Below are some examples:
Symptoms of playing specifically Half-life 2 for some people include dizziness, severe nausea, headaches, migraines, extremely tired eyes, sweating, feeling of pressure in the head, vomiting, and overall ‘food poisoning’ vibe.
Settings, age, experience with gaming, health etc. seemingly have no impact. For many people, Half-life, Half-life 2 and Portal 1 are the only games, out of hundreds or thousands of titles, that ever made them sick.
Changing Field-of-View helps some people, but many (including myself) are unaffected. Same goes for disabling any movement. Even looking at some (but not highly compressed ones) recordings of the game or at the menu screen for more than a few minutes is enough to trigger these symptoms.
My current theory is that early Source games were using some optical illusions to make the games appear higher quality.
I’d love to hear your input. I’ve been thinking about this for 3 yeas and can’t find any concrete answers.