The HGN Eye Test: How the Horizontal Gaze Nystagmus Test Works, and Why It Is Not Proof of Drinking

What the HGN test is

Nystagmus is an involuntary jerking of the eye. The HGN test asks you to follow a stimulus, the officer’s pen, fingertip, or a small light, with your eyes while you hold your head still, and the officer watches for that jerking as your eyes move side to side. Because the movement is involuntary and cannot be consciously controlled, prosecutors often present HGN as the most reliable of the three standardized field sobriety tests. That framing leaves out how much can make the eyes jerk for reasons unrelated to alcohol, and how easily the test can be administered wrong.

The six clues

The officer looks for six clues, three in each eye:

• Lack of smooth pursuit. The eye jerks instead of tracking the stimulus smoothly.
• Distinct and sustained nystagmus at maximum deviation. The eye jerks when held as far to the side as it will go.
• Onset of nystagmus before 45 degrees. The jerking begins before the eye reaches a 45-degree angle.

Four or more clues across the two eyes is the threshold for a positive result.

The protocol the officer has to follow

The HGN test is only as good as its administration, and the protocol is specific. The stimulus must be held about 12 to 15 inches from your face and moved at a controlled pace, roughly two seconds out across the full range for the smooth-pursuit passes, and held at maximum deviation for at least four seconds per eye to check for sustained nystagmus. The officer has to be positioned correctly, account for the lighting, and be able to clearly see your eyes.

Officers who move the stimulus too fast or too slow, hold it at the wrong distance, fail to hold it long enough at maximum deviation, or cannot clearly observe the eyes are not administering the validated test. Those deviations are exactly what a trained reviewer looks for on the dashcam and bodycam video, because each one undermines the reliability of the clues the officer claims to have seen.

Small protocol errors, large error rates

In 2007 NHTSA commissioned Dr. Marcelline Burns, a co-author of the original HGN research, to measure what happens when an officer’s administration varies only slightly from protocol. When the stimulus speed, height, or distance was off by small amounts, the share of people who were actually under 0.08 but still showed four or more clues (a fail) ranged from roughly 54 percent to 77 percent. Minor administration errors of the kind that are routine on a dark roadside produced false-positive rates well over half.

The medical checks the officer is supposed to do first

Before the HGN test even begins, the NHTSA protocol requires the officer to check three things: equal pupil size, resting nystagmus (jerking while the eyes look straight ahead), and equal tracking (whether both eyes follow the stimulus together).

These are medical-impairment screens. Unequal pupils, resting nystagmus, or eyes that do not track together can indicate a head injury, a neurological condition, or another medical problem, and a person who shows them should not be scored on the HGN at all, because the jerking cannot be attributed to alcohol. Officers frequently skip these checks or perform them too quickly to mean anything, which is one of the first things a trained reviewer looks for on the video.

Nystagmus has more than 47 causes besides alcohol

This is the heart of an HGN challenge. Horizontal gaze nystagmus is presented as reliable because it is involuntary, but the same involuntary jerking can be produced by dozens of conditions that have nothing to do with drinking. Documented non-alcohol causes include:

• Inner ear and vestibular disorders such as benign paroxysmal positional vertigo, labyrinthitis, vestibular neuritis, and Meniere’s disease.
• Neurological conditions including multiple sclerosis, brain stem lesions, cerebellar dysfunction, and traumatic brain injury.
• Naturally occurring nystagmus that is present at baseline in a percentage of the population.
• Certain prescription medications, including anticonvulsants, sedatives, and some antihistamines.
• Fatigue.
• Gaze-evoked nystagmus that appears at extreme lateral gaze in many people regardless of impairment.
• The flashing emergency lights present at every traffic stop, which can themselves induce nystagmus.

An officer who observes nystagmus but does not know, and does not ask, whether you have an inner ear condition, a neurological history, or a relevant medication cannot reliably attribute the jerking to alcohol. Cross-examining the officer on their knowledge of these alternative causes, and on whether they made any inquiry at all, is a standard part of an effective HGN challenge.

HGN cannot prove a specific BAC in Texas

Texas courts have held that HGN evidence is admissible to support an officer’s opinion that a person was impaired, but it cannot be used to establish a specific blood or breath alcohol concentration. A prosecutor or officer who implies that the eye test shows a particular BAC, or that it proves a number, is misrepresenting what the test can do. The HGN, at most, is one piece of an impairment opinion, and it is a piece that depends entirely on correct administration and on ruling out the many other causes of nystagmus.

Emerson v. State: what Texas law actually allows

The Texas case governing HGN is Emerson v. State, decided by the Court of Criminal Appeals in 1994. Under the Kelly standard for scientific evidence, HGN is admissible only if three things hold: the underlying science is valid, the technique is valid, and the officer applied it properly in your case. In Emerson the court settled the first two once and for all, taking judicial notice that the science and the NHTSA technique are reliable, so the State does not have to re-prove those in every case. What is still fought case by case is the third prong, whether the officer administered the test correctly on this occasion, along with the officer’s qualification (for a police officer, practitioner certification). If the officer did not follow the NHTSA protocol, the results can be suppressed.

Emerson also drew the line the State sometimes blurs: an officer may not correlate the defendant’s performance on the HGN test to a precise BAC. HGN can support an opinion that a driver was impaired; it cannot put a number on it. And before the officer can give even that opinion, the State must qualify the officer as an HGN expert by showing practitioner certification, because the testimony is treated as scientific evidence, not ordinary lay observation.

Why the right cross-examination matters

HGN is the field test most attorneys are least equipped to challenge, because it requires knowing the protocol and the medicine. Managing Partner Deandra Grant is a trained NHTSA SFST instructor who administers and grades the ACS-CHAL Forensic Lawyer-Scientist exam and the DUIDLA Board Certification Exam, and she holds a Master’s Degree in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology. She cross-examines officers on HGN from the position of someone credentialed to teach the test, focusing on whether the stimulus distance, speed, and hold time met the protocol and whether the officer ruled out the dozens of other causes of nystagmus before blaming alcohol. Learn more about field sobriety tests and how the firm approaches medical conditions that mimic intoxication.

Frequently Asked Questions

What is the HGN test?

HGN stands for horizontal gaze nystagmus. It is the eye test an officer gives at the roadside, watching your eyes follow a moving object and looking for involuntary jerking. The officer scores up to six clues, and four or more is treated as a sign of intoxication.

Is the HGN eye test accurate?

It is only reliable when administered exactly to protocol, and even then nystagmus has more than 47 causes unrelated to alcohol. Errors in stimulus distance, speed, and hold time, and the failure to rule out other causes, all undermine the result.

Can the eye test prove my BAC?

No. In Texas, HGN evidence can support an officer’s opinion that you were impaired, but it cannot be used to establish a specific blood or breath alcohol concentration.

What causes nystagmus besides alcohol?

Inner ear and vestibular disorders, neurological conditions, certain medications, fatigue, naturally occurring nystagmus, gaze-evoked nystagmus at extreme angles, and even the flashing emergency lights at a traffic stop.

Can flashing police lights affect the HGN test?

Yes. Flashing emergency lights in the peripheral field can induce nystagmus and compromise the officer’s observation, which is one reason poor administration conditions matter.

How do you challenge an HGN test?

By reviewing the video against the protocol for stimulus distance, speed, and hold time, and by establishing that the officer never ruled out the many medical and environmental causes of nystagmus before attributing it to alcohol.

What is Emerson v. State?

It is the 1994 Texas Court of Criminal Appeals case that governs HGN evidence. It allows an officer to testify about HGN clues only if the test was properly administered, and it bars using the HGN result to prove a specific BAC.

Does the officer have to check your eyes for medical problems first?

Yes. The NHTSA protocol requires the officer to check for equal pupil size, resting nystagmus, and equal tracking before the test. Those signs can point to a head injury or medical condition, in which case the HGN should not be scored as a sign of alcohol.

Talk to a Lawyer Who Can Actually Challenge the Eye Test

HGN is the field test most lawyers cannot truly challenge. Deandra Grant Law is led by a certified SFST instructor who cross-examines officers on the very test they were trained to give. Call (214) 225-7117 for a free, confidential consultation.