The horizontal gaze nystagmus (HGN) test is a roadside eye exam used by police officers to detect signs of alcohol or drug impairment. It’s one of three standardized field sobriety tests developed by the National Highway Traffic Safety Administration (NHTSA), and it works by checking for involuntary jerking of the eyes as they track an object moving to the side. When combined with the other two tests (walk-and-turn and one-leg stand), officers make correct impairment decisions in over 91% of cases at the 0.08% blood alcohol level.
Why Alcohol Causes Eyes to Jerk
Your brain uses three systems to keep your eyes steady: visual fixation, the vestibulo-ocular reflex (which stabilizes your vision during head movement), and a gaze-holding system called the neural integrator, which keeps your eyes locked in position when you look to the far left or right. Alcohol, sedatives, and anticonvulsant medications disrupt that gaze-holding system. When it fails, your eyes can’t maintain an off-center position. They slowly drift back toward center, then snap back with a corrective jerk. That involuntary snap-and-drift pattern is nystagmus.
In a sober person, the eyes smoothly track a moving object, like a finger or pen tip. This is called smooth pursuit. But when the gaze-holding system is impaired, smooth tracking breaks down. Instead, the eyes make small, jerky jumps called saccades as they try to follow the stimulus. The higher the level of impairment, the more pronounced and earlier these jerks appear.
How the Test Is Performed
The HGN test follows a strict 10-step protocol. Officers are trained to administer it in a specific sequence so nothing is overlooked. The person being tested is asked to remove glasses, stand with feet together and hands at their sides, hold their head still, and follow a small object (a pen, penlight, or fingertip) with their eyes only.
The officer holds the stimulus 12 to 15 inches from the person’s nose, slightly above eye level so the eyes are wide open. Before checking for impairment clues, the officer looks at baseline indicators: whether both pupils are the same size, whether the eyes are already jerking at rest, and whether both eyes track together when the stimulus moves quickly from side to side.
Officers are also instructed to position the person away from flashing or strobe lights (like those on a patrol car) and to account for wind or dust that might irritate the eyes, since these conditions can interfere with the test.
The Three Clues Officers Look For
Each eye is checked for three specific clues, making six possible clues total. Four or more clues indicates likely impairment at or above 0.08% BAC.
- Lack of smooth pursuit. The officer slowly moves the stimulus from center to one side and watches whether the eye follows smoothly or jerks as it tracks. A sober person’s eye will glide like a marble rolling across glass. An impaired person’s eye will stutter, like a marble rolling over sandpaper.
- Distinct and sustained nystagmus at maximum deviation. The officer moves the stimulus to the far side of the person’s gaze and holds it there for at least four seconds. If the eye jerks distinctly and continuously while held at that extreme position, it counts as a clue.
- Onset of nystagmus prior to 45 degrees. The officer slowly moves the stimulus toward the side, watching for the point where the jerking first appears. If nystagmus starts before the eye reaches a 45-degree angle from center, it’s a clue. This is considered the most reliable indicator, because in a sober person, nystagmus typically doesn’t appear until the eyes reach a more extreme angle.
After checking both eyes for all three clues, the officer may also check for vertical nystagmus by moving the stimulus up and holding it, which can indicate high doses of alcohol or certain other substances.
Why HGN Is Considered Reliable
Unlike the walk-and-turn or one-leg stand tests, nystagmus is involuntary. You can’t suppress it through effort, practice, or awareness that you’re being tested. This makes it harder to “pass” despite impairment and harder to fail because of nervousness, fatigue, or poor coordination unrelated to alcohol.
A 2006 validation study confirmed that the full standardized field sobriety test battery, with HGN as its centerpiece, correctly identified people above 0.08% BAC in more than 91% of cases. HGN alone is generally considered the most accurate of the three individual tests.
Medical Conditions That Mimic Impairment
Nystagmus is not exclusive to alcohol use. A number of medical conditions and medications can produce the same eye-jerking pattern an officer is looking for. These include inner ear disorders like benign paroxysmal positional vertigo and Ménière’s disease, neurological conditions such as multiple sclerosis and stroke, vision problems including significant nearsightedness or astigmatism, and antiseizure medications. Some people have nystagmus from birth with no clear underlying cause.
This matters because a person with one of these conditions could display multiple HGN clues while completely sober. Officers are trained to note resting nystagmus and unequal pupil size as potential indicators of a medical issue rather than impairment, but the roadside test alone can’t distinguish between the two. If you have a medical condition that causes nystagmus, this is worth documenting and raising if you’re ever subjected to the test.
HGN Evidence in Court
Whether HGN results are admissible as evidence depends on the jurisdiction and which legal standard the court applies. The two primary standards are the Frye standard, established in 1923, and the Daubert standard, established by the Supreme Court in 1993.
Under the Frye standard, scientific evidence must have “gained general acceptance in the particular field in which it belongs.” Courts applying Frye need to identify the relevant scientific community and confirm that HGN testing is generally accepted within it. The Daubert standard, used in federal courts and many states, is more flexible. It requires the evidence to be both relevant and reliable, without demanding the same threshold of universal acceptance. Most courts that have examined HGN under either standard have admitted it, though some jurisdictions limit whether it can be used to estimate a specific BAC level or only as evidence of impairment in general.
Defense attorneys commonly challenge HGN results on procedural grounds: whether the officer followed the standardized protocol correctly, whether environmental conditions (flashing lights, wind) compromised the test, or whether a medical explanation for the nystagmus exists. The strength of HGN evidence often depends less on the science and more on whether the individual officer administered the test properly.