[Dmentor]

From a quick scanning of IEEE papers on robotics prosthetic interfacing, I'd suggest electrical engineering focus in signal processing and control theory. Be prepared for lots of graduate work since EE starts very broad as an undergraduate before you start to specialize. This broad scope will actually serve you well though as you touch on a lot of fields you'll need to be aware of. Build in electives from the biomedical, mechanical, materials and artificial intelligence domains. As Chris pointed out, these systems require a lot of complexity and it is very difficult to be both deep and wide.

Oddly enough this is a very similar path to the one I took although I wound up hunting submarines for a living instead. I will tell you that if you have a EE background and can also develop software you'll have a lot of employment options.