Half-Life house Valve has detailed some of its bizarre internal experiments, including measuring players' heart rates and sweat levels, in PC Gamer's latest interview.

Valve boss, Gabe Newell revealed that the developer's started experimenting with biometrics to focus test its games - and this has lead to some surprising results.

Newell told PCG: "When you look at our games, more and more we have this representation of player state, where we think we know how you feel, essentially. And with biometrics, rather than guessing, we can actually just use a variety of things like gaze tracking, skin galvanic response, pulse rate, and so on."

He continued: "There's some surprising side-effects that we didn't expect, like what happens when you expose that information in a social gaming context. It surprises us that how much value there is to the people who are playing.

"So if you're in a competitive situation, and you see somebody's heart rate go up, it's way more rewarding than we would have thought. And if you see somebody in a co-op game who's sweating, people tend to respond to that way more than we would have thought."

The Valve man says he already sees ways to incorporate the biometric tech in retail games:

"If someone comes up with a clever way to take some non-visible light and bounce it off your retina, and read it with your web camera, and get your pulse rate that way, then that's pretty cool. Because it may be a hard problem, but if you solve it once then you're done. It's not like a recurring hard problem..."