Montana DUI Guide

The Horizontal Gaze Nystagmus (HGN) Eye Test

The eye test is the one roadside test you cannot feel yourself failing. It is also the one Montana courts treat as science that needs an expert to explain.

Written by

Misty Gaubatz, Partner, A&M Law

Licensed Montana attorney and co-founder of A&M Law. Over two decades in the practice of law focusing on family law, criminal defense, and DUI defense, with federal and state court experience and a Certificate in Mediation.

Montana Bar #122346+ years as an attorney, with over two decades working in the lawReviewed by Ashley HurlbertLast reviewed: October 9, 2026

In the horizontal gaze nystagmus (HGN) test, an officer moves a pen or small light side to side and watches whether your eyes jerk as they follow it. The officer is looking for three specific clues in each eye. In Montana the result is treated as scientific evidence, which means the State needs a qualified expert to explain it before a jury hears it. This page covers how the test is supposed to be given and how Montana courts handle it. It is part of our Montana DUI Guide.

What Nystagmus Is

The National Highway Traffic Safety Administration's (NHTSA) training manual for the standardized field sobriety tests defines nystagmus as an involuntary jerking of the eyes. Horizontal gaze nystagmus is jerking that occurs as the eyes gaze to the side. The manual says the person is usually unaware it is happening, and that alcohol and certain other drugs cause it.

HGN is one of the three tests in the standardized battery. The other two are the walk-and-turn and the one-leg stand.

How the Test Is Supposed to Be Given

The manual (2023 curriculum) sets out the administrative procedure in order:

Check for eyeglasses and have them removed.
Give the instructions: feet together, arms at the sides, head still, follow the stimulus with the eyes only.
Hold the stimulus about 12 to 15 inches from the nose, slightly above eye level.
Check for equal pupil size and resting nystagmus.
Check for equal tracking.
Check each eye for the three clues, starting with the left eye, each at least twice.
Total the clues, then check for vertical gaze nystagmus.

The pre-test checks are there for a medical reason. The manual tells officers that unequal pupils or eyes that do not track together raise the possibility of a medical disorder or injury, and that officers "are reminded to ask questions about the subject's eye and general health conditions prior to administering the HGN test." If testing continues after an abnormal finding, the manual says that "does not follow the standardized protocol" and should be acknowledged in the report.

The Three Clues

Clue	What the officer looks for	How the manual says to check
Lack of smooth pursuit	The eye jerks or bounces as it follows a smoothly moving stimulus	The stimulus is moved steadily, taking about 2 seconds from center to the side, with at least two passes
Distinct and sustained nystagmus at maximum deviation	The eye continues to jerk when held as far to the side as it can go	The stimulus is held at that position for a minimum of 4 seconds
Onset of nystagmus prior to 45 degrees	The jerking begins before the stimulus reaches about a 45-degree angle	The stimulus is moved slowly, taking about 4 seconds or more to reach 45 degrees

Each clue is counted separately for each eye, so the maximum is six. The manual states that if an officer observes four or more clues, it is likely the person's alcohol concentration is at or above 0.08, and it attributes an accuracy figure of 88% to that criterion, citing a 1998 field study. The manual attaches a condition to its figures: the validation "applies only when the tests are administered in the prescribed and standardized manner," and "if any one of the SFST elements is changed, the validity may be compromised."

It also gives officers a caution about the second clue. If the officer thinks only slight nystagmus is present, or has to be convinced it is there, "then it isn't really there."

Limits NHTSA Itself Acknowledges

Other kinds of nystagmus. The manual describes vestibular nystagmus (from the inner ear), neural types such as optokinetic nystagmus (from watching strobe lights, rotating lights, or rapidly moving traffic close by), and nystagmus from pathological disorders. It says the HGN test will not be influenced by the first two when administered properly.
Uneven results. If one eye shows all three clues and the other shows none, the manual says the person may have a pathological disorder.
Other drugs. The manual lists depressants, inhalants, and dissociative anesthetics as drug categories that can cause HGN.
Long-standing conditions. A person with a long-standing eye movement abnormality may appear abnormal to the officer and yet not be impaired.

The practical limit is that the test cannot be replayed. Body camera footage may not show the eyes closely enough to confirm a clue, but it usually shows the distance of the stimulus, the speed of each pass, how long it was held at the side, and whether flashing lights or traffic were in view.

Admissibility in Montana: Hulse and What Followed

The leading case is Hulse v. State, Department of Justice, 1998 MT 108, 289 Mont. 1, 961 P.2d 75. The Court held that HGN is not novel science, so the stricter Daubert screening does not apply, but that "the relationship between alcohol consumption and nystagmus, the underlying scientific principle of the HGN test, is still beyond the range of ordinary training or intelligence" (¶ 69). Two things are therefore required:

A showing that the officer was properly trained to give the test and gave it in accordance with that training (¶ 70).
Testimony from a witness qualified as an expert to explain the correlation between alcohol and nystagmus (¶ 72).

In Hulse the officer had 40 hours of academy training on field sobriety tests. That qualified him to describe how he gave and scored the test. It did not qualify him to explain the science, and admitting the HGN result was an abuse of discretion.

Who Counts as an HGN Expert

Later cases filled in the picture. In State v. Crawford, 2003 MT 118, the Court said the expert does not have to be a medical professional and affirmed a ruling that an officer with extensive education and instructor-level training could testify. In State v. Bollman, 2012 MT 49, 364 Mont. 265, 272 P.3d 650, the Court restated the rule: there are no "essential requirements," but "basic training on SFSTs is not sufficient" (¶ 27). Qualifications it has found significant include relevant coursework, instructor certification, certification as a drug recognition expert, and specialized training in eye anatomy and the science behind HGN (¶ 28).

A statute also speaks to who may report on roadside assessments. A report of a physical, psychomotor, or physiological assessment is admissible if made by a person trained by the Department of Justice or with training the Department recognizes (MCA § 61-8-1018(1)(c)).

What to Check in Your Own Case

Start with the video and the report side by side. Did the officer ask about your eyes, head injuries, and medical conditions? Were the pre-test checks done? How fast were the passes, and how long was the stimulus held at the side? Were patrol lights flashing in your line of sight? Then ask who the State intends to call to explain the science. Our field sobriety tests page covers the battery as a whole, and challenging field sobriety tests explains how these points are raised in court.

This page explains Montana law in general as of October 9, 2026 and describes NHTSA's published training materials (2023 curriculum). It is not legal advice about your specific case.

Montana DUI Guide DUI Defense The Walk-and-Turn Test The One-Leg Stand Test Alphabet, Counting & Other Field Tests Drug Recognition Experts (DRE)

FAQ

The Horizontal Gaze Nystagmus (HGN) Eye Test FAQ

What is the officer looking for when I follow the pen with my eyes?

Three things in each eye, according to NHTSA's training manual: whether the eye follows smoothly or jerks, whether it jerks distinctly when held all the way to the side for at least four seconds, and whether the jerking starts before the eye has moved about 45 degrees. The officer also checks for vertical nystagmus by raising the stimulus.

How many clues is a fail on the HGN test?

NHTSA's manual does not call it pass or fail. It teaches that if an officer observes four or more of the six possible clues, it is likely the person's alcohol concentration is at or above 0.08. That criterion comes from research the manual cites, and the manual says it applies only when the test is given in the standardized way.

Is the HGN test admissible in court in Montana?

Only with a proper foundation. In Hulse v. State, 1998 MT 108, the Montana Supreme Court held that the officer must be shown to be trained and to have given the test according to that training, and that an expert must explain the link between alcohol and nystagmus. Without that expert testimony, the result should not come in.

Can something other than alcohol cause nystagmus?

Yes. NHTSA's manual describes nystagmus caused by inner-ear disturbance, by neural activity, and by pathological disorders such as brain tumors, other brain damage, and some inner-ear diseases. It also says certain drugs other than alcohol cause HGN. This is one reason the officer is trained to ask about eye and general health before starting.

Do I have to take the eye test?

Field sobriety tests are treated as a search in Montana and require particularized suspicion (Hulse, ¶ 38). They are not covered by the implied consent statute that governs breath and blood tests. What declining them means in practice is covered in can I refuse field sobriety tests.

Was the Eye Test Part of Your Arrest?

Misty Gaubatz is certified in NHTSA Standardized Field Sobriety Testing and can compare the video to the manual. Call 406-830-3060 for a confidential consultation.

Call 406-830-3060 Send a Message