One of the best lectures I saw this year at GDC was from Mike Ambinder, an experimental psychologist working at Valve Software, who delivered a fascinating talk explaining how Valve are measuring biofeedback and using the results in game in various ways.
For those of us that are not
experimental psychologists, this basically means they're looking at measurable physiological aspects, like heart rate and galvanic skin response (skin conductance), and then using those measurements to do cool things, like throw in more zombies if it detects you're getting relaxed in Left 4 Dead - or even cooler things in multiplayer.
Check out our write-up
of the presentation to see a cool look behind the scenes at one of the biggest names in gaming!