Emotiv.com has developed a headset that can trackshifts in emotions during gameplay. I assume that when they say “emotions” they are referring specifically to… the level of physiological arousal, which would be used to adjust the overall stimulation level of the game to match some predefined setting that the user has created.
For example, Billy, who is 10 years old, loves it when the game is at the most exciting setting. His parents are more comfortable with a less stimulating version most of the time. This shift in “stimulation factor” could be accomplished by utilizing unconscious triggers like speed, movement, size, proximity, sound volume, pitch and tonality, as well as colour saturation and contrast.