Horizontal Gaze Nystagmus Test: Clues, Scoring, and Refusal

The horizontal gaze nystagmus test is a roadside eye exam police use to check for impairment. An officer holds a pen or penlight about twelve to fifteen inches from your face and moves it slowly from side to side while watching each eye for involuntary jerking. That jerking, called nystagmus, becomes more pronounced when someone has consumed alcohol or certain other drugs because the brain loses fine control over the muscles that move the eyes. Because the reaction is involuntary, you can’t hide it or fake your way through it. The test was developed through research sponsored by the National Highway Traffic Safety Administration starting in the late 1970s and is one of three Standardized Field Sobriety Tests officers use in the field.¹

What the Officer Checks Before Starting

Three preliminary checks come before the test itself: equal pupil size, resting nystagmus, and equal tracking.¹ Each one is meant to catch a medical issue that could look like impairment.

Equal pupil size. Noticeably different pupils can indicate a prosthetic eye, a head injury, or a neurological disorder.
Resting nystagmus. Jerking of the eyes while looking straight ahead is uncommon and usually signals a medical condition or the influence of a dissociative anesthetic like PCP.
Equal tracking. The officer moves the stimulus quickly across your field of vision to confirm both eyes follow together. An eye that lags suggests a medical problem, not alcohol.

If any of these checks come back abnormal, the NHTSA manual says the officer “may choose not to continue with the testing.”¹ Going ahead anyway is not by itself disqualifying, but it’s a departure from protocol that a defense attorney can use in court. You’ll be asked to take off glasses; the officer notes whether you’re wearing contacts. The instructions are simple: keep your head still and follow the object with your eyes only.²

The Three Passes and What Each One Looks For

The stimulus sits about twelve to fifteen inches from the bridge of your nose and slightly above eye level so your eyes stay wide open.¹ From there, the officer runs three separate passes on each eye, and each pass hunts for a specific clue.

Lack of Smooth Pursuit

The stimulus moves steadily from center to the side, taking about two seconds to bring the eye all the way out. The officer is watching for whether your eye glides along with it or skips and jumps, like a marble rolling over sandpaper. At least two full passes across both eyes are required so the officer can confirm the effect is consistent rather than a momentary twitch.³

Distinct and Sustained Nystagmus at Maximum Deviation

The officer moves the stimulus all the way to the side until no white of the eye remains visible, then holds it there for at least four seconds.¹ The hold rules out the possibility that fast movement of the stimulus itself caused the jerking. To count as a clue, the nystagmus has to be distinct and continue through the entire hold.⁴ A quick twitch that fades doesn’t qualify.

Onset of Nystagmus Prior to Forty-Five Degrees

This pass is slower, roughly four seconds from center to the forty-five-degree mark.¹ Officers estimate forty-five degrees by the outer edge of your shoulder because they don’t carry protractors. If the eye starts jerking before it reaches that angle, the officer stops the stimulus and confirms the nystagmus is both distinct and continuous before scoring the clue. Earlier onset correlates with a higher level of impairment, which makes this the most telling of the three passes.

Scoring: Six Clues and the Four-Clue Threshold

Each eye is checked for all three clues, so there are six possible clues in total.⁴ NHTSA research found that four or more clues indicates a blood alcohol concentration at or above 0.08 is likely. Applying that four-clue threshold, officers classify subjects correctly about 88 percent of the time.⁵

Officers write the clue count into the DUI arrest report along with other observations, and together those facts build the probable cause needed for an arrest.³ HGN results by themselves are generally not enough to convict, but courts widely accept them as one factor supporting probable cause.⁴

Vertical Nystagmus

After finishing the HGN, the officer may raise the stimulus and hold your eyes elevated for at least four seconds to check for vertical jerking. This is not scored as part of the HGN, but if it appears, it points to a high dose of alcohol, another central nervous system depressant, an inhalant, or PCP. Vertical nystagmus won’t show up unless horizontal nystagmus is already present.⁴

What Substances Actually Trigger It

Alcohol is the main target, but not the only substance that causes HGN. Central nervous system depressants such as benzodiazepines and barbiturates, inhalants, and PCP all disturb eye-muscle control in the same way.⁴

Several common substances do not. Caffeine, nicotine, cannabis, hallucinogens, and narcotic analgesics such as opioids do not produce the horizontal jerking officers are trained to look for.⁶ Someone who has used only marijuana, for instance, should not exhibit HGN clues, and the appearance of those clues points toward a different impairing substance.

Medical and Environmental Reasons You Might Fail Without Being Drunk

The 88 percent accuracy figure assumes the test is run correctly under reasonable conditions. Several things outside your control can produce nystagmus that looks identical to the alcohol-induced kind.

Brain tumors, other brain damage, and some inner ear disorders can each cause nystagmus, though the resulting movement tends to be pendular (back-and-forth) rather than the jerk pattern tied to alcohol.⁶ Epileptic nystagmus can appear during seizure activity. Those are among the reasons the pre-test checks exist: resting nystagmus or unequal pupils can flag a medical cause before the test even starts.

Environmental factors matter too. Optokinetic nystagmus happens when the eyes lock onto an object that suddenly moves out of view or track sharply contrasting moving images. Rotating emergency lights, strobe reflections off road signs, and fast traffic passing close by can each set it off.⁷ The NHTSA manual says a properly administered HGN won’t be affected by optokinetic nystagmus, but defense attorneys often argue the roadside conditions made proper administration impossible.

How HGN Evidence Holds Up in Court

Most courts that have considered HGN under the applicable scientific-evidence standard have found it admissible. To get the officer’s testimony in, the prosecution generally has to show the officer was properly trained, including when the training happened, how many classroom hours it involved, and how many times the officer has done the test in the field. Testimony about what the officer observed usually needs only standard training and certification; testimony explaining the underlying science can require higher qualifications such as SFST instructor status or Drug Recognition Expert certification.⁴

Defense challenges most often go after administration errors: wrong timing on the passes, cutting the four-second hold at maximum deviation short, skipping the pre-test checks, or running the test under conditions where environmental nystagmus could contaminate the results. An officer who strayed from the standardized protocol faces harder questions on the stand, and the evidence can be excluded or given less weight.

Can You Refuse the Test?

Field sobriety tests, HGN included, are voluntary in most states. You can decline without triggering the automatic license suspension that follows refusal of a chemical test like a breathalyzer or blood draw. Implied consent laws, which you agreed to when you got your license, cover chemical testing, not roadside coordination or eye tests.

Declining doesn’t send you home. The officer can still arrest you based on driving behavior, the smell of alcohol, slurred speech, or your appearance. You’ll almost certainly be asked to take a chemical test after that, and refusing the chemical test brings implied consent penalties in every state. But turning down the HGN and the other field sobriety tests leaves the officer with fewer documented observations to bring into court. Worth knowing before you’re standing on the shoulder of a highway at midnight.

1
National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing (SFST) Participant Manual
2
Washington State Patrol. Standardized Field Sobriety Testing Instructions and Clues
3
National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing (SFST) Refresher Instructor Guide
4
National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus – The Science and The Law
5
National Highway Traffic Safety Administration. SFST Refresher Participant Manual
6
Texas District & County Attorneys Association. Field Sobriety Test Review
7
National Association of Criminal Defense Lawyers. Defending OUI Cases Part 2