The Science of Locomotion in VR

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For anyone who’s ever tried a VR experience, whether it be using an HTC Vive, an Oculus Rift, Samsung Gear VR or just Google Cardboard, it’s possible you’ve experienced what many people don’t have a word for. That word you’re looking for is Vection, the feeling of moving when you are not.

How your body knows it’s moving

The human body is an interesting contraption because most of what you know about where you are in the world is based on a combination of sensory inputs that your brain processes and integrates to form a picture of your location and orientation. In no particular order…

Your eyes tell you lots about the world. They give you information about form and color, and about depth and distance. The latter is important for VR, as the observation of a parallax effect gives your brain information about your location based on that of objects in your environment.

For VR developers this isn’t a big issue, since most objects within VR are 3D models placed in fixed space. The challenge here has to do with motion, and how the user moves around in 3D space and what this feels like. For more about why this is challenging, we have to look inside your ears.

Your Ears contain something called Vestibular System, which is a group of components (bones, tubes, hairs, etc.) within the inner ear that contributes to your sense of balance and locomotion. In a nutshell, your inner ear contains a chamber full of fluid with small hairs called cilia affixed to the inside wall. When your body moves the liquid sloshes around and stimulates the cilia, and their motion tells your brain what is happening to your body (for a more detailed overview, feel free to read more on Wikipedia).

These cilia are able to detect translations and rotations (movement and turning), that give your body information about:

1. Orientation — Are you upside down or sideways?
2. Movement — You’re moving, but in which direction?
3. Acceleration — What is the rate or your movement or rotation?

A cognitive process called Proprioception lets you know where your body parts are without needing to look at them. If you shut your eyes, it’s pretty easy to tell where your hands and feel are, right?

Combining visual inputs, vestibular inputs and cognitive processes like proprioception tell us everything we need to know about where we are in the world.

The challenge for VR is that humans are evolved to actually move through physical space, and when one of our sensory systems tell us where moving and another does not, sensory misalignment occurs and our brains get confused.

Check out the GIF below. Which train is moving?

Another sense that shouldn’t be left out is auditory information. As you move through an environment, you can hear birds chirping or cars honking as you do, giving you a sense of where you are, and how you’re moving through space. While this sense is definitely useful for locomotion in the real world and VR, there isn’t that much research on how your brain can be tripped up by incongruent information (i.e. seeing a car in front of you, but hearing a non-existent car to your left). It may induce vection or affect how you move through space, but as of the writing of this article, most research that can inform VR design is focused on visual and vestibular inputs.

Motion Sickness

Feelings of vection can lead to motion sickness when we experience this sensory misalignment and our body doesn’t literally know which way is up, sideways, or forward. If you get carsick, this may have to do with a visual obstruction, meaning that you can’t see you’re moving, but your vestibular system senses the movement, and your brain can’t reconcile the two.

On the other hand, if you’ve ever gotten an ear infection, this can affect the vestibular system and you can easily experience feelings of vertigo or motion sickness even when your visual sense is telling you all you need to know.

Motion sickness can and does occur in VR, for a variety of reasons, and most of the time it has to do with the visual and vestibular systems being misaligned.

Passive vs. Active Movement

To deal with this sensory misalignment, there are a few things being done in the VR space so far, and these user experiences break down into two categories: Passive and Active Movement.

Passive Movement refers to movement that you own body does not control. An example of the former would be moving forward in a First Person Shooter (FPS) by using a thumbstick on a controller. In this instance, your eyes tell you you’re moving forward at a certain velocity and acceleration, but your vestibular system does not, so people tend to feel a bit ‘off’ after a while.

Active Movement is exactly what it sounds like, when your body is in control of the movement. The best example of this is using room sensors for Vive or Rift and physically walking around a room, crouching down and standing again. These experiences rarely lead to sickness, unless the VR environment deliberately moves objects in a funky way.

There is a spectrum of movement of course. Even when engaging in Passive Movement while playing an FPS, you’re able to look around, and your brain can sense that movement through both senses, but the second you’re moving passively and looking around actively, sensory misalignment can occur.

Strategies for VR Movement

To deal with these types of movement, and to alleviate motion sickness in VR, there are currently three main strategies:

Physically Walking Around: One of the best strategies, if you have access to the technology. This strategy reduces vection and motion sickness, giving the experience a more realistic feel.

Hand-based Controllers: As outlined above, not the best strategy. Your ears won’t know you’re moving, so if your users are sitting down while in VR, they need to be prepared for some vection.

Teleporting: This strategy is present in many FPS style games, where you’re able to physically walk around the environment (with room sensors), but if you want to move beyond the confines of your physical space, you’re able to use controllers to pick a point within your visual range and teleport to it, and from this point, walk around again. Of course if you don’t have room sensors, this strategy is limited to just standing in place, then teleporting to the next spot, and repeat.

Proxy Devices: There’s been a few options tried over the years, including cycling in VR (which leads to vection), the use of a Wii Balance Board to move around in the environment like you’re on a Segway (see video below), or treadmills that allow you to run around in place (top image). Again, because active physical motion in the real world results in changes sensed in your vestibular system, even these devices don’t physically make you move forward. You’re essentially pantomiming the actions that will move you around in the real world, but you’re not actually moving, so sensory misalignment still occurs.

The Future of Locomotion

There are a number of researchers actively working in the space of motion sickness and movement in VR, both for academic and commercial applications, so it will be interesting to see what they come up with in the next few years.

Given that VR does induce vection in certain circumstances, which can lead to motion sickness, the best thing we as designers can do is simply to be aware of this effect on our users and design accordingly so that they experience the least amount of discomfort as possible. The tradeoff here has to do with immersion v. comfort. While it’s true that walking around in an environment (either physically or with a hand-based controller) is incredibly immersive, the effect on the user might be negative, so giving the user a variety of ways to move around in the environment you’ve built is probably the best way. Just like in the real world, we have so many ways of moving around, whether it be walking, car, bus or bike, and the same should be true for VR.

If you’re an active VR user, as most tutorials and manuals can tell you, and if you’re experiencing vection, don’t stay in there for too long, because the longer you stay in, the more likely your brain will rebel because you’re feeding it incongruent information.

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Immersion is another aspect of VR Experience Design, so if you’re interested in passive and active interaction with objects in VR, check out the article below.

Direct v. Indirect Manipulation in VR