Physics BS to Cognitive Science PhD?

CyberShot

I'm aware of ZapperZ sticky on switching from X undergrad to physics grad. But, I'm wondering about the other way around? Does it depend on the program?

I'm a 3rd year physics major, who took a quarter off due to depression, and was very close to switching my major to computer science, but for some reason, my heart just won't let me give up the thing I love. Now, let's say once I graduate, I want to escape the physics PhD trap, and get involved in more cutting edge stuff like AI, brain-computer interface, etc. How hard will it be, say, to get into MIT's Cognitive Science or AI program?

Obviously, I would need to take some computer science courses. But how much? What are your guys' thoughts of keeping my physics degree until I'm really sure I want to switch?