According to what I remember from college. The Amigdala controls the flight or flight mechanism. The Hypocampus is the memory center of the brain. I'm not really an expert, but my guess is that they'll be looking for it in either the hypocampus, or else the amigdala. I suppose they could find something in the speech center center of the brain. On discovery channel: A new lie detector test uses an MRI scan. It shows that when a person is lying there is hyperactivity in the speech center of the brain.
So this is probably the application of the MRI Lie Detector test.
If you don't really know but shoot your mouth off anyway, does that make me a liar?