I am applying this year and asked this question to a few nsgy faculty members. From the advice I have received, neurosurgeons strongly prefer letters from other neurosurgeons. However, if you did a lot of research with someone and have made the choice to emphasize this, then you may want to ask your PI to write you a letter. However, this does not prevent you from getting 2 other neurosurgeon letters from your own institution and more when you do aways.
