Of the three standardized field sobriety tests New York officers use during a DWI investigation, the Horizontal Gaze Nystagmus (HGN) test is the one most people don’t fully understand — and also the one most likely to be challenged in court. That combination matters.
What Nystagmus Actually Is
Nystagmus is the involuntary jerking of the eye. Your eyes do it naturally at extreme angles of gaze, but alcohol and certain other substances cause this jerking to begin earlier — at less extreme angles — and to be more pronounced than it would be in a sober person. The HGN test is designed to measure involuntary jerking and use it as an indicator of impairment.
The officer administers the test by holding a small object — usually a pen, a penlight, or their finger — approximately 12 to 15 inches from your face. They move it slowly and horizontally from one side to the other while watching your eyes. They’re not asking you to do anything in particular; your eyes move involuntarily. What they’re looking for are specific “clues” in each eye, assessed in a defined sequence.
The Six Clues Officers Are Trained to Score
The NHTSA-standardized HGN test is scored on a total of six clues — three per eye. An officer finds each clue (or doesn’t) in each eye separately, working through the sequence in a specific order.
| Clue | What It Means |
|---|---|
| Lack of smooth pursuit | Eye jerks or bounces rather than following smoothly as the stimulus moves |
| Distinct nystagmus at maximum deviation | Sustained jerking when eye is at the extreme edge of its range |
| Onset of nystagmus prior to 45 degrees | Sustained jerking when the eye is at the extreme edge of its range |
The third clue — onset prior to 45 degrees — is considered the most significant. NHTSA research has claimed that observing four or more clues out of six correlates with a BAC at or above 0.08%, though the accuracy of that claim has been contested in courts.
Why the HGN Test Is the Most Legally Controversial FST
The HGN test is fundamentally different from the Walk-and-Turn or One-Leg Stand tests. Those are performance tests — the officer watches what you physically do and documents it. The HGN test asks an officer to make a medical neurological observation about an involuntary physiological response in your eyes. That distinction is legally significant.
In New York, HGN evidence has been treated with more scrutiny than the other two FSTs. Several courts have required that the prosecution lay a proper foundation before HGN results are admitted — including establishing that the administering officer received adequate training, that the test was performed according to NHTSA protocol, and in some cases that expert testimony supports the scientific basis for the test.
Compare that to breathalyzer accuracy, which faces its own set of challenges, or to the difference between types of chemical testing — the HGN test has a different evidentiary foundation than any of those, and that’s precisely where defense attorneys focus.
What Can Cause False Positives on the HGN Test
This is where the defense challenge becomes concrete. Nystagmus is not caused exclusively by alcohol. A number of conditions and factors can produce the same eye-jerking that officers are trained to associate with impairment:
- Prescription medications, including some anticonvulsants, tranquilizers, and muscle relaxants
- Over-the-counter antihistamines or cold medications
- Inner ear disorders and vestibular conditions
- Head or brain injuries
- Eye muscle fatigue
- Natural, pre-existing nystagmus (some people simply have it)
- Caffeine and nicotine (can affect smooth pursuit slightly)
- Strobe lighting or flashing lights from police vehicles in the driver’s field of view
The officer is required to ask whether you have any medical conditions before administering the test. Whether they did, what you said, and what was documented all become part of your attorney’s analysis.
What Officers Are Required to Do — and Often Don’t
Protocol errors in administering the HGN test are more common than people realize, and they become critical when the officer’s testimony about your eye movements is central to the prosecution’s case.
Officers are required to hold the stimulus at the correct distance — approximately 12 to 15 inches from the face, positioned above eye level. They must move it at the correct speed — slow enough to properly evaluate each clue. They must evaluate both eyes, in the correct sequence, for each of the three clues. They must allow sufficient time at maximum deviation to observe distinct nystagmus. They must maintain the correct angle when assessing the 45-degree threshold.
Any departure from this protocol — moving too quickly, holding the stimulus at the wrong angle, skipping the proper sequence, performing the test in poor lighting — undermines the reliability of the results and gives your attorney grounds to challenge the foundation for the officer’s conclusions.
How It Fits Into the Broader Stop
The HGN test is almost always administered once you’ve been asked to step out of the vehicle, after the officer has made initial observations during the traffic stop. It’s usually the first of the three FSTs administered, and it’s the only one a driver has virtually no ability to consciously influence — your eyes move involuntarily regardless of what you’re trying to do.
What you say before, during, and after the test matters. If you have a condition that could cause natural nystagmus, this is one of the limited situations where providing that information — calmly and specifically — may be in your interest. Your attorney will advise on this.
Note that declining the HGN test specifically, or declining all FSTs, is legally permitted. FSTs are voluntary in New York. Unlike refusing the official chemical test at the station, refusing field sobriety tests does not trigger automatic administrative penalties. The officer may note the refusal in their report, but no per se license consequences attach.
If You Were Arrested After an HGN Test
An HGN result is not the end of your case. It’s one piece of evidence among several, and it’s the one with the most established legal challenges. When you speak with an attorney before the chemical test at the station or after your arrest, relay everything you can about how the test was administered — the lighting, the officer’s technique, whether the proper distance and speed were used, whether you were asked about medical conditions, and any health issues that may have affected your eyes.
These details are how a DWI defense attorney builds the challenge to HGN evidence before trial. NHTSA’s own research acknowledges the test’s limitations, and New York courts have been among the more critical when it comes to the scientific foundation for HGN results.
The NHTSA Standardized Field Sobriety Testing manual governs how the test is supposed to be administered. When officers deviate from it, that deviation matters.
Disclaimer: This overview is for informational purposes only and does not constitute legal advice. Every case is unique — contact the DWI TEAM defense attorneys for personalized guidance.
