Brain freeze may be caused by direct cooling of the roof of the mouth near the hypothalamus, leading to vasomotor changes and pain receptor activation.

“Good question, the thing is that the roof of your mouth is very close to your hypothalamus. So if indeed it's a Popsicle that's giving you the brain freeze, it may be a direct cooling effect from the roof of your mouth and if you put your tongue there, you're insulating the roof of your mouth. I don't know, I'm guessing. - But what's the source of the brain freeze? It at vasoconstriction? - It's a vasomotor change. Whether it's constriction, I think it's more likely an increase in blood pressure, which will essentially cause an expansion of the arteries and activate pain receptors.”