Key points
- NHTSA defines horizontal gaze nystagmus as an involuntary jerking of the eyes as they gaze to the side, which the person is usually unaware of and is powerless to stop or control.[1]
- Officers look for three clues in each eye, for a maximum of six; NHTSA teaches that four or more clues indicate a BAC likely at or above 0.08.[2][3]
- NHTSA's manual says nystagmus can also come from brain injury, some inner-ear diseases and other conditions, and teaches a medical check before the test.[4][5]
- Michigan's statute names the HGN test and allows a person qualified in giving it to testify, subject to a proper foundation of qualifications.[6]
The horizontal gaze nystagmus (HGN) test is the eye test in the standardized field sobriety battery: the officer moves a stimulus (a small object to follow) side to side and watches the driver's eyes for jerking. NHTSA defines horizontal gaze nystagmus as "involuntary jerking of the eyes, occurring as the eyes gaze to the side," and says alcohol and certain other drugs cause it.[1]
This page explains what the test looks for, how NHTSA's manual teaches officers to give and score it, what the manual itself says can cause nystagmus besides alcohol, and how Michigan law treats testimony about it. The other two roadside tests are covered in the main field sobriety tests guide.
What nystagmus is
Nystagmus is jerking of the eyes. NHTSA's training material says that with horizontal gaze nystagmus the person "is usually unaware that it is happening" and "is powerless to stop it or control it."[1] HGN is the first test given in the SFST battery.[1]
NHTSA also says HGN "is not the only kind of nystagmus." It groups the causes into three general categories: vestibular, neural, and pathological disorders and diseases.[4] Pathological causes listed in the manual include brain tumors, other brain damage and some inner-ear diseases, which the manual says "occur in very few people and in even fewer drivers."[4] Resting nystagmus, meaning jerking while the eyes look straight ahead, is described in the manual as usually indicating a pathological disorder or high doses of a dissociative anesthetic drug such as PCP.[7]
How the test is given
NHTSA's 2013 participant guide teaches "10 steps in the systematic administration of the Horizontal Gaze Nystagmus test."[8]
- Check for eyeglasses and have the subject remove them if worn.[5]
- Give instructions: feet together, hands at the side, keep the head still, follow the stimulus with the eyes only, and keep looking until told the test is over.[8]
- Position the stimulus about 12 to 15 inches in front of the nose and slightly above eye level.[8]
- Check for equal pupil size and resting nystagmus.[8][5]
- Check for equal tracking, meaning whether the eyes follow the object together.[8][5]
- Check each eye for lack of smooth pursuit.[8]
- Check each eye for distinct and sustained nystagmus at maximum deviation.[8]
- Check each eye for onset of nystagmus before 45 degrees.[8]
- Total the clues.[8]
- Check for vertical nystagmus.[8]
The medical check before scoring
Steps 4 and 5 are a screen for medical causes. The manual says: "If the eyes do not track together, or if the pupils are noticeably unequal in size, the chance of medical disorders or injuries causing the nystagmus may be present."[5] It also tells officers to check both eyes independently, and says that if one eye shows all three clues distinctly while the other shows no nystagmus, "the person may be suffering from one of the pathological disorders covered previously."[9]
How the test is scored
| Clue (checked in each eye) | What NHTSA describes |
|---|---|
| Lack of smooth pursuit | The eye jerks while following the moving object, rather than moving smoothly.[8][2] |
| Distinct and sustained nystagmus at maximum deviation | Jerking that is distinct and sustained when the eye is held as far to the side as it will go for at least four seconds.[2] |
| Onset of nystagmus prior to 45 degrees | The jerking begins prior to 45 degrees.[2][8] |
The maximum is three clues per eye and six in total.[2] NHTSA's 2013 manual says: "if you observe four or more clues it is likely that the subject's BAC is at or above 0.08. Using this criterion you will be able to classify about 88% of your subjects accurately."[3]
Reading the 88% figure carefully
The NHTSA figure is about classifying subjects as at or above 0.08 BAC, not a measurement of driving ability, and it comes from the 2013 edition of the manual read for this guide.[3] NHTSA also says the field sobriety tests "are a tool to assist you in seeing visible signs of impairment and are not a pass/fail test."[10] A frequent claim online is that a certain number of clues "proves" intoxication; the training material does not say that.
What Michigan law says about HGN evidence
Michigan defines a "standardized field sobriety test" as one of the tests validated by NHTSA, and a test counts as standardized only if given "in substantial compliance with the standards prescribed by" NHTSA.[11] The NHTSA manual lists HGN as one of the three standardized tests.[12]
The Vehicle Code names HGN expressly. Under MCL 257.625s, "A person who is qualified by knowledge, skill, experience, training, or education, in the administration of standardized field sobriety tests, including the horizontal gaze nystagmus (HGN) test, shall be allowed to testify subject to showing of a proper foundation of qualifications."[6] That section was added by 2016 PA 242, effective September 22, 2016.[6]
The Evidence Benchbook describes People v Peebles (1996) as allowing an officer to testify as an expert on field sobriety tests where the evidence shows knowledge, training and experience regarding the tests at issue.[13]
The statute's wording raises two questions in a given case: whether the witness is qualified and has a proper foundation of qualifications, and whether the test was given in substantial compliance with NHTSA standards so that it is a "standardized" test at all.[6][11] Where the testimony is expert testimony, the court acts as gatekeeper under MRE 702.[14] Challenges of that kind are raised through the motion process explained in the motions to suppress guide.
HGN and drugs
NHTSA says alcohol "and certain other drugs" cause horizontal gaze nystagmus.[1] The drug recognition expert protocol described in People v Bowden includes an eye examination and an examination of pupil sizes in different lighting, as separate steps from the roadside battery.[15] See the drug recognition experts guide and the drug-impaired driving evidence guide.
Where HGN fits in the investigation
HGN is one part of the roadside investigation, along with the officer's observations, the other field tests and any preliminary breath test. An officer may arrest based in whole or in part on a PBT result.[16] The legal standard for arrest is explained in probable cause for an OWI arrest, and the full sequence in what happens at an OWI traffic stop. At trial, Michigan's chemical-testing rules do not limit other admissible evidence about whether a person was impaired.[17] The evidence guide covers how roadside observations and test results are presented.
Common misunderstandings
| Belief | What the sources say |
|---|---|
| "Only alcohol causes nystagmus." | NHTSA lists vestibular, neural and pathological causes, including brain injury and some inner-ear diseases.[4] |
| "Four clues means I was over 0.08." | NHTSA says four or more clues make it "likely" the BAC is at or above 0.08 and that the criterion is about 88% accurate.[3] |
| "Any officer can testify about HGN." | MCL 257.625s requires a person qualified in giving the tests and a proper foundation of qualifications.[6] |
Common questions
What does the HGN test actually measure?
It looks for nystagmus, an involuntary jerking of the eyes as they move to the side. NHTSA's manual says alcohol and certain other drugs cause it, and that a person is usually unaware of it and cannot stop it. The officer counts specific clues in each eye.[1][2]
Can something other than alcohol cause nystagmus?
Yes. NHTSA's own manual says there are other kinds of nystagmus, grouped as vestibular, neural, and pathological disorders and diseases, and that brain tumors, other brain damage and some inner-ear diseases can cause it, though it says these are rare among drivers.[4]
What is the HGN 'score' and what does it mean?
NHTSA's maximum is six clues, three per eye. Its 2013 manual teaches that four or more clues indicate a BAC likely at or above 0.08 and says that criterion classifies about 88% of subjects accurately. That is a statement about predicting an alcohol level, not a pass/fail result.[2][3][10]
Is HGN evidence allowed in Michigan courts?
Michigan's statute says a person qualified by knowledge, skill, experience, training or education in giving standardized field sobriety tests, including HGN, shall be allowed to testify subject to a proper foundation of qualifications. When an officer testifies as an expert, the court acts as gatekeeper under MRE 702 to make sure the testimony is relevant and reliable.[6][14]
Do I have to take my glasses off for the test?
NHTSA's procedure starts with telling the subject to remove eyeglasses, if worn. The manual says it does not matter whether the subject can see the stimulus with perfect clarity, as long as the subject can see it at all.[18]
Sources
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, p 8-18; Glossary — nj.gov
- NHTSA, DWI Detection and SFST Participant Guide, HS 178 R5/13, Session 8, pp 8-38 to 8-39 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, p 8-39 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, pp 8-19, 8-28 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, pp 8-31 to 8-33 — nj.gov
- MCL 257.625s — Michigan Legislature
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, p 8-27 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, pp 8-34 to 8-38 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, Session 8, p 8-38 — nj.gov
- NHTSA SFST Participant Guide, HS 178 R5/13, p 8-75 — nj.gov
- MCL 257.62a — Michigan Legislature
- NHTSA, DWI Detection and SFST Participant Guide, HS 178 R5/13, Glossary — nj.gov
- People v Peebles, 216 Mich App 661, 667–668 (1996) — Michigan Courts
- MRE 702; People v Kowalski, 492 Mich 106, 120 (2012) — Michigan Courts
- People v Bowden, 344 Mich App 171 (2022) (slip op at 2) — Michigan Courts
- MCL 257.625a(2)(a) — Michigan Legislature
- MCL 257.625a(7) — Michigan Legislature
- NHTSA, DWI Detection and SFST Participant Guide, HS 178 R5/13, Session 8, p 8-34 — nj.gov
This page is general legal information about Michigan law, published by the office of Attorney Shawn Haff. It is not legal advice and reading it does not create an attorney-client relationship. Laws change and every case turns on its own facts — check the sources linked above and talk to a licensed Michigan attorney about your situation. In an emergency, call 911. For confidential help with alcohol or drug use, the SAMHSA National Helpline is 1-800-662-4357.