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Valve Index VR equipment running a flight simulator.

Addressing the virtual reality of motion sickness in the digital world

Virtual Reality: The Frontier of Simulation 

First, let’s talk about Virtual Reality, or VR. In an ever-changing world, technology seems to continually advance and this is no different for the world of screens. Interacting with various types of screens has become a normal part of everyday life, whether they be used to make phone calls using touch dialing (phones used to have buttons that you pressed), or presented to you once you entered a restaurant to order a meal. You probably used a screen to find your gate number that last time you took a flight, or maybe you have used an interactive map to find a store location at the mall. There are television screens, computer screens, phone screens, movie screens, marquees, and all types of screens in varying sizes, resolutions, and functionality. The larger these screens have gotten, the more immersive they have been able to become. But when this just isn’t enough, maybe mounting it on your head will be.

Over the course of the last few years, Virtual Reality has become the tech du jour in regards to immersive experiences. From training, gaming, remote operations, and various other concepts, the technology has rapidly gained popularity in a very broad consumer range. Since 2016, VR technology has really picked up steam, and just like most technology, it keeps on improving in some way or the other, always upping the specs, always creating conversations. It has even become a social interaction device with many different corporations enmeshing people in various “verses.” But on the contrast, it has stirred controversy with many individuals concerning whether it’s going to broaden our horizons, or make us get dizzy, or a mix of both, and this conflict has good reason. VR is a major catalyst for motion sickness and is associated with many different health problems, both short and long-lived, especially when used for long periods of time. But there are many different components that play into how sick one will get after a session in VR, and when properly accounted for, VR has the potential to be extremely practical in many ways. It can improve the health of many through therapeutical enhancement and surgical precision, allow for new possibilities in industrial calculations and automotive visualizations, and make way for fully immersive vehicular simulations, all without the dizzy spells that can make us feel sick.

Motion Sickness: It Exists 

Motion sickness is caused by many factors, both natural and artificial. It is not technically an “illness,” but symptoms of motion sickness can make you feel very ill. Primarily, motion sickness is caused by excessive and unpredictable movement in the body. It can typically occur in rocky vehicles, such as aircraft, boats, cars, rockets, etcetera, making one feel very queasy and nauseous. However, the body can use internal senses in combination with other factors, primarily vision, to adapt to external G-forces over time, which is demonstrated by Air Force pilots and NASCAR drivers. When the eyes look at a two-dimensional screen showing a three-dimensional environment, as when watching the television, playing video games, and most notably participating in virtual reality, discrepancies in the visual environment can eventually falsely match to one’s equilibrium and other internal senses. Most noticeable during camera movement and motion in VR, the body, attempting to counter what it thinks is incoming G-force, starts getting tipsy, tightening the stomach, leaning in response, and many other reactions which only make matters worse, and excess cognitive activity can intensify these reactions even further.

One’s point-of-view on virtual reality can be quite averse when their first try at it left them feeling dizzy and nauseous, and with the wrong equipment for the wrong software, severe motion sickness can be unavoidable, leaving you with another VR renouncer. However, one’s experience can be completely different if they use the proper combination of components, such as hardware and software, and it helps if the user has at least some tolerance for motion sickness.

What is the Solution? 

There are many different ways to lessen the effects of motion sickness during a VR session, especially during Virtual Locomotion. Simulating certain G-forces by swinging the arms, stepping in place, or physically moving around are just a few ways to ease the effects. Some organizations have developed other solutions to these issues. Software such as Natural Locomotion solve this issue by translating “swinging arm” movements into movement inputs that would normally be controlled by a finger. Some games & programs are doing this already. Hardware like the VIVE Tracker and the Tundra Tracker have been designed as devices the user mounts to their body that can be used to translate all body movements, rather than just the hands, allowing the user to see his or her own body and limbs in VR, which can reduce confusion in the mind. These technologies all share the same objective, which is to mimic the user’s body in virtuality as closely to reality as possible. Countless gimmicks and contraptions have been fabricated to try and combat motion sickness in VR, some of which have made it to market, but the most critical factors that affect or influence how motion sickness will occur, and at what intensity, revolve around the VR hardware technology being used.

The Equipment: It Makes a Difference 

While there are many different elements and configurations in a VR setup, the essentials normally remain the same and include the Head Mounted Display or HMD, the interaction apparatus, spatial tracking, and a computer for generating the image and processing inputs and outputs. When choosing the technology, there is no “one size fits all” configuration. The user’s needs dynamically adjust according to the environment, both virtual and actual, and the details that need to be known are very expansive.

The Headset: The Window to Virtuality 

The HMD is the single largest influencer in any VR configuration, and a lot of numbers subsist on the spec sheet, but there are four things in particular that are affiliated with motion sickness the most: refresh rate, field of view, resolution, and comfortability. Even if the computer used to generate the simulation is high-spec, a low-spec HMD can keep things very limited.

Refresh Rate: The Most Critical Number 

Refresh rate determines how many times a second the image in the VR headset refreshes. Essentially, it should sync up with the frame rate of the VR content. When the refresh rate is low, e.g. 72Hz, it can cause “ghosting” at areas of contrast, choppy animation, and an iridescent flicker. These factors can cause motion sickness during locomotion, quick head movements, and other fast-moving scenarios. A refresh rate of 90Hz is acceptable in most circumstances, and even higher refresh rates, e.g. 120Hz, typically eliminate these issues completely, at the cost of high power consumption, excessive heat generation, and more stress on the computer. This makes high refresh rates uncommon in standalone headsets, but not impossible to find in PC Virtual Reality (PCVR) Headsets. One company in particular, known as Pimax, has developed a few PCVR HMDs with refresh rates of up to 180Hz, among other unusually high specs, which altogether make the headset very heavy. As for standalone HMDs, some VR developers, such as Pico and VIVE, have designed headsets with refresh rates of 90Hz, which combined with other high specifications, inevitably require an extra powerful battery and computer, in turn, making them quite expensive and heavy too. Overall, refresh rate is the most influential factor for motion sickness during a VR experience, but getting the best refresh rate can come at a cost.

Field-of-View: The Peripheral Definer 

Field-of-View, or FOV, refers to how wide the view of the HMD will be. Wider FOV rapidly increases peripheral vision, making the experience much more immersive, and it enhances equilibrial precision, potentially reducing motion sickness dramatically. The equilibrium relies on many different body parts, such as the spinal cord, inner ears, and the eyes, among other things. When the head of the body moves, the peripheral nerves account for objects that the body moves past (furniture, decorations, ground texture, etc.), even if that person may not actively reckon them. When wearing an HMD, the peripheral nerves mostly detect a black tunnel leading to the world, which even though the user attempts to ignore it, can impart motion sickness so slowly that the user may not feel anything until a few minutes pass by. Most HMDs have an FOV of about 100 degrees to 120 degrees. But some developers, such as Valve, have designed canted lenses that increase FOV up to around 130 degrees, which is a significant improvement for the peripheral. A proprietary HMD known as the Rapture, used exclusively with the VR experience attraction destination, “The VOID,” has patented curved display panels that offer up to 180 degrees FOV, and one company in particular, Pimax, has widened these lenses altogether allowing the FOV to an hit an impressive 200 degrees, which is almost the FOV of human vision. The main drawback of high field-of-view, however, is that pixel density decreases the wider it gets, reducing clarity. Depending on the individual wearing the headset, if FOV gets too wide, distortion may be present in the peripheral, and depending on the user’s eye qualities, it can give false information to the peripheral nerves and trigger motion sickness. So to a more contrastive point, the one time high FOV can backfire is during virtual locomotion, because that is when the peripheral is getting false motion information. In response to this, some VR software programs can digitally decrease the FOV during virtual locomotion or rotation, which can solve this problem, and some host software that comes with certain headsets can allow the user to digitally reduce FOV manually, depending on the program to be used.

Resolution: A Beguiling Number 

VR resolution typically refers to how many pixels per inch are in both of an HMD's internal displays combined, meaning when the HMD’s resolution is marketed at 4K, it’s really just a pair of 2K displays, although some companies do specify their resolution “per eye.” While resolution tends to be the “big number” of any HMD, it is not necessarily the most important number on the spec sheet. Take a headset with 3K displays and a FOV of 200 degrees, for example. The number of pixels physically in the screen may be on the high end, but the compaction of the pixels will be similar to a headset with 1.5K displays and a FOV of 100 degrees. Pixels per inch (PPI) specifies how many pixels are in one inch on a display and is typically used in reference to flat screens, but in the VR world, pixels per degree of vision (PPD) is a more popular term. It divides the HMD’s resolution by the field of view running along the same line, preferably diagonally, to approximate how many pixels are in 1 degree of the FOV on the screen from the eyes. These numbers are the true specification on the definition of the displays, and a change of 10 PPD +- makes a significant difference. But unfortunately, PPI & PPD specs are not always listed on the spec sheet because it may vary for different users, meaning that the customer may have to approximate these numbers on there own. For VR experiences that include small words and complex, minute details that the user needs to see from a distance, (realistic flight simulators, historical galleries, mechanic simulators, etc.), it is recommended to have an HMD with at least 35 PPD and up to a very high 60 PPD, or more. But the average user should be fine at 20 PPD or even 10 PPD for software that is not detail dependent, e.g. basic geometry manipulation, simple vehicular simulators, arcade recreation, etc. PPD can be the most weighty spec on an HMD, and some eccentric VR developers, such as Varjo, have invented particularly expensive PCVR HMDs with PPD ratings of up to 70, while still maintaining 115 degrees FOV and 90Hz refresh rate, among quite a few other features. This makes for an exceptional experience in equipment training, advanced education, and virtual showcasing.

The downside with most high PPD headsets is that they are very resource hungry, and they render the entire screen at high PPD, which is very wasteful considering the fovea (the center of the eye where vision is best) is the only area that can detect the clarity of the screen. The outer peripheral cannot detect the clarity. However, some companies like Varjo and Vrgineers, have integrated eye tracking into the headset and use techniques such as “foveated rendering” to reduce the workload on the GPU by tracking the user’s eyes and rendering the spot on the screen where the eye’s fovea are pointing and leaving the surrounding area at a progressively decreased PPD. These techniques should not affect the peripheral equilibrium at all, and if applied properly, can allow for high pixel density ratings alongside substantial FOV.

Comfortability: Not an Exact Science 

Comfortability is a very, very broad subject in the VR industry, and many different things contribute, such as weight balancing, headset adjustment procedures, interpupillary distance (IPD), any obstructions the user may encounter, and many other factors. All these things play into how fatigued the user will be during and after a VR session and how much motion sickness the user will experience, especially when dealing with heavy headsets. Standalone headsets have the potential to be comfortable because there is no cord running from the back of the headset to the computer, meaning the user can face in any direction without coiling up in the cord or yanking the cord during big arm movements. But these headsets can be heavy due to the powerful and weighty portable battery and high-performance computing that must be built into the unit. On the other hand, if the battery pack is placed on the back of the headset, it can provide a counterweight which can lead to better front and back balance, making it more comfortable than other types. Tethered PCVR headsets do have a long cord running to its resources, but since it does not have to incorporate the power source and computing capabilities into the headset, these units are usually lighter in weight. Interpupillary distance (IPD) is a big make-or-break of an HMD. IPD determines how far apart the lenses are laterally from each other, and needs to be adjusted to line up the lenses with the pupils of the eyes as close as possible. Some HMDs, such as the Vrgineers line of products, and also the Varjo family, can track the user’s eye and adjust the IPD automatically. The wider the IPD of a headset, the more people that it can accommodate. Unfortunately, IPD tends to be particularly limited for some headsets, primarily because the lenses are the only part that move, not the displays themselves, meaning they can only go so far apart. Other factors that contribute to comfort include the audio delivery system, such as if it’s on the ear or off the ear, the process for the user to adjust the HMD to his or her head, how warm it gets around the user’s face, if it causes excess perspiration, and the list keeps on going. Overall, making a VR system as adjustable as possible will maximize user comfort and minimize motion sickness.