DUI Eye Test | HGN and North Carolina DWI Cases

The “DUI eye test” refers to the Horizontal Gaze Nystagmus, or “HGN.” It is one of the three “verified” Standardized Field Sobriety Tests (SFSTs) recognized by the National Highway Traffic Safety Administration (NHTSA), along with the walk-and-turn and the one-leg stand.

HGN is a “sobriety test” where an officer holds a stimulus, usually a pen, a fingertip, or a small flashlight, in front of the suspect’s face and asks the driver to follow it with their eyes without moving their head. The officer is watching for an involuntary jerking or twitching of the eyes. Some officers describe it as looking like windshield wipers going back and forth on a dry windshield. That jerking motion is referred to as a nystagmus and it is purportedly an indicator of appreciable impairment by alcohol and possibly other substances.

The test sounds simple. The science behind it is not. Whether HGN is actually a reliable indicator of impairment in any given case is a separate question. Even when scientifically reliable, the accuracy and reliability of field sobriety testing largely depend on the training, experience, and objectivity of the charging officer. That matters when the test results are summarized in a police report, described on the witness stand, or used to support probable cause for arrest under North Carolina’s impaired driving statute, N.C.G.S. § 20-138.1.

At a Glance | The DUI Eye Test (HGN)

Issue	Why It Matters
What HGN measures	Involuntary eye jerking that can become more pronounced when alcohol or certain other substances are present
What officers look for	Up to six “clues” across both eyes, with four or more typically described as a sign of impairment
Accuracy claims	NHTSA-sponsored research reports relatively high accuracy under controlled conditions, but real-world roadside performance can vary
Other causes of nystagmus	Medical conditions, medications, fatigue, head injuries, and certain environmental factors can produce, mimic, or be misinterpreted as nystagmus
North Carolina law	HGN evidence may be admissible when offered by a properly trained officer, but training, administration, and the underlying science can all be tested in court

1. What the HGN Eye Test Actually Measures

HGN is not necessarily an accurate measure of how much someone has had to drink. It is a measure of how the eyes move. Nystagmus, generally speaking, is the involuntary jerking of the eyes. Many, if not most, people have some degree of nystagmus at wide viewing angles. Alcohol and certain other central nervous system depressants can make that jerking more pronounced.

Careful, consistent administration of the Horizontal Gaze Nystagmus matters. In fact, a driver with HGN clues is not always impaired. A certain percentage of the population has a natural nystagmus. The officer reports a physiological observation, then draws a conclusion. Like any other human endeavor, sometimes a police officer conducting a roadside DWI investigation gets it right. Sometimes they don’t.

It’s helpful to understand different kinds of nystagmus. Put simply, that jerking of the eyes is not always related to drinking alcohol or is the result of some other type(s) of potentially impairing substances.

There is also vertical nystagmus, which officers sometimes check for as an indicator of high doses of certain substances, and there is resting nystagmus, which may relate to other medical or physiological conditions. The roadside HGN test is relatively focused, narrow, and dependent on the officer following very specific protocols. When the protocol is not followed, NHTSA police officer training materials indicate the reliability of field sobriety test results may be compromised. Of course, that’s also true for the other standard “SFSTs,” like the One-Leg stand and the “Heel to Toe” (the Walk and Turn test).

It is also important to understand that the HGN test is part of the autonomic nervous system. The person being tested (the defendant potentially accused of DWI) cannot feel HGN happening. Put simply, you cannot voluntarily control whether your eyes “jerk” when they move back and forth while watching the officer’s finger. You can control whether you actually focus on the stimuls or whether you move your head during testing.

Unlike the walk-and-turn, where a driver may know they stepped off the line, or the one-leg stand, where a driver may know they put their foot down, HGN is basically invisible to the person being tested. The driver has no way to evaluate the officer’s observations in real time. That is one reason why body-worn camera and dash camera footage can matter so much in HGN cases.

2. The DUI Eye Test | Six Clues

Consistent with NHTSA training protocols, the officer is supposed to follow a defined procedure. Before the test even begins, the officer should check for equal pupil size, equal tracking, and resting nystagmus. Those pre-test checks exist because if the eyes are not tracking equally, or if there is resting nystagmus, the test results may not mean what the officer thinks they mean.

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

Lack of smooth pursuit (left eye). As the stimulus (finger, pen or small flashlight) moves side to side, the eye should follow smoothly. Jerky tracking is a clue.
Lack of smooth pursuit (right eye). Same observation, other eye.
Distinct and sustained nystagmus at maximum deviation (left eye). When the eye is held at its furthest point to the side, the officer is looking for jerking that is both clear and sustained for a period of seconds.
Distinct and sustained nystagmus at maximum deviation (right eye).
Onset of nystagmus prior to 45 degrees (left eye). The officer is looking for jerking to begin before the eye has rotated 45 degrees from center.
Onset of nystagmus prior to 45 degrees (right eye).

Four or more clues, under NHTSA training, is typically described as a sign that the driver may be appreciably impaired. That is a probability statement based on validation studies, not a guarantee about a specific person on a specific night. Furthermore, at least in North Carolina, that probability or evidence relating to the clues indicating a .08 or higher BAC is often inadmissible as evidence in court.

The procedure looks simple on paper. In practice, a lot can go sideways. The stimulus needs to be held at the correct distance, usually about 12 to 15 inches from the driver’s nose, and slightly above eye level. The pace of the movement also matters. The hold at maximum deviation matters. The angle estimation at 45 degrees matters. Lighting matters. The driver’s head position matters. Whether the driver is wearing glasses or contacts may matter. Whether passing traffic, emergency lights, or strobe effects are visible in the driver’s peripheral vision can matter because optokinetic effects can produce eye movement known as an “optokinetic nystagmus” that is unrelated to alcohol.

That is where Body Worn Camera (BWC) video can become important. A police officer report, charging officer affidavit, or supporting narrative may say something like the driver “exhibited six out of six clues.” Video may show the test being conducted on the shoulder of NC 12 with a patrol car’s blue lights flashing in the background, the stimulus moving too quickly, or the hold at maximum deviation lasting shorter than the required time. None of that automatically wins a case. But, it is the kind of detail that OBX defense lawyers carefully review and, when appropriate, challenge at trial.

3. The Accuracy Question

NHTSA-sponsored research has reported relatively reliable accuracy figures for the HGN test. Validation studies, particularly the 1998 San Diego field validation study, have been cited for the proposition that the HGN sobriety test, when administered correctly, can identify drivers at or above 0.08 with a certain level of reliability. Those figures appear regularly in officer training materials. That’s not necessarily true in North Carolina, where the HGN eye test may serve as the basis for both probable cause for arrest (the arrest decision) and during the trial itself as part of the “not guilty/guilty” determination by the finder of fact.

The fuller (and frankly more fair) picture is substantially more complicated. The validation studies were conducted under specific conditions (not roadside), by officers who knew their work was being evaluated, and with research assumptions that have been consistently criticized in peer-reviewed literature. Critics point out that the studies do not always separate HGN from the other tests, that “accuracy” is sometimes defined in ways that overstate the test’s predictive value, and that real-world roadside conditions rarely match research conditions.

None of that means HGN is entirely junk science. It does mean that an alleged observation of jerking eyes on the side of the road is not the whole story, and is one reason why DUI defense lawyers consider whether:

The officer was trained according to current NHTSA standards
The officer’s training (and refresher training) is current and documented (within two years, as recommended by NHTSA)
The pre-test “screening” checks were performed
The stimulus (finger/pen) was held at the correct distance and moved at the right pace
The timing at maximum deviation was sufficient (at least four seconds)
The angle (the “onset of nystagmus”) estimation was reliable
Environmental conditions, including emergency lighting, may have produced optokinetic effects
The test was recorded on a body-worn camera or a “dash camera,” and what the recording actually shows

There is a temptation, on both sides of a DWI case, to treat HGN as either decisive proof of impairment or as unreliable junk science that should be ignored. Neither assumption serves the truth or true justice in court. HGN is one piece of evidence. It can be a meaningful, reliable piece of evidence and indicator of impairment. That said, it’s not perfect or unassailable in every instance.

4. Medical, Environmental, and Other Causes of Nystagmus

Alcohol is not the only thing that can cause or contribute to nystagmus. A number of medical conditions and other factors can produce eye movements that resemble those officers are trained to look for. A non-exhaustive list includes:

Inner ear conditions, including vestibular disorders
Certain neurological conditions, including multiple sclerosis and the after-effects of stroke
Head injuries, TBI – Traumatic Brain Injury, and concussion, including injuries that occurred long before the traffic stop
Certain prescription medications, including some antiseizure and antianxiety drugs (which may or may not be imairing substances)
Some over-the-counter medications
Being Tired, fatigue
Caffeine and nicotine, though their role, nature, and extent of resulting nystagmus are subject to debate
Congenital nystagmus, which is present from birth
Eye conditions affecting muscle control or tracking

There are also environmental factors that may contribute to what the officer sees. Strobing emergency lights, passing headlights, wind, and cold air can all affect the testing environment. Optokinetic nystagmus, in particular, can be triggered by moving visual stimuli in the driver’s peripheral vision, which is one reason NHTSA training instructs officers to position the driver away from active traffic and turn off emergency lights when possible.

There is not a list of magic excuses. A driver does not get out of a DWI case by saying, “I have an inner ear problem.” It does illustrate why HGN, by itself, rarely tells the whole story. A driver who has been in a serious car accident earlier in the year, who is taking a prescribed medication, who is exhausted from a long drive to the Outer Banks, and who is being tested next to a patrol car with active blue lights is in a different situation than the situation contemplated by the validation studies in a research lab with perfect lighting, level floors and no flashing lights. If you are charged with DWI in North Carolina, your case deserves to be reviewed with that context in mind.

It is also worth noting that defendants (the person charged with impaired driving) do not always know what is medically relevant. A driver may not think to mention a decade-old head injury, a recent ear infection, or a medication taken hours before driving. Those facts may turn out to matter. And that’s why retaining legal counsel early on in the process, giving your lawyer time to investigate, document, and prepare, can make a difference.

5. HGN Evidence | North Carolina Court

In North Carolina, HGN has been the subject of significant appellate attention. The North Carolina Supreme Court in State v. Godwin, addressed the admissibility of HGN evidence and recognized that HGN testimony is treated as scientific evidence subject to foundational requirements. Subsequent cases and statutory developments have continued to affect how HGN testimony is offered and challenged.

Under N.C.G.S. § 20-138.1, the State must prove the elements of impaired driving beyond a reasonable doubt. HGN evidence may support an officer’s opinion of appreciable impairment and may contribute to probable cause for arrest, but it does not eliminate the State’s burden or prevent the defense from challenging how the test was conducted, how the officer was trained, and how the result is being used.

North Carolina Rule of Evidence 702 governs the admissibility of expert and specialized opinion testimony during trial. The reliability of the underlying methodology, the application of that methodology to the facts of the case, and the qualifications of the witness offering the testimony can be subject to a certain level of cross-examination and scrutiny. That is part of what criminal defense lawyers do.

HGN testimony often sounds impressive when an officer describes “six out of six clues” on the stand. Jurors and judges are human. Numbers feel certain. The defense response is rarely to dismiss the test without digging a bit deeper and pointing out flaws. It’s important to review the body camera footage (if there is video available), examine the officer’s procedure step by step, and put the result in context with the rest of the alleged evidence of “appreciable impairment.”

A Final Word on the DUI Eye Test

HGN is not nothing. It is also not everything. The honest answer to the question of whether the DUI eye test is accurate is that it depends on the officer, the conditions, the driver, the protocol, and the way the results are interpreted. Any SFST result that looks decisive on a charging officer’s affidavit and revocation report may look very different on video, and a test result that looks weak on paper may turn out to be backed up by other evidence the defense has to take seriously.

If you are facing a DWI charge on the Outer Banks, where roadside conditions and the surrounding coastal areas in Northeastern North Carolina can complicate any field testing, the HGN eye test is ordinarily one piece of the case.

Glover Law Firm represents clients facing DWI charges in Dare County and throughout the Outer Banks, including Hatteras, Manteo, Nags Head, Kill Devil Hills, Kitty Hawk, Duck, and surrounding communities. If you have questions about how HGN evidence may affect your case, call OBX Lawyer Danny Glover to schedule a consultation.