prisms glasses that you wear. The concept for PCS is that it helps to take some strain off the brain. I think used more for dizziness than headaches. They say its when the eyes are trying too hard to stay in focus (i think its called convergence...when the eyes want to go outward).

see if you can watch this video: (if not google debby feinberg)

http://www.youtube.com/watch?v=dYKvbYpM8b0