Question: What is the HGN test in Texas?
Answer: The HGN is a horizontal gaze nystagmus test. It’s an involuntary jerking of the eyeball. It’s something that you can’t feel, it’s something that you can’t see from the inside. It’s just any type of central nervous system depressant can cause the eyeball to have a jerky motion. Alcohol is one of those things that can increase it. It’s the test that they give is a three-part test: they look for equal tracking, onset of nystagmus prior to 45 degrees, and then distinct nystagmus when it’s held out way over on to the side of your—when you’re looking over to the side as far as you can.
And the interesting thing about that test, it’s something that nobody can see often other than the police officer. So, that’s the one test that we’re relying on the police officer’s testimony alone as to the results, unlike the walk-and-turn and the one-leg stand and the Romberg balance. The jury can often see on a video how a person does on those tests. But on the HGN, that’s something that the fact finder, or the jury, can’t see the jerking of the eyeballs. So, that one’s just kind of required to take the officer on his word as to whether it’s true or not. Sometimes it’s a little more believable from an officer than others.