Ideally it would be in neurosurgery. We are a small field and people know each other. Research relevant to the field is easier to gauge in value, and research letters from friends and respected colleagues carry more weight or at least be more easy to interpret (i.e. if a neurosurgeon say's some kid is the bees knees out of 20+ applicants that have passed through his/her lab, that has weight). Other fields may care less about the topic of research, but unless it is MD/PhD basic science stuff neurosurgeons prefer neurosurgery research.