Biology 202
2000 Third Web Report
On Serendip

Since we see two slightly different pictures through each of our eyes, a disparity must occur. This disparity is what enables us to perceive depth, although we can perceive depth pretty accurately when using only one eye. Human perception, on the whole, is pretty precise in that the depth cues received from the world are in agreement (2). But what happens when we toy with our depth perceptions? Like when we use virtual reality systems? Virtual reality systems combine and confuse our perception of what is real and what it not in an illusionary system (4). In the broader sense, virtual reality belongs to a category named "Mixed Reality" which widely defines the concept of any aspect of the real world presented with images of a "virtual environment" (2).

Such systems combine our senses of touch, sight and sound with a computerized image, sometimes presenting us with images that would be normal in everyday life. Virtual reality systems, nowadays, are being used everywhere from within the entertainment industry to simulated surgery (2). Companies like Nintendo and Sega play an integral role in the entertainment aspect of this technological dimension. However, certain criteria must exist in achieving the best virtual reality experience. This usually includes a HMD, or a head mounting display, which is a helmet with a pair of earphones. The helmet has the computer generated visual scene while the earphones cover the audio portion of this experience. The helmet holds a tracking device that traces the direction your eyes are looking at and then "alters the visual scene according to the orientation of the user" (7). A joystick or a glove pad also accompany the system.

Virtual reality attempts to make this adventure as realistic as possible. Through the use of the HMD, joystick or glove, and tracking device, one can feel that they are truly part of this computerized scenario. However, within the nervous system, a conflict of inputs occurs and the person can experience cybersickness or barfogenesis (4), a more crude term some people prefer. The real question then comes into mind: is there a real difference between simulator sickness and regular motion sickness?

There is some speculation concerning the symptoms of motion sickness and cybersickness. The first one is that the motion sickness is usually associated with nausea, sweating, dizziness, eye strain and sleepiness (10). Simulator sickness includes a lot of the same symptoms, but generally causes more eye strain, headaches and fatigue (10). In addition to these distinctions, some scientists have defined that simulator sickness results from "incorrect presentation" of the simulation (2). If your perception is equivalent to the simulation, and the virtual reality system is motion based, then the sickness you might feel is categorized as motion sickness.

Scientists are having difficulty differentiating between motion sickness and simulator sickness because of several problems. The first problem is that no one will ever experience the same kind of sickness and then the symptoms always vary. Sometimes we cannot observe the symptoms because they are internal. The symptoms also occur over a span of time, some being immediate, others occurring after several hours. Then, inconsistency in getting sick often occurs- sometimes you get sick and other days, you're fine. Although there is a list of similar symptoms both in motion sickness and in barfogenesis, the symptoms never occur consistently (2).

Scientists and researchers are still searching for more definite answers, but there are some theories that attempt to explain why people suffer from cybersickness and how it differs from motion sickness. There are two theories that attempt to explain why this occurs. The first theory is that confusion occurs from the inputs. The nervous system expects one specific input to occur, but a different kind happens instead. This is called the computational lag theory (4). This theory is described as lag in the computation and display of the virtual world to the viewer" as well as an "incompatibility between the actual and expected visual input to the system" (4). This theory states that there is some kind of doubt in explaining the lapse in time between the perception of the input and the origin of the input.

The second theory is called the vestibular- ocular incompatibility theory or sensory theory. This is the theory upon which many researchers are looking into. For example, I went to a Sony IMAX theater and saw a movie about a World War II pilot. He took us on wild expeditions between dozens of mountains, through flocks of birds and even over cliffs. The movie was exciting and extremely realistic. I felt as if I were in the plane, flying between those same valleys and cliffs. Unfortunately, I found myself gripping my stomach and holding back slight gagging noises. I was getting nauseous from the film. My eyes wanted me to "stop flying," while my body said, "Chris, you're not moving at all."

The vestibular -ocular structures are contained in the inner ear ( one structure in each ear). The main purpose of the vestibular- ocular function is to report the movement of the head and then send signals regarding motor responses, and eye movements(10). The nature of the vestibular-ocular reflex( VOR) is to maintain an image on the retina even when your head moves (10). The VOR function along with four other types of eye movements (smooth pursuit, , saccade, optokinetic and vergence) stabilize the image on the fovea (10). But before the VOR function maintains an image on the fovea, two things occur: convergence and accommodation (2). In convergence, your pair of eyes turn or rotate in opposite directions so they can then focus on the object. The focusing on the object is called accommodation. In this step, the lens of the eyes change their focal length (2) so the image is perceived appears clearly in the brain.

Through accommodation and convergence, the brain works with several distance or depth cues which allow us to perceive our three dimensional world. In a mixed reality system, only some of the depth cues are available through the HMD (2). Let us consider the combination of depth cues when a real figure is placed in a virtual reality setting. When we look into a virtual reality system, sometimes conflicts arise in interpreting different depth cues. In a mixed reality system and in real life, the depth cues are prioritized and agree with physical inputs. In such a scenario, we do not have to worry about a conflict in depth cues since the weaker ones were ignored (2).

On the other hand, when two different cues take the same precedence, our brains combine the cues, creating an "intermediate percept" (2). This is where a conflict occurs. The real figure will not be properly aligned with the virtual setting. The only solution to this conflict is that the brain must decide where these figures exist in relation to each other. The spatial perception becomes a little blurry and those viewing the mixed reality system cannot decipher just how far away the real object is. According to a recent study, if exposed to the same mixed reality scenario, the viewer can say if the real figure is close or far away but cannot get any more specific than that (2). In this case, a lot of uncertainty arises and the viewer is hesitant (depending on previous exposure to mixed reality viewing) to even place the real figure really close or really far. Many times, they will find the medium and place the real object somewhere in the middle (2).

During the studies, the researchers are asked to reach out and "touch" the object. Most of the time, the viewer inaccurately reaches out too far or too close, depending on their display mode (the different settings and combinations of real objects and a simulation environment) and the amount of experience they have working with mixed reality (2). The display mode, if kept constant, can help the viewer decrease the disparity in depth. If they see their own body in a different display mode from the previous one they had been working with, then they usually cannot identify what their hand can touch if asked to reach for a figure. This happens because the accommodation of the eyes focuses on their hand and then cannot clearly focus on the background.

Such instances, like the one mentioned above, can contribute to the sensory theory conflict that can cause cybersickness. Some researchers find that the Gestalt psychology can help explain the problem of cybersickness and how it differs from motion sickness (4). A gestalt has been defined as "a figure or form that is not a property of an object observed but represents these organization of sensations by the brain" (4). The brain creates and pieces together the two images it receives from each eye. The brain "sees" a three dimensional image, while the retina features a two dimensional image.

The nervous system, in this case, makes certain assumptions (4) about the world when the brain creates a three dimensional image by putting together the images that our foveae detect. These assumptions originate in our brain but the brain expects that these assumptions are accurate from prior experience and from the internal functions that occur within the nervous system that allows us to have vision (4). In a virtual reality system, the illumination perpetually changes the shape, lighting and brightness of the objects in view (2). The images that are then formed on the fovea cannot be stabilized because these changes occur.

The vestibular system attempts to fix the conflicts between the sensory systems, but in virtual reality systems, the conflict that occurs between our physical perception and what our eye see can be very confusing for the nervous system. Researchers have suggested that training and some medications can alleviate the symptoms of the cybersickness, but when we look at the situation and assess what causes the sickness, there are variables that differentiate it from regular motion sickness. The "display mode" for life is more constant, whereas the display mode and its depth cues in virtual reality always changes, especially when it is for entertainment value.

Although researchers are still attempting to differentiate ( or not) motion sickness and cybersickness, they do suggest one form of help. They suggest a series of training and repeated exposure to the environment that causes the sickness (4). This training involves the I function and conscious mental preparation which involves telling your body and brain that physical inputs will not match what your eyes perceive. The simulated environment that you must accustom to will challenge your depth cues and in time will hopefully stop making you sick.

WWW Sources

2)Pereptua l Issues in Augmented Reality , This helped me work through the confusion I had about depth cues and what happens when multiple ones are perceived.

3)Psycho logy Explorer , This site was good for quick reference terms .

4)Virtual Reality and Cybersickness , I found that this page best supported my question.

5)Virtual Reality: Overview of Its Application To Neurology ,This listed the applications of virtual Reality.

6)VR Terminology , This site allowed me to look up VR terms quickly.

7)A Note On The Legal Issues Surrounding Virtual Reality Technology , This site suggested that extended period of exposure to VR can help cybersickness.

8)Studies into the Visual Effects of Immersion in Virtual Environments , This site was helpful because it explained all of the major instruments and components in a VR system.

9) Can your eyes make you sick?: Investigating the Relationship between the Vestibulo-ocular Reflex and Virtual Reality , This gave the best explanation of the VOR complex.

10) Oculomotor Changes within Virtual Environments, This site explained the conflict between the HMD’s and the position of the eye.


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