If you were stopped for DUI, the officer almost certainly moved a pen or finger in front of your eyes and watched them. That is the Horizontal Gaze Nystagmus (HGN) test, and officers treat it as the strongest of the roadside tests. I am Joel Brand, and while HGN can look scientific, it is also one of the easiest field sobriety tests to challenge. Here is how it works and where it breaks down.

What HGN actually is

Nystagmus is an involuntary jerking of the eye as it moves to the side. Alcohol and some drugs can exaggerate it, and the officer is looking for that jerking as a sign of impairment. The catch is that nystagmus has dozens of natural and medical causes that have nothing to do with alcohol.

How it is supposed to be done

Under the federal (NHTSA) protocol, the officer holds a stimulus about 12 to 15 inches from your face and slightly above eye level, then looks for three specific clues in each eye: lack of smooth pursuit, distinct jerking at maximum deviation, and onset of jerking before 45 degrees. Six clues across both eyes is the threshold the officer is taught to treat as a fail. The procedure, the timing, and the stimulus position all have to be exactly right, and frequently they are not.

The three clues, and why the timing matters

Each of the three clues has to be checked in a precise way and held for a set amount of time. For lack of smooth pursuit, the officer is supposed to move the stimulus slowly across your full field of vision and watch whether the eye tracks like a marble rolling across glass or jerks like a windshield wiper. For nystagmus at maximum deviation, the officer must hold your eye at the far corner for at least four seconds. For onset before 45 degrees, the officer must move the stimulus at a controlled speed and estimate the angle at which jerking begins, with no protractor, in the dark, by eye. Rushing any of these steps, which officers routinely do, produces clues that are not really there. When I get the video, the first thing I time is how fast and how steadily that pen actually moved.

Why HGN is so often wrong

  • It is administered roadside by a non-doctor. Officers are not optometrists or neurologists, yet HGN is essentially a medical eye exam.
  • Many things cause nystagmus. Fatigue, caffeine, nicotine, eye conditions, inner-ear problems, head injuries, and a long list of medical conditions and common medications all produce it.
  • Roadside conditions corrupt it. Flashing patrol lights, passing traffic, and the driver's head movement all skew the result.
  • Small procedural errors invalidate it. Moving the stimulus too fast, holding it at the wrong distance, or miscounting the passes undermines the whole test.
  • It cannot give a number. California courts limit how HGN is used; an officer cannot translate it into a specific BAC, only offer it as one piece of opinion evidence.

Nystagmus you were born with, and other innocent causes

A surprising number of people have natural nystagmus that has nothing to do with alcohol. Congenital nystagmus, for example, is present from birth and is permanent. There is also a phenomenon called optokinetic nystagmus, which is triggered by moving lights and visual stimulation, exactly the environment of a DUI stop, with strobing patrol lights and headlights from passing cars. Certain prescription and over-the-counter medications, neurological conditions, and even severe fatigue can all produce eye movements that an officer will read as alcohol impairment. The officer has no way to tell at the roadside whether the jerking he sees is from a drink or from biology, and he is not qualified to make that medical judgment in the first place. When I learn that a client wears glasses or contacts, has had eye surgery, takes medication, or has any neurological history, that information often becomes a central part of explaining the so-called clues to a jury.

Why officers like HGN, and why that is a problem

Officers favor HGN because, unlike the walk-and-turn or one-leg stand, the subject cannot see their own performance and cannot tell whether they are "passing." That makes it nearly impossible for a driver to dispute in the moment, and it makes the officer's word almost the entire evidence. But that same feature is the weakness: the result rests entirely on what one officer claims he saw in your eyes in the dark, with no objective recording of the eye movement itself, no number, and no second observer. When the only proof is one person's interpretation of a subtle, involuntary movement that no one else can see and no camera reliably captures, that proof is fragile, and a careful cross-examination can expose just how little is really there.

How I challenge HGN

I look at the officer's training and certification, every deviation from the NHTSA protocol, the conditions at the scene, and the innocent and medical explanations for what the officer saw. HGN rarely stands on its own, and when it is the centerpiece of the impairment case, discrediting it can unravel the whole charge. It pairs with the broader attack on the roadside tests in the unfair field sobriety test defense, alongside the walk and turn and one-leg stand, and it ties into the most common police mistakes at a DUI stop. See my full top DUI defenses.

You can decline field sobriety tests

For drivers 21 and over, the roadside tests, HGN included, are generally voluntary, and there is usually no penalty for politely declining them. That is different from the post-arrest breath or blood test, which is mandatory and carries real consequences for a refusal. I explain the distinction in whether to refuse field sobriety tests.

Did an eye test help convict you? Let's look.

If HGN is driving the case against you, it deserves a hard, informed challenge. Use the free case analysis on this page, or call me directly at (888) 271-6644. I answer my own phone, 24/7.