Horizontal Gaze Nystagmus is basically involuntary eye movement, and it's got some interesting science behind it.

Nystagmus and DUI: The Inside Scoop Officers use Horizontal Gaze Nystagmus (HGN) and Vertical Gaze Nystagmus (VGN) tests on the field. The deal is, the more your eyes wiggle involuntarily, the more likely it is you're under the influence. Citek, Ball, & Rutledge in 2003 found a pretty solid link between the number of eye wiggles and Blood Alcohol Content (BAC) levels.

In 1986, Halperin & Yolton figured out that these eye movement tests could correctly spot about 80% of folks with a BAC of 0.10% or more. That's a better track record than some of the more old-school tests.

In 2001, a research pointed out some issues. The accuracy of the HGN test can get a bit shaky, especially if your BAC is going up or down. And if you're just plain tired, that can throw the results too. Plus, not all officers stick to the rulebook when they're doing these tests.

Gilbert in 1999 mentioned another key point - the officer's training and experience matter a lot. If they're not up to speed, the reliability of these nystagmus tests can take a hit.

Wrapping It Up So, nystagmus tests can be a pretty good way to tell if someone's had too much to drink. But, it all boils down to how well the test is done and who's doing it. Things like changing BAC levels and how closely officers follow the testing procedures can make a big difference in DUI cases.

Just a heads up, this isn't legal advice. Every DUI case is its own beast, so if you're in this situation, you'll want to chat with a lawyer who knows their stuff.