Finally

http://www.livescience.com/19834-brain-freeze-blood-flow-migraines.html

Cause of Brain Freeze Revealed

Date: 22 April 2012 Time: 05:00 PM ET

Jennifer Welsh, LiveScience Staff Writer

Date: 22 April 2012 Time: 05:00 PM ET

"They saw that increased blood flow to the brain through a blood vessel called the anterior cerebral artery, which is located in the middle of the brain behind the eyes. This increase in flow and resulting increase in size in this artery brought on the pain associated with brain freeze.
When the artery constricts, reining in the response to this increased flow, the pain disappears. The dilation, then quick constriction, of this blood vessel may be a type of self-defense for the brain, the researchers suggested."