motion sickness. However, differences do not mean that these theories are mutually exclusive; their proponents agree that the conditions causing the symptoms of motion sickness involve multimodal sensory information, while they at the same time the authors disagree as to where this information comes from with respect to these sensory modalities (Harm, 2002).
FACTORS THAT INFLUENCE CYBERSICKNESS:

There are three main categories of likely factors influencing the intensity of cybersickness generated by a virtual environment (psicologia.net; North & al., 1995; Stanney, 1998): the individual’s characteristics, system’s characteristics and the task’s characteristics.

For the moment these three factors serve as a guide in terms of applications of virtual therapy. However, the development of a causal theory of cybersickness would allow for better prediction of the combination of factors likely to create side effects following exposure to virtual reality (Stanney & al., 1995). Despite the many suggestions that have been brought forth in order to explain the origin of cybersickness, a definitive explicatory theory on this subject remains unclear.

1. Individual's characteristics:

  • Physical characteristics: age (below the age of 12), gender (female), ethnic origin (China), postural stability and state of health (insomnia, ear infection, medications).
  • Prior experience with virtual reality and nature of tasks required (cybersickness diminishes with experience).
  • Perceptions: the style, the degree of concentration and the ability to effectuate mental rotations.

The type of side effect and its degree of severity vary greatly among users of virtual reality systems. Certain individuals seem to feel a brief discomfort or faintness at the beginning of the experience only to later adapt, whereas others feel the onset and impact of side effects only gradually over the course of the experience. According to Howart & Costello (1996), certain subjects report experiencing a linear augmentation of symptoms during the course of immersion; this may suggest that the subjects would have greater difficulty adapting to a new environment (at a behavioral and neurobiological level) when compared to other individuals, this difficulty of adaptation however, diminishing with repeated exposure and habituation.

Age appears to also influence one's susceptibility to cybersickness. Riva et al. (Psicologia.net) suggests that the sensitivity level peaks in children between the ages of 2 and 12 and tends to diminish rapidly between the ages of 12 and 21 and more gradually after the age of 21.

One explanation is based on the fact that technical aptitudes (e.g. spatial vision, orientation, spatial memory, etc.) are less developed in children. The studies cited by North et al. (1996) suggest that the difficulties encountered could be specifically linked to one’s ability to navigate in the virtual world. In such a case, directive assistance is recommended in order to assure and maintain spatial navigation.

2. System's characteristics :

  • Screen: luminosity, resolution and contrasts should be adjusted in function of the tasks required in order to attain an optimal level of performance; the flickering of certain screens ("flicker") has a frequency of 8 to 12 Hertz and may become distracting, contributing to eye fatigue. Nevertheless, the equipment that is currently in use does not pose significant problems in this regard.
  • The weight of the helmet: an increase of physical symptoms may occur when the subject wears a heavy helmet for a long duration (Howarth, & Costello, 1996). However, given that equipment has become increasingly more sophisticated, lighter helmets can be used to avoid such problems.
  • Temporal delay: the delay of head movements and of the corresponding image appearing on the screen are also a source of conflict between visual and vestibular perception. This conflict can present itself either when the visual stimulation is presented in the absence of vestibular stimulation, or when there exists a delay between the vestibular sensations of movement and the perceived movement of the screen. Moreover, if the movement of the visual scene becomes disturbed versus the movements of the head. In this case, the quality of the equipment allows for the correction of the problem.
  • "Visual Stress: the use of a stereoscopic screen can provoke visual stress, because even a minimal change in the position of the helmet on the head can create a significant impact at the level or of the convergence of the eyes (“gaze angle”). This can be solved by wearing the helmet in such a way as to ensure greater stability on the user’s head. The use of a binocular screen does not seem to create this type of visual stress (Mon-Williams et al., 1988).

3. Task's characteristics :

  • Control of movements : The control of movement (and their speed) in the virtual environment diminishes the appearance of symptoms.
  • Characteristics of visual image: Different characteristics of the visual image can also diminish the appearance of symptoms: the quality of the field of vision, the content of the scene and/or of the observed region, etc.
  • Interaction with the task: a session duration of less than 10 minutes or of more than 40 minutes can lead to nausea; the seated position is thus preferable to standing, etc.
CONTROL FACTORS

In addition to technological advancements, certain general precautions can also be taken in order to attenuate the intensity of cybersickness during a virtual therapy session:

  1. The differences between symptoms of anxiety and symptoms of cybersickness should be explained to the user (the symptoms of cybersickness are directly linked to the virtual environment). If the person feels a symptom of discomfort that appear to be related to their adaptation to the virtual environment. If the person feels a discomfort related to adaptation to VR (and not related to symptoms of anxiety) the session should be terminated immediately. This rule does not necessarily apply to cases of in vivo exposure because the fact of terminating the session could reinforce the individual’s tendency to avoid fear-provoking situations.
  2. The duration of the exposure should be 20 to 30 minutes and the client should wait 20 minutes after the session before leaving to go home.
  3. Individuals suffering from serious medical problems (cardiac conditions, epilepsy), psychotic disorders or those consuming psychotropic substances capable of inducing significant physiological or psychological effects should not participate in virtual reality sessions.
  4. Participants should be seated on a chair or placed next to a railing so that they may hold on. It is also possible to position the helmet in such a way so as to allow the user to partially see his or her own body.
  5. The tracker should be adjusted as a function of the client’s sensitivity. If the image is too rapid or inversely if there is too much delay between the user’s head movements and those of the image, the therapist may adjust the speed in order to diminish the intensity of cybersickness. In addition, it is preferable that the apparel be activated five minutes before beginning the session and the helmet be placed horizontally onto the head of the client to avoid rapid image movement or movement at a different angle when the helmet is being positioned.

By following these precautions, the degree of immersion diminishes but the degree of psychological and physical personal security increases, thereby keeping the potential risk of discomfort to a minimum (North et al., 1996).

In addition to these general recommendations, our Laboratory has developed a more complete protocol in order to promote the reduction of cybersickness, thus allowing the therapist to insure that the client experiences maximum control and security (please see ‘protocol: long version’ and ‘protocol bref version’)
REFERENCES:

DiZio, P., & Lackner, J.R. (2002). Proprioceptive adaptation and aftereffects. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp. 751-771). Mahwah : IEA.

Kennedy, R.S., Lane, N.E., Berbaum, K.S., & Lilienthal, M.G. (1993). Simulator Sickness Questionnaire: An enhanced method for quantifying simulator sickness. International Journal of Aviation Psychology, 3(3), 203-220.

Harm, D.L. (2002). Motion sickness neurophysiology, physiological correlates, and treatment. InK.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp. 637-661). Mahwah : IEA.

Hettinger, L.J. (2002). Illusory self-motion in virtual environments. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp.471-491). Mahwah : IEA.

Lawson, B.D., Graeber, D.A., & Mead, A.M. (2002). Signs and symptoms of human syndromes associated with synthetic experience. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp. 589-618). Mahwah : IEA.

Marieb, E.N., & Laurendeau, G. (1993). Anatomie et physiologie humaine. St-Laurent : ERPI.

May, G.J., & Badcock, D.R. (2002). Vision and virtual environments. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp. 589-618). Mahwah : IEA.

Money, K.E., Lackner, J., & Cheung, R. (1996). The autonomic nervous system and motion sickness. In B.J. Yates & A.D. Miller (Eds.). Vestibular autonomic regulation (pp.147-163). Boca Raton, FL: CRC Press.

Riccio, G.E., & Stoffrengen, T.A. (1991). An ecological theory of motion sickness and postural instability. Ecological Psychology, 3, 195-240.

Société internationale de réhabilitation vestibulaire (2002). Le labyrinthe membraneux et osseux.. Retreived on August 22, 2002, from: http://www.vestib.org/chap4anatphysio/omivestib.htm.

Stanney, K.M., Kennedy, R.S., & Kingdon, K. (2002). Virtual environment usage protocols. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp.721-730). Mahwah : IEA.

Stoffregen, T.A., Draper, M.H., Kennedy, R.S., & Compton, D. (2002). Vestibular adaptation and aftereffects. In K.M. Stanney (Eds.) Handbook of virtual environment : Design, implementation, and applications (pp. 773-790). Mahwah : IEA.

Stoffrengen, T.A., & Riccio, G.E. (1991). An ecological critique of the sensory conflict theory on motion sickness. Ecological Psychology, 3, 151-194.

Treisman, M. (1977). Motion sickness: An evolutionary hypothesis. Science, 197, 493-495.

Welch, R.B. (2002). Adapting to virtual environments. In K.M. Stanney (Eds.) Handbook of virtual environments: Design, implementation, and applications (pp.619-636). Mahwah : IEA.