Why is 90 FPS recommended for Virtual Reality games?

Question

Origin of the question

I have recently finished an internship at a (small) game development company which is working on a Virtual Reality game that is also used in medical rehabilitation. From what I have heard and read online, you generally want to achieve at least 90 FPS in a VR game because otherwise the player can get motion sickness or dizzyness, etc.

I am aware that not everyone is getting sick but from what I have read, you still want to aim for that 90 FPS to be sure, although I am unable to find the exact reason for this number. (it feels like an industry standard)

Questions

"Why is 90 FPS an industry standard when developing a VR game?" and "Why can people get sick when playing on lower frame rates?"

When searching online, I keep reading about 'studies' that show that people will be more likely to feel sick or dizzy when playing a VR game below 90 FPS but I never see the actual reason (source) as to why that is.

I can only make assumptions based on what I know already but I would like to know the exact reason (or as much of a reason as possible) and hopefully a source as to why 90 FPS is chosen (and not 80 or 100) and what causes some people to become sick from lower framerates.

sidenote: I am referring to VR games for devices as the 'Vive', 'Rift', etc. (as there are also VR Simulations for pilots, etc. which also have elements such as simulated movement that can impact sickness)

Answer 1

I'm going to start by answering your second question, which has a more concrete answer.

Why can people get sick when playing on lower frame rates?

In the real world, when you turn your head one direction, objects move across your field of view in the other direction. Consider the drawing below. When the person turns their head to the left, the tree is visible in their right field of view, and vice versa. Your brain expects this to happen whenever you move your head. The response time of your visual system is low enough that we can consider this to be instantaneous.

Imagine that someone has a stick mounted on their head holding a photograph of a tree in front of their face. In this situation, when the person turns their head left and right, the tree always remains in the center of their field of view, as shown below.

Now imagine if we put a screen on the stick instead of a static photograph. If we can change what is displayed on the screen when the person turns their head, then we can make the image of the tree behave as if it were an actual tree, moving across the field of view in the opposite direction of the turning head, as in the drawing below. This is basically how VR headsets work.

However, the visual system is finely tuned to this behavior and knows that it should happen pretty much instantly any time you move your head. If VR frame rates are too low, your head can turn faster than the image can keep up.

When your brain detects this discrepancy between the image it sees and the movement of your head, it thinks, "Hmm, something is poisoning my nervous system; I should get rid of whatever I just ate." This is the nausea and discomfort some people feel in VR, which is exacerbated by low frame rates.

Why is 90 FPS an industry standard when developing a VR game?

Because people tend to feel sick when the frame rate is below 90 FPS.

This is not a magic number; 89 FPS would be fine. I'm sure some neuroscience articles have identified the maximum amount of time that your brain will tolerate between frames, but VR developers most likely just pushed the frame rate higher until most people weren't feeling sick, as suggested in the comments. Also mentioned in the comments is the fact that a 90 FPS VR display will work well in conjunction with a 60 FPS monitor, since usually both are active at the same time.

Unfortunately I was not able to find any authoritative study demonstrating the 90 FPS "standard." However, some articles do mention the 90 FPS figure. If you are really curious, you may have to do some digging. If so, here are some places to start:

Temporal Resolution Multiplexing: Exploiting the limitations of spatio-temporal vision for more efficient VR rendering.

The Problem of Persistence with Rotating Displays.