It depends on what you want to do, and what you neuro residency will be like. If you want to be a neurointensivist, you'd better learn some medicine, actually, you'd better learn a ton of it. If your neurology residency has roughly 50-60 inpatients with every comorbidity known to man, you'd better learn some medicine at least so you can survive residency, because you aren't getting medicine consults on all of them, and they WILL go into AF with RVR if they smell fear.
If your residency is clinic/consult driven, or you are mostly interested in a specialty with less need for a broad internal medicine background, then you can probably focus on neuro during internship without losing out on much.
Please note, however, that if you've read many of my posts, you know I am a strong proponent of a broad knowledge of internal medicine and would never personally advocate for short-changing your medical education during internship. Not everyone agrees with this viewpoint. But remember, you are going to be a neurologist for your whole life, and there is a reason why until recently, neurology was a medical subspecialty.