Really, shadowing is more like watching a car drive by a few times than it is like sitting in the passenger seat. Clinical rotations are more like the passenger seat, you get to feel all the motion of the vehicle and play with the radio but very little of the responsibility is yours.
As far as neurosurgery goes, I doubt many medical students get to do much besides sitting in the passenger seat. No offense but I sure wouldn't want some MS3 digging about in my noggin just for a hands on learning experience. I've also read that other specialties, like ophthalmology, don't give med students much hands on experience either.
The best it really will get for choosing a specialty is figuring it out from rotations that you get to do in MS3 and early M4. So, make the most of what you get.
There are also some online test that can give you an idea of what specialty you might like.
http://www.smbs.buffalo.edu/RESIDENT/CareerCounseling/interior.htm?self-assessment.htm
http://www.med-ed.virginia.edu/specialties/
For me, I have an idea
about what I might want to do. I called a local doc's office that does that type of specialty, told them that I'm an entering MS1, and asked to get some shadowing experience that is geared towards 'getting my feet wet'. They were all like, 'Hellz yeah, we would love to have THE Onetyme in our office!'
From where we are, I guess that is all we can do.