This is the March version of the SteamVR "Vive" headset co-created by HTC; a newer dev kit has already shipped to eligible SteamVR developers.

Sam Machkovech

By this time next year, consumers should have their choice of a variety of virtual reality (VR) gaming systems. Sony's Playstation VR (formerly Project Morpheus) has been announced for the first half of 2016, Facebook's Oculus Rift is scheduled for Q1 2016, and Valve/HTC's Vive has promised a limited release during the 2015 holiday season. While the VR systems on the horizon are largely being marketed to gamers, everyone involved in their production sees the technology as having a much wider appeal. These companies dream of applications in virtual travel, training, and entertainment, to merely name a few.

Enthusiasm among people who have experienced the new VR systems is high. Jaded reviewers who have been introduced to "the next big thing" countless times over the years often come away giddy with excitement. But as with any new technology, there are some potential issues. Most obvious, VR can produce feelings of dizziness and nausea. But perhaps less obvious, those effects are symptoms of a deeper problem that stems from the way VR interacts with the system that allows people to keep their balance. Vision is an essential component of the balance system, and viewing a virtual world in place of the real one can induce mild feelings from vertigo and motion sickness to extreme nausea and loss of balance. And while any user of the technology may experience these symptoms, the balance problems are likely to be worse for one particular demographic—older people.

Balancing excitement with balance

They call it "presence," although no one is sure exactly what that means. VR simply creates the illusion of being present in a virtual environment. The illusion rests on a convincing 360-degree visual presentation combined with the ability to interact with objects in the virtual world using a handheld controller. When it all comes together seamlessly, the sense of being present in the virtual world can be overwhelming. This sense of actually being in a place you know is not real can lead to a euphoria, one that has been reported time and again by people who have experienced it.

Again, gamers are viewed as the most likely early adopters of these new VR systems. But once the technology is in the hands of consumers, people will discover many other uses for it. Class field trips could become quaint experiences from yesteryear, when students can be virtually present anywhere in the world without leaving the classroom. Training across a wide variety of occupations can be enhanced when the trainee learns in a virtual environment that is very similar to the real work environment. The opportunities for the entertainment industry are endless. Why watch a movie on a screen in your living room when you can be present in the world where the action is taking place? Walk through any museum you like taking as much time as you like with any piece of art that you like, explore the ruins of Machu Picchu at your own pace, watch the World Series from seats behind home plate or the first-base dugout, whichever you prefer. Do it in the evening, in your living room, with presence that is so real it feels like you're there.

But seemingly every report chronicling the development of the new VR systems has been accompanied by stories about the corresponding dizziness and nausea. And while these seem like simple symptoms, dizziness and motion sickness are complex, multi-causal phenomena that are not fully understood.

Researchers largely accept that dizziness and nausea involve disruption of the balance system. In this vein, motion sickness can be viewed as an early warning signal, telling us something in our current situation lies outside of the balance system's boundary conditions. Nausea that ramps up from uncomfortable to incapacitating is a pretty extreme early warning signal.

Why do so many experience these sensations and why can it be so extreme? In short, keeping your balance is an essential function. Balance allows us to maintain an upright posture while moving around and interacting with objects in the Earth's gravity field. And maintaining balance relies on the interaction of three sensory systems: proprioception (a sense of your body parts and strength), the vestibular sense (dealing with spatial orientation), and vision. Information from these systems is integrated in the cerebellum and spinal cord through a network of complex feedback loops. Together this system operates below the level of conscious awareness when functioning normally, but we have means of telling when something in the system is out of whack—feelings of nausea and dizziness.

Proprioception provides the foundation for balance by providing information about how the different parts of the body are positioned in space with respect to one another and with respect to the body's center of gravity. Receptors in the joints, muscles, and skin tell you about the nature of the surface that is supporting your body, as well as where your body's center of gravity lies with respect to that surface. For example, if you lean forward while standing on a hardwood floor, receptors in your toes, ankles, and the balls of your feet provide critical information: your weight is being supported by your feet, you are standing on an unyielding surface, and the body's center of gravity has shifted forward.

The vestibular system, located in the inner ear, provides information about the motion of the head (both linear and rotational) and how it's positioned with respect to gravity. It helps maintain balance through a number of reflex actions. For example, the vestibulo-ocular reflex keeps your eyes focused on a target when your head moves. Without this focus, people often become dizzy when their head spins around.

The vestibulo-spinal reflex is another example. Stand quietly with your eyes closed and your feet together. If you concentrate on the sensation from your feet, you will feel your weight subtly shifting from front to back and side to side. These tiny movements are called sway.

You don't sway too far and fall over because the vestibulo-spinal reflex makes use of proprioceptive information from your feet and ankles to counteract sway. It does so by adjusting muscle tone in the legs and trunk to bring the body's center of gravity back toward the center of its base of support. Sway increases as people get older, one of the many factors that increases the likelihood that older people will lose their balance.

The brain uses visual information to make sense of the information provided by the vestibular and proprioceptive systems. When necessary, it prepares the body to execute movements that prevent a loss of balance.  For example, seeing a set of descending stairs in your path prepares the system to execute the changes in gait and body angle that walking down stairs necessitates. Visual, vestibular, and proprioceptive information are combined when you begin the descent so you can walk down the stairs without falling. (This is no small thing as you well know if you've ever stepped off what you thought was the last step at the bottom of the stairs only to discover that the floor wasn't where your foot was prepared for it to be.)

Vision also serves an important corrective function. When proprioceptive and vestibular information are inadequate or in conflict, visual input is used to resolve matters and keep the body in balance. Any task that challenges balance, such as standing on one foot, is more difficult if you close your eyes.

Disruption of any of these three systems can produce dizziness, motion sickness, or loss of balance. So while VR typically doesn't affect the proprioceptive or vestibular systems directly, it has a radical effect on vision. Replacing vision of the real world with vision of a virtual world stresses the balance system in ways that can produce problems.

When a VR experience works as planned, the visual input about the virtual world is coherent with the real-world input from the vestibular and proprioceptive systems. The user is able to experience the extraordinary feeling of being present in the virtual world without discomfort.

When the visual input is in conflict with the input from the other two systems, the result can be what we discussed above: disorientation, nausea, and loss of balance. In many circumstances, the disruption of balance by VR is mild and inconsequential. However, it can reach levels that are severe enough to result in staggers and falls—and this problem may be exacerbated for older users.

VR challenges balance because the visual information provided by the VR system is an unreliable indicator of the body's actual status with respect to gravity. Balance can be lost when the adjustments needed to maintain balance don't get made because of misleading visual information.

Think about the example of walking down stairs that was mentioned earlier. When you descend stairs, you lean back to counter changes in the relative positions of the body's center of gravity and its base of support. Suppose you are rapidly descending stairs in the virtual world while standing upright in the real world. You may begin to lean back because vision tells you that you are going down stairs.

If the virtual experience is compelling enough, it may override the proprioceptive information that tells you you're standing still. The vestibulo-spinal reflex doesn't correct for the backwards lean—you stagger and possibly fall.

Balance can also be lost when visual information from the virtual world triggers postural changes that are unnecessary. The VirZoom demo that took place at the most recent E3 in Las Vegas provides a good example. VirZoom is a system that allows the user to move through a virtual world by pedaling and steering a stationary bicycle. Many people who tried VirZoom found it thrilling.

They also found it dangerous. Bicycle riders typically lean into the turn when taking a corner. During the demo, visual input overwhelmed vestibular and proprioceptive input, and users tended to lean into virtual turns, losing their balance on the stationary bike.