I remember being wowed sometime back in the 60s or 70s reading about a prototype aiming system for helicopter guns using some kind of helmet mounted sensors that tracked the pilot's eyeball. The guns (or maybe rockets) were aimed where the pilot looked, and fired when the pilot blinked in a certain overstated way.
The helmet was pretty big, but that was a long time ago. So the technology exists (?) to register which monitor the user is looking at.