In one test, Valve measured how much players sweat while playing Left 4 Dead, as correlated to their levels of arousal – just as Valve boss Gabe Newell specified back in March, concerning biometrics in the Steam Box. Another experiment gave players four minutes to shoot 100 enemies, and the game would move more quickly as the player showed signs of nervousness.
Valve also created a successful version of Portal 2 controlled with players' eyeballs, but it was necessary to separate aiming and viewpoint – where your eyes are looking and where your head is facing – for that to work properly.
The Steam Box will host some sort of biometric scheme, Newell said in that March interview.
"What we've found is you can directly measure player state and it turns out to be very useful," Newell said. "You need to be able to directly measure how aroused the player is, what their heart rate is, things like that, in order to continue to offer them a new experience each time they play."