The part of one's brain that actually does the thinking-as opposed to the areas that deal with sex ("Me want FUCK!"), food ("Me want peanut butter cups."), relationships ("Me want FUCK!"), seeking shelter/safety (frequently negotiated via demand for/willingness to provide sex and peanut butter cups), fashion sense ("You're going to wear THAT?").
Taking the college entrance exam really strained my "thinkulator"-I can tell because I notice a "fizzing" sensation in my head, then my eyes and ears start crossing and uncrossing.