Whether MIT and Caltech screen out introverts is not obvious. But it definitely pays at these schools to be able to interact comfortably with other students. The Psets are HARD and a collaborative environment between the students to solve these problems is encouraged, even expected. You are definitely better off at MIT and Caltech being an extrovert, rather than an introvert. Trying to go it alone, because you are reluctant to interact with your classmates, is really tough.