Right. It's called a "gaze nystagmus" test. Sober or intoxicated, you can track something being moved in front of your eyes at a reasonable, moderate speed. When you're sober, your eyes track smoothly. When you're intoxicated, they will track in a jerky, start-stop-start-stop manner. When holding at maximum deviation, the eyes "bounce" (nystagmus) in intoxicated people but will hold steady in sober people. There are certain pathologies that can cause this, so the officers will do additional tests at roadside to confirm intoxication. Barring those pathologies, it's a reliable test because lack of smooth pursuit and nystagmus at maximum deviation do not occur after having only one or two drinks.It's called a field sobriety test and the reason is a tricky ear/eye interaction i learned about years ago in a class i wasn't paying attention to. The cop is looking for the smooth tracking of the eye and a bit of a wobble of the iris because the feedback to the brain from the balance receptors in the ear are thrown off by alcohol. I want to say nastigmus, or something like that. I know that can't be right, i wasn't paying attention. Probably hungover or stoned.
It's present in all sorts of other diseases and pathologies too, and other external stimuli. Not the most accurate determination of whether someone is sober or not.
Your turn, Mothra.