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. Generally, police consider it the most reliable of the three tests.
(The National Highway Traffic Safety Administration (NHTSA) concluded, based on its research, that the three standardized FSTs were reliable tools for determining whether a driver has a blood alcohol concentration (BAC) of .1% or more.)
“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 one such factor, according to the NHTSA. That’s why police use the HGN test in DUI investigations.
Usually, officers administer the HGN test by having suspects follow a pen with their eyes—so nonlawyers sometimes call it the "pen test."
How Is the HGN Test Administered?
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:
- “I’m going to check your eyes.”
- “Keep your head still and follow this stimulus with your eyes only.”
- “Keep following the stimulus with your eyes until I tell you to stop.”
The officer then positions the stimulus—typically, a pen or the officer’s finger—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.
Before looking for signs of inebriation, the officer is supposed to check to see if the driver’s pupil sizes are the same and whether the driver’s eyes track equally (meaning, one eye doesn’t lag behind the other while following the stimulus). Pupils of different sizes or unequal tracking can indicate the driver has a medical disorder or injury—conditions that can cause nystagmus. For drivers with these types of conditions, the HGN test is a less reliable gauge of intoxication.
What Are HGN Clues?
While moving the stimulus, the officer watches the driver’s eyes for three “clues” of intoxication:
- lack of smooth pursuit,
- nystagmus at "maximum deviation," and
- “onset of nystagmus prior to 45 degrees.”
The officer counts the number of clues for each eye. For example, if the officer sees lack of smooth pursuit in both eyes, that counts as two clues. According to the NHTSA, the existence of four or more clues indicates the driver’s BAC is most likely above .10%.
Lack of Smooth Pursuit
Generally, an unimpaired person can smoothly follow a moving object with their gaze, much like a mounted security camera can smoothly pivot from side to side. “Lack of smooth pursuit” means the driver's eyes jerk or bounce as they attempt to follow the stimulus. However, it’s imperative that officers move the stimulus at a constant rate to avoid inducing this scorable clue.
Nystagmus at Maximum Deviation
To look for this clue, the officer moves the stimulus to the side until the driver's eye can’t go any further. This position is called the “maximum deviation.” The officer holds the stimulus in this spot for at least four seconds. Jerkiness in the eye while it’s in this extreme position counts as one clue.
Nystagmus Prior to 45 Degrees
“Nystagmus prior to 45 degrees” refers to when the driver’s eyes exhibit jerkiness as the officer moves the stimulus between the starting point at the driver’s nose and 45 degrees to the driver’s right or left. Typically, officers use the driver’s shoulder as a reference point for the approximate 45-degree mark. Nystagmus at any point prior to reaching the shoulder is scored as a clue.
Is HGN Evidence Admissible in Court?
Generally, officers are allowed to testify in court about observations they made during an HGN test, but not before describing the testing procedures and their qualifications to administer the test. However, some states courts consider HGN results scientific evidence. These states require expert testimony on the scientific reliability of HGN before an officer is permitted to testify about a driver’s performance.
Get In Touch With An Attorney
DUI laws differ by state. And the facts of a given case have everything to do with the outcome. If you’ve been arrested, get in touch with an experienced DUI attorney who can explain the relevant law and procedure to you.