With many futurists and companies expecting virtual reality to play a large part in the computer industry in the years to come the need to find the cause of the problem of cybersickness is great (Ellis, 1991). The applications of immersion virtual reality include everything from military training to medical practice to robotic manipulation in unsafe environments (Regan & Price, 1994). It is important to the virtual reality industry to find a cure or a solution to the problems of cybersickness. If the problem of cybersickness is not solved potentially 60% of healthy users will be incapacitated by the throes of cybersickness (Stern, Hu, Anderson, Leibowitz & Koch, 1990; Regan & Price, 1994).
Immersion virtual reality connotes an illusionary world in which visual, tactile, and sound stimulation causes a user to lose the sense of what is real and what is simulated (Strauss, 1995). There are two general theories for the causes of cybersickness. The first theory, the computational lag theory, involves a lag in the computation and display of the virtual world to the viewer. This results in an incompatibility between the actual and expected visual input to the nervous system (Strauss, 1995). The second theory is the vestibular-ocular incompatibility theory or the sensory conflict theory (Regan & Price, 1994). There is a difference between the ocular input and the vestibular input that results in an response of nausea by the nervous system. Sensory conflict theory bears a similarity to the symptoms induced by motion sickness and vestibular malfunction (Yardley, 1992).
The computational lag theory while providing an explanation for the causes of cybersickness can be combated by either reducing the fidelity of the virtual reality system or by increasing the computational power of the system. Both approaches have shown that cybersickness symptoms still exist even when there is no computational lag in the system (Strauss, 1995; Regan & Price, 1994; Wickens, 1992). The only group of people completely immune to the symptoms under all circumstances have no functioning vestibular system (Yardley, 1992). Therefore the sensory conflict theory explains a larger frequency of the cases of cybersickness.
The sensory conflict theory states that the sickness in simulators could be attributed to the unexpected absence of vestibular signals to accompany the visual field motion - a visual-vestibular conflict (Yardley, 1992). This theory is derived from van Holstās (as cited in Yardley, 1992) reafference principle, and modifications of this concept by Held (as cited in Yardley, 1992). Reafferance implies that the brain predicts the outcome, expected consequences, of a self-produced movement (Allard, 1993).
Diagram of von Holsts' Model of Reafferance (GIF, 5k) (Allard, 1993)
Cybersickness symptoms can be divided into two independent classes. The first group of symptoms arise from the disruption to perceptual and sensorimotor activities involving the vestibular system such as disorientation, disequilibrium, and inappropriate vestibulo-ocular or vestibulo-spinal reflexes (Yardley, 1992). The second group consist of a largely autonomic response, including drowsiness, salivation, sweating and vomiting, which also appear to have a perceptual origin since they are triggered by the virtual environment (Yardley, 1992). The anatomy and perceptual components of the two types of symptoms are discussed below.
Diagram of Neural structures associated with the vestibular system (GIF, 17k) (Frank, 1995)
The visual pathway is a point to point projection from the retina to the dorsal nucleus of the lateral geniculate body of the thalamus, and from this nucleus to the visual cortex of the occipital lobe (Barr & Kiernan, 1993). There are three systems projecting to a number of different areas of the occipital cortex. Magnocelluar system detects motion and spatial relationships and contributes to stereopsis. The parvocellular interblob system detects the form of objects. And the parvocellular-blob detects color of the object (Kandel, 1991). These three systems originate in the retina and project to the occipital lobe Brodman areas 17, 18, 19 and continue to other cortical areas (Kandel, 1991).
Diagram of Visual pathways and Occipital cortex (GIF, 11k) (Kandel, p. 447, 1991)
The perception of three-dimensions has part of its roots Gestalt psychology. Gestalt psychology is the idea that the act of perception creates a Gestalt - a figure or form that is not a property of an object observed but represents these organization of sensations by the brain (Kandel, 1991). The brain creates the three-dimensional experiences from the two-dimensional images by organizing sensations into stable patterns, or perceptual consistencies (Kandel, 1991). The brain makes certain assumptions about what is to be seen in the world, expectations that seem to derive in part from experience and in par from the built-in neural wiring for vision (Kandel, 1991). In virtual reality the ambient illumination changes, the size shape, and brightness of the two-dimensional images that are projected onto the retina(Kandel, 1991).
Object-Centered Cues (Monocular Cues) create Far-Field Depth Perception
Observer-Centered Cues (Stereoscopic Cues) create near-Field Depth Perception
Stereopsis requires that the input from the two eyes be slightly different; disparity-selective neurons have been detected all along the magnocellular pathway (Kandel, 1991).
The vestibular system serves to resolve conflicts between other sensory system. In virtual reality the conflict is between proprioception and the visual inputs. Because there are no predefined limits on what a virtual environment may or may not be there may be large discrepancies between the proprioceptive feedback and the visual input to the nervous system. The receptors in the utricle and saccule respond to the pull of gravity and to the inertial movement caused by linear acceleration and deceleration (Barr & Kiernan, 1993). The receptors in the ampullae of the semicircular canals respond to rotation of the head, i.e. angular accelerations and decelerations (Barr & Kiernan, 1993). The vestibular nuculei have connections with the cerebellum and reticular formation.
The reticular formation autonomic responses through the intermediolateral column to sympathetic ganglion chain to blood vessels and sweat glands resulting effects include pallor, and cold sweats. Through the nucleus ambiguus to the pharynx/larynx resulting in regurgitation. Through the phrenic nucleus to the diaphragm resulting in vomiting. Connections through the salivatory nuclei to the major and minor salivary glands resulting excessive salivation. And the reticular formation has motor output to the dorsal efferent nucleus X to the gastrointestinal tract resulting nausea and vomiting (Dodd & Role, 1991).
There are two theories concerning the causes of cybersickness. The computational lag theory and the sensory conflict theory. They attribute the cause of cybersickness to two different factors that act on the body and the nervous system in a very similar manner. However when the problems of one theory are resolved there are still subjects who experience cybersickness (Yardley, 1992). The only group of individuals who never experience the problem are those with no functioning vestibular system (Yardley, 1992). This leads us to believe that because the limitations of computing power can be overcome, it is only a matter of cost to eliminate the computational lag. However the problems of cybersickness are still present, the sensory conflict theory is the most plausible theory at this moment in time.
There are many methods have been provided for the solution to cybersickness/motion/simulator sickness, they include drug therapy to reduce vestibular output, drugs to control nausea, and individual environment training (Yardley, 1992; Roenick & Lubow, 1991). The likelihood of the usage of drugs to control the symptoms of cybersickness in the workplace where immersion virtual reality is being used to control robots in life-threatening situations, or to train pilots and drivers is highly unlikely due to the safety concerns and the adverse side-effects the drugs may have on the performance and reliability of the operator (Roenick & Lubow, 1991). Therefore the question is can we train individuals to be tolerant to cybersickness?
Almost all individuals eventually adapt to motions or situations which initially provoke sickness; continued exposure to a particular nauseogenic environment leads to a gradual reduction in the disorientation and associated symptoms (Yardley, 1992). Research has shown that the notion that both specific and general components to tolerate motion environments, like virtual reality, can be learned through individual training (Dobie & May, 1990).
Last updated: January 26,1996: Dave Crow