The horizontal gaze nystagmus test is a field sobriety test used to investigate potential DUI suspects. The officer asks the suspect to follow an object with his eyes. If there is nystagmus, or jerking of the eyes, it is considered a sign of intoxication.
Law enforcement officers use a variety of field sobriety tests (FSTs) to evaluate whether a driver is impaired or not. One of the most “scientific” of these is the horizontal gaze nystagmus test, or HGN test. This test is considered scientific because it has been widely tested both in laboratory and field conditions. When properly administered, it has a 77% reliability rate and can help officers make a decision on whether to arrest you for DUI.
What is horizontal gaze nystagmus or HGN?
HGN is a twitching or jerking of the eye. There are actually several types of nystagmus, but HGN is twitching of the eye when looking horizontally to the left or right. Most sober people have little or no HGN until they look all the way to the far left or far right, as far as the eye can move. Even then, a sober person typically has only faint nystagmus.
When someone has been drinking, however, they exhibit more HGN. Their eye will twitch even if looking less than 45 degrees to the left or right, and it will twitch more noticeably.
Nystagmus is an involuntary movement. That means that you cannot control it and, in most cases, you don’t even know when it’s happening.
How is the HGN test done?
To do the HGN test an officer will ask you to follow an object (such as a pen) with your eyes, without moving your head. They have to give you clear instructions and administer the test correctly in order for it to be considered reliable. The officer will then look for several things:
- First, they check for any resting nystagmus when your eyes aren’t moving at all. This is simply to evaluate whether you have a condition that causes nystagmus other than being intoxicated.
- Then they observe how smoothly your eyes can track the object. If either eye cannot track smoothly, that is the first “clue” to intoxication.
- When your eyes reach the far left and far right, they look for nystagmus. If there is any, it should be faint. If there is pronounced nystagmus in either eye, however, that is the second clue.
- Last, they check for nystagmus when you are not looking as far to the left or right—a 45-degree angle or less. If there is any nystagmus at all, that is the third clue.
If any of these clues are present in either eye, that counts as one “point” on the test, and each point counts against you. You could end up with as many as 6 points total (three clues for each eye), but it only takes 4 or more points to fail the test.
If you fail, officers will assume you have a blood alcohol concentration (BAC) of .10% or more, which is higher than the legal limit for driving.
Can I be convicted of DUI based on the HGN test?
Yes and no. Technically, the court considers the HGN test more reliable than any other field sobriety test, and failing it does count as valid evidence against you. It’s important to attack this evidence in every way possible.
On the other hand, an HGN test is much weaker evidence than, for example, a breath test or blood sample. It would be rare to be convicted based only on the HGN. A good DUI lawyer will work to question the credibility of your HGN test and undermine its value as evidence.
How do I attack the HGN test as evidence in my DUI case?
HGN tests suffer from two major weaknesses:
- Even if administered correctly, they are only approximately 77% accurate. That means that nearly one fourth of all people who fail the test are actually sober.
- Law enforcement are notorious for doing the test incorrectly, either because of poor training or not following the procedures.
Both of these weaknesses can mean that the HGN is not accurate in your case, or that it can at least be called into doubt. Some of the reasons the HGN may be inaccurate include:
- You are on medication that causes nystagmus, including seizure medication and a variety of depressants.
- You have suffered a brain injury in the past, or may have suffered one in your accident.
- You have nystagmus even when not moving your eyes.
- You were wearing contact lenses that irritated your eyes and caused twitching as you moved your gaze.
- You normally wear corrective lenses that help with an eye “twitch” but were not wearing them during the test.
- There were bright flashing lights nearby during the test, such as police squad car lights.
- The officer did not hold the pen or other object at the correct 12-15 inch distance from your face.
- The officer did not properly instruct you.
- You moved your head as well as your eyes and the officer did not clarify and start over.
- The officer did not correctly estimate where “45 degrees” is to the left or right.
- The officer’s eyes were lower than your eye level during the test.
- The officer did not move the pen or other object in a smooth, steady manner.
Any of these factors can be used to discredit the HGN test in your case. And, even if the HGN stands, it is not a full-proof evidence of DUI. A good lawyer can often win a DUI case regardless of the HGN results.
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