Yes, in Acts 2, the purpose of the tongues was a sign - it got people's attention.
The MRI's I've seen are when someone is speaking in a known language, but one not learned by the speaker. I've heard this several times in my own church. My friend, who is our most powerful intercessor, speaks in several known languages without knowing them. She has spoken French, Hebrew, (both of which I understand a little), and she has also prophesied to a Ukrainian congregation, I believe in Ukrainian. I'd have to check - I wasn't there at the time. I've also heard someone speak in a tongue, and known beyond a doubt what they were saying, but not recognize the language. God just told me, "This is what they are saying - interpret." The one time I hesitated too long, someone else spoke up, and said word for word what I had gotten.