Horizontal gaze nystagmus (HGN) is one of the three standardized field sobriety tests (FST) that police use to determine whether a driver has had too much to drink. The horizontal gaze nystagmus test generally involves the officer instructing the driver to follow a pen or some other object with his or her eyes. Generally, police consider it the most reliable of the three DUI field sobriety tests.
Here's more about how the HGN test works and the factors that might affect its accuracy.
"Nystagmus" describes a condition where the eyes make repetitive and involuntary jerking movements. HGN refers to the occurrence of this condition as the eyes gaze to the side.
Lots of factors—including fatigue and stress—can exacerbate nystagmus. Alcohol consumption is apparently another one of these factors. So, that's why police use the HGN test in DUI investigations.
Failing the HGN or another FST can get you arrested and have implications in your DUI case—though you generally don't have to participate in these tests if you don't want to.
First, the officer is supposed to minimize all visual distractions, including having the driver face away from rotating emergency lights and passing traffic. If the driver is wearing glasses, the officer will normally ask the driver to remove them. Next, most officers tell the driver something like:
The officer then positions the stimulus—typically, a pen—about 12 to 15 inches from the driver's nose, slightly above eye level. While moving the stimulus slowly across the driver's field of vision, the officer watches the driver's eyes.
While moving the stimulus, the officer watches the driver's eyes for three "clues" of intoxication:
The officer counts the number of clues for each eye.
The National Highway Traffic Safety Administration (NHTSA) did a number of studies where officers gave FSTs to people with known blood alcohol concentrations (BACs). The goal was to determine how well officers could identify a person with a BAC of .1% or more. According to the NHTSA, the studies showed officers could correctly identify these intoxicated drivers 77% of the time using the horizontal gaze nystagmus test. The NHTSA also said that when officers gave multiple FSTs to a person, they were able to accurately classify those with .1% BACs about 80% of the time.
There are lots of factors that can affect FST reliability. For instance, an officer not following NHTSA guidelines when administering an FST can lead to inaccurate results. And some people have physical or mental disabilities that might hinder their FST performance. Defense attorneys often challenge FST results based on these kinds of factors.