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    DUI Defense Division

    DUI Eye Test – Horizontal Gaze Nystagmus – How It Works

    Authored by Neil Shouse
    Criminal Defense Attorney | UC Berkeley • MIT • Harvard Law School
    Last Reviewed: November 12, 2026

    Quick summary

    The horizontal gaze nystagmus (HGN) test is the DUI eye test: an officer moves a pen or small light from side to side and watches your eyes for an involuntary jerking that alcohol can cause. It is one of three standardized field sobriety tests police use to decide whether to arrest you for DUI.

    3 key things to know:

    1. Officers look for 6 clues, 3 in each eye. Police training says 4 or more clues means a BAC of 0.08% or higher, a call it claims is right 88% of the time.
    2. In California you can politely decline the HGN test and the other field sobriety tests. The breath or blood test after an arrest is different, and refusing it has penalties.
    3. Lights, medical conditions, some medications and a rushed test can all cause false clues, and California courts don’t let an officer turn HGN results into a BAC number.
    HGN Test | Horizontal gaze nystagmus | HGN | Nystagmus Test | Horizontal gaze nystagmus test

    In this article, our California DUI defense lawyers answer 9 common questions about the HGN test:

    1. What is the HGN test?

    The horizontal gaze nystagmus (HGN) test is a roadside eye exam that police use when they suspect DUI. “Nystagmus” means an involuntary jerking of the eye. “Horizontal gaze” means it shows up as your eyes look to the side.1

    Everyone’s eyes jerk a little when they look far to the side. Alcohol and certain drugs make the jerking stronger and make it start sooner, before the eye has turned very far. You can’t feel it or control it, and it doesn’t blur your vision, so most people never notice it.1

    Why alcohol makes your eyes jerk

    Alcohol affects the cerebellum, the part of the brain that holds your gaze steady. In a 2017 lab study, drinking roughly doubled how fast the eyes drifted back toward center when people tried to look to the side, and the eye then “jumps” back out, which is the jerk the officer is looking for.2

    One of three standardized field sobriety tests

    The HGN test is one of three standardized field sobriety tests created by the National Highway Traffic Safety Administration (NHTSA). The other two are the one-leg stand and the walk-and-turn test. “Standardized” means officers are trained to give and score them the same way every time.1

    The test also picks up some drugs. Nystagmus can be caused by central nervous system depressants (alcohol, sleeping pills, anti-anxiety drugs), inhalants and dissociative anesthetics such as PCP. Marijuana, stimulants and opioids generally don’t cause it. If an officer suspects drugs, the case usually turns into a drug DUI investigation.3

    Key point: In California, you can politely decline the HGN test and the other field sobriety tests. The breath or blood test after an arrest is a different matter (section 5).

    Police officer giving the horizontal gaze nystagmus test to a driver
    The HGN test is usually the first field sobriety test an officer gives at the roadside.

    2. What are the 6 clues of the HGN test?

    The officer checks 3 clues in each eye, for a possible total of 6. Under NHTSA’s scoring, 4 or more clues means the officer should conclude your BAC is 0.08% or higher.4

    In the order officers check them:

    Clue (checked in each eye) What the officer looks for What else can cause it
    1. Lack of smooth pursuit Your eye jerks or “bounces” instead of gliding as it follows the pen Moving the pen too fast or unevenly; age; some medications
    2. Distinct and sustained nystagmus at maximum deviation Clear jerking that continues while your eye is held as far to the side as it goes for at least 4 seconds Normal “end-point” jerking many sober people have; fatigue nystagmus if held 30+ seconds
    3. Onset of nystagmus before 45 degrees Jerking starts before your eye has turned about 45 degrees, while some white still shows at the outer corner The officer misjudging the 45-degree angle; some sober people’s normal eyes

    Clue 1: Lack of smooth pursuit

    A sober eye follows a slowly moving object smoothly, like a marble rolling across glass. When it can’t keep up, it makes small catch-up jumps. The officer moves the pen at a steady speed, about 2 seconds out to the side and 2 seconds back.4

    Clue 2: Jerking at the far side

    The officer holds the pen far enough to the side that your eye can’t turn any further, and keeps it there for at least 4 seconds. The clue counts only if the jerking is distinct and keeps going. A slight jerk that fades does not count.4

    Clue 3: Jerking before 45 degrees

    The officer moves the pen slowly, taking about 4 seconds to reach 45 degrees, and stops when the jerking begins. If it starts before 45 degrees and some white still shows at the outer corner of your eye, the clue counts.4

    3. How is the HGN test given?

    A properly given HGN test follows a fixed sequence of at least 14 passes of the pen or light, checking your left eye first each time.45

    1. Questions first. The officer should ask whether you wear glasses and whether you have any eye or medical problems. Glasses come off for the test.
    2. Positioning. You should face away from flashing lights and passing traffic. The officer holds the pen or penlight 12 to 15 inches from your nose, slightly above eye level, and tells you to follow it with your eyes only.
    3. Medical check. The officer looks for equal pupil size and resting nystagmus, then makes one pass per eye to see if your eyes track together. Unequal pupils or tracking can mean a head injury or other medical problem, and the test should stop.
    4. Smooth pursuit. Two passes per eye, about 2 seconds out and 2 seconds back.
    5. Maximum deviation. Two passes per eye, holding the pen at the far side for at least 4 seconds.
    6. Onset before 45 degrees. Two passes per eye, about 4 seconds to reach 45 degrees.
    7. Scoring. The officer adds up the clues, from 0 to 6.
    8. Vertical gaze check. Many officers then raise the pen and hold your eyes up for at least 4 seconds to look for vertical nystagmus.

    Done this way, the test takes time. By one count from defense lawyers who study NHTSA’s manual, the 14 passes take at least about 82 seconds. A test that is over in 20 or 30 seconds on a body camera was not given the standardized way.5

    A driver stands at a roadside at night while a hand holds a small penlight to the side of his face.
    During the HGN test, you follow a pen or penlight with your eyes while the officer watches for jerking.
    Timeline of the HGN test in 7 steps: medical questions and lighting, a medical check, then smooth pursuit, maximum deviation held 4 seconds and onset before 45 degrees, each eye twice, scoring where NHTSA treats 4 of 6 clues as BAC 0.08% or higher, and a vertical gaze check. At least 14 passes in all.
    The standardized HGN test takes at least 14 passes of the stimulus. Sources: NHTSA SFST curriculum (2/2023); National College for DUI Defense.

    Vertical gaze nystagmus

    Vertical gaze nystagmus is jerking when the eyes look up. It is not one of the 6 HGN clues. Research with police officers found it shows up only at high levels of impairment and only along with HGN clues, so officers treat it as a sign of a high dose of alcohol or of certain drugs.3

    4. How accurate is the HGN test?

    NHTSA says that 4 or more HGN clues correctly identify a BAC of 0.08% or higher 88% of the time. That figure comes from a single 1998 field study of 297 drivers stopped by San Diego police.6 In the early 1980s, NHTSA’s figure was 77% accuracy at the old 0.10% limit.7

    Bar chart of NHTSA accuracy claims for each field sobriety test at 0.08% BAC: HGN 88%, one-leg stand 83%, walk-and-turn 79%, from one 1998 San Diego study of 297 drivers, 234 of whom tested at 0.08% or higher.
    NHTSA accuracy figures for each field sobriety test. Source: Stuster and Burns, San Diego validation study (NHTSA, 1998), as charted by the California Highway Patrol.

    What the 88% figure doesn’t tell you

    Most drivers in the San Diego study were well over the limit: 234 of the 297 tested at 0.08% or higher, and their average BAC was about 0.12%. When nearly everyone is drunk, guessing “drunk” is usually right. Among the 83 drivers who were actually under 0.08%, officers using all three tests still judged 24 of them to be over the limit, about 29%.6

    Independent scientists who re-analyzed the San Diego data in 2005 concluded the tests are much less accurate than NHTSA reported for drivers near the 0.08% line, and are mainly useful for spotting drivers well above it. A 2010 review of the vision science found that the onset of nystagmus at or before 45 degrees can occur in some sober people.89

    Example: John has two beers with dinner in Pasadena and is pulled over for a broken taillight. The officer sees 4 HGN clues and arrests him. His breath test comes back at 0.05%. The “88%” figure did nothing for John; he was in the group the test gets wrong.

    5. Do I have to take the eye test in California?

    No. California law does not require you to take the HGN test or any other field sobriety test before an arrest, and there is no license suspension for declining. You can say politely, “I’d prefer not to do any field sobriety tests.”

    Three different tests get mixed up at a DUI stop:

    • Field sobriety tests (including HGN): voluntary. The officer is gathering evidence to decide whether to arrest you.
    • The roadside breath test (PAS): California law treats the preliminary alcohol screening test as a field sobriety test, and the officer must tell you that taking it does not satisfy the chemical test you may be required to take later.10 It is mandatory for drivers under 21 and for anyone on probation for DUI.11
    • The breath or blood test after arrest: required. By driving in California you have agreed to take a chemical test if lawfully arrested for DUI.12 Refusing means a 1-year license suspension for a first refusal, and if you are later convicted of DUI the judge must add refusal penalties, which for a first DUI can include 48 hours to 6 months in jail.1314 See chemical test refusal.

    For more on declining the roadside tests, see are you required to do a field sobriety test?

    6. Can HGN results be used in court?

    Yes, but only as one sign of impairment, never as a BAC number. In People v. Leahy, the California Supreme Court held that HGN is a “new scientific technique,” so prosecutors could not use an officer’s HGN testimony until a court found the test generally accepted by the relevant scientific community under California’s Kelly test.15

    A year later, in People v. Joehnk, a California appeals court found that the scientific community does accept HGN as a useful part of an intoxication assessment, together with an officer’s other observations.16

    The officer can explain HGN to the jury

    In People v. Randolph (2018), a California appeals court held that a trained, experienced officer may testify to what your HGN performance means without a separate expert witness. The court added that you can still challenge the officer’s evaluation with an expert of your own.17 In practice, that makes a defense expert and the body-camera video more important.

    No BAC number from your eyes

    California courts do not let an officer use the HGN test to estimate your blood alcohol level. In People v. Loomis, the court found no scientific acceptance of HGN for measuring BAC, and the experts in Joehnk agreed it can’t be used that way.1816 An officer can testify that you showed clues, but not that the angle of onset put you at a particular BAC.

    7. What can make the HGN test wrong?

    Many things besides alcohol can cause the clues, and the test is easy to give wrong. According to San Bernardino DUI lawyer Michael Scafiddi:

    “the reliability of the horizontal gaze nystagmus test is in serious question. Any good DUI defense attorney should therefore be able to expose several weaknesses in the procedures / administration of the horizontal gaze nystagmus test in a DMV hearing and/or in the courtroom.”

    Flowchart with 5 questions: whether the officer asked about glasses, eyes and health; whether flashing lights or passing cars were in view; whether the pen was 12 to 15 inches away, slightly above eye level and moved smoothly; whether the test took at least 82 seconds; and whether you have an eye, ear or brain condition or take medication. Each problem answer points to a possible defense.
    Five questions that can show your HGN clues came from something other than alcohol. Sources: NHTSA SFST curriculum; National College for DUI Defense; Rubenzer and Stevenson (2010).

    Medical conditions and medications

    Nystagmus can come from inner ear problems, head injuries, eye conditions, seizure medications and other depressant drugs. A big difference between your left and right eye may point to an injury rather than alcohol. Expert testimony from a neuro-ophthalmologist can show that your nystagmus is normal for you.1

    Sober people show nystagmus more often than many jurors would guess. In a 2023 study of drivers stopped by police in Norway, 25% of the drivers who had no alcohol or drugs in their blood still showed nystagmus on a roadside exam, compared with 53% of drivers who had been drinking.19

    Example: Maria has an inner ear condition that makes her eyes jerk when she looks far to the side. An officer in Riverside sees clues at maximum deviation and onset before 45 degrees, but never asked about her health. Her medical records can explain the clues.

    Fatigue

    One forensic study reported that fatigue makes end-point nystagmus worse.20 A later study, though, found that staying awake for 24 hours did not add field sobriety clues on its own; only alcohol did.21 So tiredness is worth raising, but it works best alongside other problems with the test.

    Lights, traffic and where you were standing

    NHTSA tells officers to turn you away from flashing lights and passing traffic. Watching flashing or fast-moving lights can trigger “optokinetic” nystagmus, a normal jerking that has nothing to do with alcohol. If the patrol car’s lights or the headlights of passing cars were in your line of sight, the clues may not mean what the officer says.15

    A driver looks at a pen held in front of him while police lights and passing headlights glow behind it.
    Flashing lights and passing cars in your line of sight can make your eyes jerk on their own.

    The pen, the speed and the 45-degree angle

    The pen should be 12 to 15 inches from your nose and slightly above eye level, moved at a steady speed. If the officer moves it jerkily, your eyes will look jerky too. The officer also has to estimate the 45-degree angle by eye, which leaves room for error.4

    To be fair, a 2007 NHTSA-funded study found that moderate changes in the pen’s speed, height and distance did not change HGN results. That makes a rushed or skipped step, or lights in your view, a stronger argument than an inch or two of distance.22

    Example: On the body-cam video, the officer in Ana’s case waves the pen across her face twice and calls it 6 clues. The whole test lasts 25 seconds. That is far short of the standardized procedure, and her lawyer can argue the test was never really given.

    What we look for on the body-camera video

    In our experience, the HGN test is rarely given by the book, and the video usually shows it. One thing we’ve found is that officers often skip the medical questions and hurry the passes.

    A forensic study of patrol-car videos reached the same conclusion: officers rarely followed the minimum HGN procedure they were trained on.20 When we review the video, we check:

    • Medical questions: did the officer ask about glasses, eye problems and health?
    • Your position: were patrol lights or traffic in your line of sight?
    • Left eye first: did each check start with your left eye and repeat for each eye?
    • Timing: did the officer hold the far-side position for 4 seconds and take about 4 seconds to reach 45 degrees?
    • Total time: did the test take at least about 82 seconds?
    • Distance and height: was the pen 12 to 15 inches away and slightly above eye level?

    Your lawyer can get the dash-cam and body-cam footage through discovery. For other ways to fight the charge, see our DUI defenses page.

    8. Does the HGN test matter at the DMV hearing?

    Yes. At the DMV’s administrative hearing on your license suspension, the hearing officer can rely on field sobriety evidence, HGN included. In Coffey v. Shiomoto, the California Supreme Court upheld a suspension where the driver’s breath and blood results pointed to a rising BAC, because her driving, red eyes, alcohol odor and failed field sobriety tests (she showed a lack of smooth pursuit in both eyes) supported a finding that she was over 0.08% when she drove.23

    So poor HGN results can hurt you at the DMV even when your chemical test was close to the limit, or support a rising blood alcohol defense when they were good. One thing we’ve found is that pointing out HGN mistakes on the video can take that evidence off the table at the DMV hearing as well as in court. Learn more about DMV hearings in DUI cases.

    9. Questions and answers

    What does HGN stand for?

    HGN stands for horizontal gaze nystagmus, an involuntary jerking of the eyes as they look to the side. Police use the HGN test as a DUI eye test.

    What do “drunk eyes” look like compared with sober eyes?

    Drunk eyes may look red or watery, but the HGN test looks for something you can’t see in a photo: the eye jerking or bouncing as it follows a moving object or looks far to the side. Sober eyes usually glide smoothly and hold steady until they reach the far side.

    Can you fail the HGN test when you’re sober?

    Yes. Medical conditions, some medications, fatigue, flashing lights and a badly given test can all produce clues, and studies have found nystagmus in some drivers with no alcohol or drugs in their system.

    How accurate is the HGN test?

    NHTSA says 4 or more clues identify a BAC of 0.08% or higher 88% of the time, based on a 1998 San Diego study. Independent reviews have found it much less reliable near the 0.08% limit.

    Do I have to take the HGN test in California?

    No. Field sobriety tests, including the HGN test, are voluntary in California. The breath or blood test after a DUI arrest is required, and refusing it brings a license suspension and other penalties.

    Can you control or stop nystagmus?

    No. Nystagmus is involuntary, so you can’t stop it or practice your way out of it. That’s also why most people don’t know they have it.

    Charged with DUI after an HGN test? Our California DUI defense lawyers can review the body-camera video, the officer’s report and your medical history to look for problems with the test. Call us or fill out the form on this page for a free consultation.

    Legal and scientific references:

    1. National Highway Traffic Safety Administration, DWI Detection and Standardized Field Sobriety Testing, Participant Manual (rev. 2/2023), Session 8 (horizontal gaze nystagmus; categories of nystagmus; medical impairment).
    2. Romano F, Tarnutzer AA, Straumann D, Ramat S, Bertolini G. Gaze-evoked nystagmus induced by alcohol intoxication. J Physiol. 2017;595(6):2161-2173: alcohol intoxication “induces a two-fold increase of centripetal eye-drift.”
    3. Citek K, Ball B, Rutledge DA. Nystagmus testing in intoxicated individuals. Optometry. 2003;74(11):695-710: “The presence of VGN at any test posture occurs only in the presence of signs of HGN and only at high levels of impairment.” See also NHTSA Participant Manual, note 1, Session 8 (depressants, inhalants and dissociative anesthetics).
    4. NHTSA Participant Manual, note 1, Session 8 (administrative procedures; standardized clues; 4 or more clues indicates BAC at or above 0.08%), as summarized in A. Palacios, SFST seminar handout (2025).
    5. National College for DUI Defense, Horizontal Gaze Nystagmus (HGN): a properly administered test “will have at least 14 passes of the stimulus and take a minimum of 82 seconds.”
    6. Stuster J, Burns M. Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent (NHTSA, 1998); decision matrix at 0.08% (24 of 83 drivers under 0.08% estimated at or above 0.08%) reported in NHTSA Traffic Tech No. 196 (March 1999).
    7. California Highway Patrol, SFST Timeline: “HGN 4/6 clues present = 77% accuracy at .10% or greater”; “HGN 4/6 clues present = 88% accuracy at .08% or greater.”
    8. Hlastala MP, Polissar NL, Oberman S. Statistical evaluation of standardized field sobriety tests. J Forensic Sci. 2005;50(3):662-669: accuracy “is much poorer than that indicated by Stuster and Burns.”
    9. Rubenzer SJ, Stevenson SB. Horizontal gaze nystagmus: a review of vision science and application issues. J Forensic Sci. 2010;55(2):394-409 (onset of nystagmus may occur in some sober subjects at 45 degrees or less).
    10. California Vehicle Code 23612(h)-(i) VC: a preliminary alcohol screening test “is a field sobriety test and may be used by an officer as a further investigative tool.”
    11. California Vehicle Code 13388 VC (drivers under 21); California Vehicle Code 23154 VC (drivers on probation for DUI).
    12. California Vehicle Code 23612(a)(1)(A) VC: “A person who drives a motor vehicle is deemed to have given his or her consent to chemical testing of his or her blood or breath … if lawfully arrested.”
    13. California Vehicle Code 13353(a)(1) VC: the DMV shall “[s]uspend the person’s privilege to operate a motor vehicle for a period of one year.”
    14. California Vehicle Code 23577(a)(1) VC; California Vehicle Code 23538(a)(1) VC (confinement “for at least 48 hours, but not more than six months”).
    15. People v. Leahy (1994) 8 Cal.4th 587, 591-592, 605-607, 611: HGN tests “involve a ‘new scientific technique'” subject to Kelly; once accepted, “no reason exists why police officers should be deemed unqualified to administer and report the results.”
    16. People v. Joehnk (1995) 35 Cal.App.4th 1488, 1495-1496, 1505, 1508: “[w]e conclude the evidence offered by the prosecution in this case establishes HGN testing is so accepted.”
    17. People v. Randolph (2018) 28 Cal.App.5th 602 (as modified Nov. 14, 2018): an officer “with adequate training and experience in performing the nystagmus test … without additional expert testimony, may now testify as to the significance of a defendant’s performance on an HGN test.”
    18. People v. Loomis (1984) 156 Cal.App.3d Supp. 1, 5-7; see Leahy, note 15, at 605.
    19. Strand MC, Gjerde H, Høiseth G. Nystagmus among suspected amphetamine impaired drivers. J Forensic Leg Med. 2023;95:102502 (nystagmus in 25% of drug-negative and 53% of alcohol-only apprehended drivers; Norwegian clinical test of impairment).
    20. Booker JL. End-position nystagmus as an indicator of ethanol intoxication. Sci Justice. 2001;41(2):113-116: “Fatigue exacerbates one component of the HGN test, end-position nystagmus”; police officers “rarely comply with the minimum requirements.”
    21. Citek K, et al. Sleep deprivation does not mimic alcohol intoxication on field sobriety testing. J Forensic Sci. 2011;56(5):1170-1179.
    22. Burns M. The Robustness of the Horizontal Gaze Nystagmus Test (NHTSA, 2007): “The variations did not alter the occurrence of, or the observations of, HGN.”
    23. Coffey v. Shiomoto (2015) 60 Cal.4th 1198: “non-chemical-test evidence of plaintiff’s intoxication may be relevant and thus admissible.”

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