About 15% of migraine sufferers have a warning that the migraine headache is coming on. They experience a change in brain function called an “aura”. This is usually a visual symptom, such as an arc of sparkling zig-zag lines or a blotting out of vision or both. 

But any other brain-related symptom may occur, such as numbness of one side of the face and hand, weakness, unsteadiness, or altered consciousness. In past years these symptoms were thought to be caused by spasm of blood vessels supplying parts of the brain, and the headache was thought to be due to subsequent expansion (dilation) of blood vessels in the head. We now know that it is not that simple. The aura is due to changes that take place in the cortex, the outer layer of the brain. 

(Seymour Solomon, M.D., Headache Unit, Montefiore Medical Center, New York, (NY Headache, vol. 6, issue 4, December 1995)).