"Right now we have to [observe players] simply by watching them and then we have to guess as to how certain things are affecting the player," Newell stated in an essay appearing on Edge. "We want to take that even further in the future," he wrote.
Getting scientific, Newell wrote, "One of the areas that we're really interested in is testing biometrics on player state." Biometrics is the practice of measuring activity in the human body such as heart rate--or, in this case, brain activity.
"There are new technologies where we can wire players up with EEGs and actually have direct exposure to their physical reactions to the games," the Valve head continued. EEG is the technology that measures brain activity by placing electrodes on the scalp. The product of an EEG is usually a chart with spikes and troughs, like a seismometer.
The benefit to Valve is precise knowledge of what works, and what doesn't, when making a game. "We can know for sure of something is actually frightening the player--their heart rate is going up ... appropriate parts of their brains are being activated," Newell asserted.
Concluding, Newell called the possibility "super exciting."