Vergence-accommodation conflict:
https://medium.com/vrinflux-dot-com/vergence-accommodation-c...
Also, lack of positional sensing (versus angular) on mobile VR devices is a much bigger contributor to discomfort.
I'm not sure why you're so keen on vergence accommodation, but it seems that you're focusing on a tiny effect, when the large effects are known.
Just because there's little in the peer-reviewed literature, does not mean that this is poorly understood by practitioners.
No one Ive met can stay in VR for hours regardless of game. Most of my friends who have tried or have VR devices get sick after 45 minutes. Admittedly they are not VR enthusiasts or hard core gamers however.
I'm not saying this qualifies as any sort of degree nor was I saying you should count my anecdotes as facts.
It is simply my belief that the vergence-accommodation conflict needs to be solved before VR will be adopted by the masses. I think vergence-accommodation neurological process should stay coupled as it does in nature to provide a completely natural feeling and non strained experience for the user. The research here is scant. Perhaps someone is aware of some ongoing research on this topic.
EDIT> I have a relatively low tolerance for motion sickness.
I've read that this is a problem for astronauts IRL when they're in free-fall next to a slowly rotating or panning object that occupies a lot of their field of view. So, that would be a kind of existence proof that it's not vergence, but rather optical flow that is decoupled from the subject's actual motion.
It sucks, but basically FPS games are out for VR, until you can actually run around, leap, etc.
EDIT> Another example -- during a scene in Apollo 11 where the camera (and hence my view) was orbiting the spaceraft while my IRL body was still, I started to feel queasy. I was able to reduce the sensation by looking at the growing space-craft as it started to fill more and more of my field of view -- it was relatively stationary to me. This again speaks to optical flow, and motion that is out of whack with what the player's ears are telling them. Another cheat along these lines is to use a cockpit that is fixed relative to the player's view to reduce the amount of visual area that is giving wrong motion cues.
EDIT2> I'd be interested to know whether you can last longer in a game that uses only your own IRL motion + instant teleport. So, no thumbsticks, mouse, keyboard locomotion -- as these are all known chunder inducers. Unfortunately non-VR games with bolt-on VR are like the worst-case for comfort, but also often the games that we really want to see in VR.
My educated guess is that it actually is quite important and the key to making VR more comfortable for long term use.
I think it is the primary issue. I am a software developer who works in the neuroscience field. I am not an MD but I have read enough research to know you cannot definitively say what the cause is, and that disrupting such natural processes in the brain are likely to have consequences. To me it seems obvious that it's the primary source of the headaches and nausea commonly associated with VR.
For example I have certainly felt sick in the real world from roller coasters. But I have never felt the type of sick I get when I put on a VR helmet. It's another feeling all together. And it comes on differently than any other sick feeling I've had. This is anecdotal of course. I digress.
If VR doesn't solve this issue, VR doesn't go anywhere IMHO. But it's obvious they will solve it in time and as someone who closely follows the field it seems to be one of their primary concerns.
You need foveated rendering to start with but im sure you know the rest. If you want VR, well you need actual VR. Its not enough to trick your vision superficially. You have trick the whole system. I digress.
Yes, even in a decent VR experience you will get headaches after an hour or two, possibly or even probably from vergence-accomodation conflict. But that's entirely secondary - you won't last more than a couple minutes on a virtual roller coaster.