“[W]hile standing and in the middle of a sentence, I had an incredibly strange, weird moment of comparing real life to the VR,” wrote the video-game developer Lee Vermeulen after he tried Valve’s SteamVR system back in 2014. He was mid-conversation with a coworker when he started to feel off, and the experience sounds almost metaphysical. “I understood that the demo was over, but it was [as] if a lower level part of my mind couldn’t exactly be sure. It gave me a very weird existential dread of my entire situation, and the only way I could get rid of that feeling was to walk around or touch things around me.”

It seems that VR is making people ill in a way no one predicted. And as hard as it is to articulate the effects, it may prove even harder to identify its cause.

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The notion of virtual-reality devices having a physical effect their users is certainly familiar. Virtual-reality sickness, also known as cybersickness, is a well-documented type of motion sickness that some people feel during or after VR play, with symptoms that include dizziness, nausea, and imbalance. It’s so common that researchers say it’s one of the biggest hurdles to mass adoption of VR, and companies like Microsoft are already working rapidly to find ways to fix it.

Some VR users on Reddit have pointed out that VR sickness begins to fade with time and experience in a headset. Once they grew their “VR legs,” they wrote, they experienced less illness. Van Schneider has noticed the same thing. “[The physical symptoms] usually fade within the first 1–2 hours and get better over time,” he wrote. “It’s almost like a little hangover, depending on the intensity of your VR experience.” Indeed, VR sickness is often referred to as a “VR hangover.”

The dissociative effects that van Schneider and others have written about, however, are much worse. In an attempt to collectively self-diagnose, many of the internet-forum users have point ed to a study by the clinical psychology researcher Frederick Aardema from 2006 — the only study that looks explicitly at virtual reality and clinical dissociation, a state of detachment from one’s self or reality. Using a questionnaire to measure participants’ levels of dissociation before and after exposure to VR, Aardema found that VR increases dissociative experiences and lessens people’s sense of presence in actual reality. He also found that the greater the individual’s pre-existing tendency for dissociation and immersion, the greater the dissociative effects of VR.

Dissociation itself isn’t necessarily an illness, Aardema said. It works like a spectrum: On the benign side of the spectrum, there is fantasizing and daydreaming — a coping mechanism for boredom or conflict. On the other side, however, there are more pathological types of dissociation, which include disorders like derealization-depersonalization (DPDR).