During your second year do you have time to do some shadowing and so forth. This way you'll get some idea if this is what you want. Then, when is your surgery rotation? If that is early enough then you'll know if surgery is for you or not at least somewhat early. And finally, the real important thing to know, are you sure you are not more keen on the things that are more competitive than neurosurgery (dermatology, ENT, radiation oncology, ortho, urology, possibly radiology). Because if you might be into one of those, then it is more critical that you figure out that neurosurgery is not for you early so you can start doing the things you need to do to match well into those fields.