I actually think the thought process of imagination would be made stronger without the limitations or percieved experiances. Our imagination is somewhat limited to what we know to be true. If you have no other experiances to learn from, your thought possibilities are endless.
However, without being able to know langauge, those thoughts would be totally alien to us.
Having said that, it is also possible that the lack of any senses, the brain would somehow adapt and use other parts of its brain to make up for the loss. Like a blind person being able to hear better.
I think this would open the brain up to its raw potential.
Its like a computer, with free will that is not limited by its programming or hardware limitations and is free to do anything it wants.
I think it would be total darkness at first, but somehow a spark would ignite, a flash of light when the head is hurt, something. The brain can still sence temprature change for example, because it is a direct manipulation of the brain. Not to mention all the experiments done on the poor kid. I think something would spark and once it did, it would send the ball rolling. A brain like that must be hungry for knowledge, and once the ball gets rolling i think it would be an infinite roller coaster ride.