Police officers lean on the Horizontal Gaze Nystagmus test because it feels scientific. A few swipes of a penlight across your field of view, a series of “follow my finger” prompts, and an officer claims to see involuntary jerking of the eyes that supposedly signals impairment. In court, prosecutors routinely treat HGN as a cornerstone of probable cause. But HGN is not breath or blood. It is a human observation test layered with training limitations, environmental variables, medical confounders, and sometimes a shaky foundation under state evidence rules.
A seasoned DUI Lawyer approaches HGN with skepticism and a checklist. The goal is not to pretend nystagmus does not exist. The goal is to force the state to prove, step by step, that what the officer saw was real nystagmus, that it was measured correctly, and that it correlates to intoxication in this defendant, on this night, under these conditions. Any gap in that chain can undermine the test’s value.
What HGN is and why officers use it
Horizontal Gaze Nystagmus is the involuntary rhythmic jerking of the eyes as they track a stimulus sideways. Everyone has nystagmus at some point, even stone sober. The central question is when the jerking begins, at what angle it becomes visible, whether it is sustained, and how the pattern changes with alcohol or other depressants. The National Highway Traffic Safety Administration (NHTSA) folded HGN into the standardized field sobriety tests after validation studies in the 1970s and 1980s. In those studies, trained officers who followed a specific protocol had a higher probability of correctly identifying a blood alcohol concentration above 0.10 than they did with nonstandardized methods.
Those validations came with important caveats. NHTSA stresses that HGN must be administered under specific conditions and scored according to a strict checklist. Even then, it is a probabilistic indicator, not a direct measurement of BAC. The standard cutoffs in older studies relate to 0.10 BAC, not the 0.08 threshold used today in most jurisdictions. Modern field use often drifts from the protocol, and courts vary widely in how they treat the test under evidence rules and Daubert or Frye standards.
A Criminal Defense Lawyer who reads the reports closely will notice: officers routinely gloss over the details that made the research meaningful. That is where a Defense Lawyer can make headway, whether moving to exclude HGN entirely or to strip it down to modest weight.
What the officer is supposed to do
The NHTSA walk-through for HGN is less mysterious than it appears. The officer positions the stimulus 12 to 15 inches from the subject’s nose, slightly above eye level. The subject is instructed to hold the head still and follow the stimulus with the eyes only. The officer checks for equal pupil size and equal tracking, then performs three distinct “clues” in each eye: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees. That yields a maximum of six clues.
Timing is the quiet killer of accuracy. For smooth pursuit, the stimulus should travel from center to side in roughly two seconds. At maximum deviation, the stimulus remains at the far edge for at least four seconds. For onset, the stimulus should move from center to 45 degrees in about four seconds, then back in the same cadence. Each pass should be scored precisely. Deviations from that cadence can create false positives. If you sweep too fast, you may provoke saccades, which are normal, not nystagmus. If you hold too short at maximum deviation, you may miss nystagmus entirely. The protocol matters.
An experienced DUI Defense Lawyer arrives in court with a stopwatch and the training manual. Jurors understand clocks. If video shows lightning-fast passes, a shaky flashlight, or a stimulus held at eyebrow level while the defendant squints into flashing LED bars, that is a story a jury can follow.
The problem with the real roadside
The validation studies loved controlled conditions. Real roads do not. Patrol cars flood the scene with warning lights. Oncoming traffic adds glare and movement. It might be 3 a.m., 28 degrees, and windy. The officer may stand the driver on a sloped shoulder, next to a rumbling diesel. Dry eyes, contacts, fatigue, or a sinus infection can complicate tracking. HGN is sensitive to some prescription medications, to vestibular disorders, and to simple head position. If the chin is tilted or the head is not level, the angle estimates go off. A subtle head turn can mimic earlier onset.
Those are not hypothetical gripes. They are the kinds of details that change the test. The difference between onset at 45 degrees and onset at 35 degrees is not a lot of tape measure in bright daylight. At night, with a moving target, it is often guesswork. If the officer did not use a reference like the width of the subject’s shoulder or a known marker to estimate 45 degrees, cross-examination can expose that the angle was a guess.
I once watched dashcam of a stop where the officer stood three feet below the driver on a roadside ditch. The stimulus bobbed, the camera shook, and the passenger-side light bar washed the driver’s face in strobe. The officer still marked six clues. When the jury saw that video in real time with a freeze frame on the head tilt and the flickering shadows across the eyes, the state’s confidence in HGN evaporated.
Where medical science intersects and diverges
Criminal Defense Law is filled with tests that look scientific, then blur when moved out of the lab. HGN is one. Alcohol does enhance nystagmus in many people. But other causes exist. Inner ear dysfunction, concussion history, certain seizure disorders, and even barbiturates can produce or exaggerate horizontal nystagmus. Stimulants can change eye movements differently. Tetrahydrocannabinol is not known to cause HGN, yet mixed drug cases often turn on sloppy interpretations.
A defense grounded in medical records can be compelling. If an assault lawyer defends a client whose eye trauma came from a prior incident, the detective’s HGN narrative may crumble. If a drug lawyer handles a case involving benzodiazepines, the jury needs to hear that central nervous system depressants can cause HGN even absent alcohol. The officer should have asked about medical conditions and contacts, and should have noted resting nystagmus, if present, before moving the stimulus. If that baseline step is missing, the rest of the test becomes suspect.
Training versus practice
NHTSA certifies officers through courses that cover HGN administration, but years later many cannot recite timing standards. Retraining is uneven. Some departments update forms and cue cards, others do not. In cross, I sometimes ask the officer to demonstrate the sweep they used. If the four-second hold at maximum deviation turns into a one-second flick, the jury does not need a Ph.D. to see the disconnect. If the officer testifies they performed “vertical” nystagmus checks without explaining why, or they conflate HGN with “eye wobble” in general, that becomes grist for the mill.
A Criminal Defense Lawyer can also compare reports over time. If the same officer writes “six out of six clues” in nearly every DUIs they process, regardless of environment, lighting, or subject age, that pattern undermines credibility. Objective measurements should vary. A flat line often signals rote paperwork rather than careful observation.
Admissibility battles: science in the courtroom
Different states treat HGN differently. Some allow officers to testify about HGN as evidence of impairment generally, but not to state or imply a specific BAC. Others require expert testimony to link HGN to alcohol concentration. A few demand a foundation that the officer was trained under NHTSA and followed the protocol. In Daubert jurisdictions, defense motions often focus on validation quality, error rates, and whether the officer’s field application matches the research methods. In Frye jurisdictions, the emphasis shifts to general acceptance.
No matter the standard, the prosecution must lay a proper foundation. That means training, protocol adherence, and conditions that do not undermine the test. A motion in limine that forces the state to limit HGN testimony to what the officer actually observed, not what it “means,” can shift the trial narrative. When judges rule that HGN does not prove a BAC above 0.08, prosecutors usually pivot to totality of circumstances. That opens room to argue that driving behavior, speech, balance, and time estimates matter more than a penlight across the eyes.
Building the cross-examination
A tight cross is specific. Avoid general attacks like “the test is junk.” Jurors tune out. Anchor the questions in the officer’s own training and the video. If you can, replicate the timing in court using the video’s timestamp. Jurors instinctively understand seconds.
- Foundation: “You were trained under NHTSA, correct?” “You learned specific timings for each pass, correct?” “You took no refresher course since [year], correct?” Conditions: “You conducted the test facing your emergency lights?” “There was oncoming traffic?” “You positioned the defendant on a sloped gravel shoulder?” Method: “You held the stimulus 12 to 15 inches from the nose?” “You estimated that without measuring, correct?” “You kept the chin level?” “No video shows you checking head position, correct?” Timing: “From center to the edge, you moved in about one second, not two, correct?” “You held at maximum deviation for roughly one second, not four?” “We can watch the clip together and count?” Scoring: “You marked lack of smooth pursuit in both eyes. You agree normal saccades can appear with fast stimulus movement?” “You marked distinct nystagmus at maximum deviation. You did not note whether it was sustained for at least four seconds?” “For onset prior to 45 degrees, you did not use any reference for angle?”
Those questions keep the focus where it belongs: on whether this officer did this test correctly. A DUI Defense Lawyer who follows that thread often finds several breaks.
When to hire your own expert
Not every case warrants an expert. In marginal cases, the dashcam might be enough to blunt HGN. But if the court allows the state to lean heavily on HGN, and the video is limited, a defense expert in oculomotor physiology or a former DRE instructor can provide context. The best experts do not overstate. They acknowledge that HGN can correlate with impairment, then explain how small errors in execution produce misleading results. Jurors DUI Lawyer reward candor.
In a murder lawyer’s practice, you might use ophthalmology experts to challenge witness identification or gunshot residuals; in a DUI case, the eye is again the battleground, but for a different reason. That cross-disciplinary insight helps. Medical professionals can describe vestibular-ocular reflexes in plain language. They can explain how alcohol affects the central nervous system and how other substances differ. They can also flag conditions like latent nystagmus or amblyopia that never made it into the police report.
Video is your friend, even when it’s messy
Many jurisdictions now capture HGN on body-worn camera or dashcam. The footage rarely shows the subject’s pupils crisply. That is fine. The goal is not to second-guess the officer’s scoring frame by frame. The goal is to evaluate setup, head position, environment, timing, and whether the officer actually did what they claim. If the camera shows the officer whipping the pen back and forth, that tells a story. If the officer tells the jury they held at maximum deviation for four seconds, and the video shows a quick bounce, credibility takes a hit.
I once synced the bodycam clock to a courtroom stopwatch visible to the jury. We watched the officer’s hand move from center to left in about one second. We watched the “hold” at the edge for about one and a half seconds. On the second pass, the officer’s flashlight dipped, and the subject blinked rapidly in direct glare. The officer still scored six clues. The jurors traded a few glances that said more than any objection.
Medical and environmental alternative explanations
Defense is not just attack. It is also explanation. Jurors like coherent stories. If the defendant had a head cold, wore hard contact lenses after 14 hours awake, and stood facing flashing strobes, those facts explain eye movement without intoxication. If the defendant took prescribed clonazepam for an anxiety disorder, you can show the drug’s known effects on the vestibular system. If the defendant works swing shift and had four cups of coffee before leaving work at midnight, fatigue plus caffeine can produce saccadic intrusions that officers mistake for lack of smooth pursuit.
None of this requires a medical lecture. It requires a clear link to the night’s conditions. Criminal Law is about concrete facts, not lab hypotheticals. If there is a prior diagnosis, introduce the record through a custodian. If there is a simple environmental factor, pull the weather data, map the shoulder’s slope with a phone inclinometer, or photograph the area at the same time of night to demonstrate glare patterns. Jurors trust tangible details.
The limits of what HGN can say
Even in its best form, HGN is an impairment clue, not a chemical test. It does not measure a number. It does not tell you how much a person drank or when. In mixed-drug cases, it can mislead. Prosecutors sometimes imply that six out of six clues equals intoxication. That shorthand sounds crisp, but it skips steps. HGN does not diagnose intoxication. It contributes to a probable cause matrix. Courts that keep it in that lane do better justice.
Make the prosecution connect HGN to the rest of the case. If driving behavior was clean, the stop was for a rolling stop at a quiet intersection, speech was normal, and balance tests occurred on uneven gravel in the cold, HGN should not carry the day. If an assault defense lawyer would not hang a whole case on a single shaky eyewitness, a DUI Lawyer should not let a single shaky eye test dominate either.
Strategic decisions before trial
Plea posture changes when HGN gets weaker. If a breath test is out because of a machine error and HGN becomes the state’s anchor, you have leverage. That leverage increases if your pretrial motion trims HGN testimony to “I saw eye movement” without the scientific gloss. Pretrial hearings are often where cases resolve. Prosecutors read the room. If the officer’s training looks rusty and the video undermines timing, a negotiated outcome improves.
Conversely, if your client performed poorly on all standardized tests and the video is damning, you may not want to spend trial capital on HGN. Juries have limited patience. Pick the strongest points. Sometimes that means focusing on why the stop was unlawful, why the post-stop observations were biased, or why the breath machine’s mouth alcohol safeguard failed.
Practical steps for defendants facing HGN evidence
A defendant should not try to relitigate HGN on their own at arraignment. They need a Criminal Defense Lawyer who understands both the science and the courtroom dynamics. Still, there are practical steps that help the defense team evaluate the case swiftly.
- Ask for all video, including body-worn camera from every officer on scene, not just the arresting officer. Request the officer’s training certificates, field sobriety refresher logs, and department policy on field tests. Document any medical or eye-related conditions, prescriptions, and contact lens use, including brand and wear time. Visit and photograph the scene at a similar time of night to capture lighting, slope, and traffic patterns. Time the stimulus movements off the video using a stopwatch or video analysis app to check cadence and holds.
Those steps preserve details that disappear with time. They also equip your Criminal Defense Lawyer to make smart early moves, including targeted discovery and motions.
How HGN fits with other field tests
HGN rarely stands alone. Officers also use the Walk and Turn and the One Leg Stand. Those tests are less tied to esoteric physiology and more to balance and divided attention. They have their own pitfalls, especially on poor surfaces or with older defendants. If all three tests are weak, the state will argue robustness. If HGN is the only strong test, the defense has a story to tell: balance looked fine, speech was normal, driving pattern was ordinary, and only a pen test in tough conditions suggested impairment.
In drug cases, officers may attempt a Drug Recognition Expert protocol that includes more eye checks, blood pressure, and pupil size in varied light. DRE opinions are more expansive and therefore more fragile on cross if the steps were rushed or skipped. A drug lawyer experienced with DRE can often show that the protocol was a checklist exercise performed in a noisy booking room instead of a clinical setting.
Jury education without jargon
Jurors are smart, but nobody enjoys a lecture on saccade latency. Keep the language simple. Show, do not tell. Use the video. Use clear analogies: “Think of the eyes like a camera on a gimbal. If the base is tilted or the wind is shaking the mount, the picture jitters even with a good camera.” Demonstrate timing with your hand and a slow count the jury can follow. Explain that many people have minor eye movements that look jerky if you whip the target around. Tie everything back to what the officer did, not what the literature says in the abstract.
Ethical lines and credibility with the court
Good Criminal Defense respects the difference between pointing out doubt and manufacturing it. Do not assert medical conditions you cannot support. Do not overstate research that cuts both ways. Judges and jurors spot overreach. A measured approach holds more weight: acknowledge that HGN can be useful when done right, then show the parts that were not done right here. That credibility helps in other cases too, whether you practice as a DUI Lawyer, an assault lawyer, or handle serious felonies where expert testimony decides the outcome.
The broader lesson: standardized does not mean infallible
Standardization helps, but it does not cure context. Field tests remain tools in the hands of humans working at night, often under stress. Criminal Law asks whether the state proved its claim, not whether the officer followed a checklist in spirit. When HGN becomes a linchpin, defense counsel should lean into the details: training, timing, environment, physiology, and the precise limits of what the test can say. Juries respond to specifics.
In practice, the most effective challenges pair the officer’s own training against their performance, use the video as an anchor, and offer a common-sense explanation for what the jury sees. When that happens, HGN shrinks to its proper size, one clue among many. And in a system that must deal in proof beyond a reasonable doubt, that shift matters.
Final thoughts for defendants and counsel
If you face a DUI charge where HGN features prominently, move quickly. Video requests age poorly. Memories fade. A short delay can cost crucial context. Pair experienced counsel with a clear plan. In many cases, HGN will end up as background noise once the jury sees the conditions and timing for themselves. In others, a successful motion will limit HGN to descriptive observations without the scientific gloss. Either route can change outcomes.
For defense teams, keep the playbook sharp. Review the latest NHTSA manuals. Track local appellate decisions on HGN admissibility. Maintain a library of exemplar videos that demonstrate proper cadence and head position for use in cross. Cultivate relationships with medical experts who can speak plainly. When a case turns on the eyes, preparation and clarity win.
The law gives every defendant the right to demand reliable evidence. HGN can be part of that story, but only if the state proves it was done right. When it was not, a careful, grounded challenge can level the field. That is the work of a Criminal Defense Lawyer committed to rigorous advocacy, whether in a straightforward DUI or a complex case where every detail counts.