Chaos;Head employs a unique system, where instead of usual choices presented in visual novels you get Delusion Triggers. In other words, our main character Nishijou Takumi at certain points in the game will start loosing mental health and a warning in a beeping sound and two lifelines - green and red - will appear. If you select the green trigger on the left Takumi will experience a positive hallucination, the red trigger on the right, however, will give him a "trippy" paranoid delusion. The third choice is not to select any of the triggers and just generally click on the screen. In that case Taku will rein in his delusions and won't experience a "trip".
source: /beliar-cos.blogspot.com/2011/06/chaoshead-review.html#more