Making eyes track

I am building a set of eyes, using two servos per eye to give me the up/down and left/right motion. Each eye is 18" in diameter. I am planning on using the SSC-32 to manually control the motion of each eye; either together or seperately to give me that ole cross-eyed look.

What input sensor would be best to pick up motion and then track the motion, making the pair of eyes follow an object? And could that sensor work with the SSC-32 to control the pair?

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