Horizontal gaze nystagmus is the eye exercise. The officer holds a pen or fingertip in front of your face, moves it side to side, and watches your eyes for an involuntary jerking called nystagmus. The theory is that alcohol and other central nervous system depressants make that jerking start earlier and look more pronounced. Of the three standardized exercises, this is the one Florida treats as scientific evidence, which under State v. Meador, 674 So. 2d 826, 836 (Fla. 4th DCA 1996), means the State has to lay a predicate before a jury hears a word about your eyes.
There are exactly six clues, three in each eye, and they are scored the same way every time. The officer looks for a lack of smooth pursuit as your eye tracks the stimulus, for a distinct and sustained jerking when your eye is held all the way to the side at maximum deviation, and for the jerking starting before your eye reaches 45 degrees. Four or more clues across both eyes is the decision point the training uses.
Six clues, three per eye, and a fixed procedure. A stimulus moved too fast or held too briefly is the single most common way the eye exercise goes wrong.
The procedure is precise, and precision is where it fails
The manual sets the procedure down to the inch and the second. The stimulus is held 12 to 15 inches from your face and slightly above eye level. Before any of it, the officer is supposed to check that your pupils are equal and that you do not have resting nystagmus, because unequal pupils or a jerk at rest can signal a medical issue or head injury rather than alcohol. The stimulus has to move at a set speed and pause at maximum deviation for at least four seconds. Those numbers are not suggestions. They are the conditions the exercise was validated under.
The most common error is speed. An officer who sweeps the stimulus across in one or two seconds, instead of taking the full four, will see what looks like an early onset of nystagmus in almost anyone, because the eye cannot keep up with a stimulus moving that fast. As an NHTSA-qualified instructor, I time the officer’s movements on the bodycam frame by frame, and a rushed pass is something a jury can see for itself once it is pointed out.
Where an HGN reading goes wrong
| What the standard requires | The common deviation | Why it matters |
|---|---|---|
| Screen for equal pupils and resting nystagmus first | Screen skipped or not recorded | A medical cause is never ruled out before the eyes are scored |
| Stimulus 12 to 15 inches out, slightly above eye level | Held too close or off center | Position alone can create the jerk the officer counts |
| Hold at maximum deviation at least 4 seconds | A quick sweep with no hold | Fast movement mimics sustained nystagmus that is not there |
| At least 4 seconds to reach 45 degrees | Onset judged by eye, too fast | Onset angle is estimated, not measured, so speed skews it |
Every line here is something the officer controls and the report rarely captures, which is why the body-worn video matters so much on this exercise.
Your eyes jerk for many reasons that have nothing to do with alcohol
Nystagmus is common and has a long list of causes. Fatigue, certain prescription and over-the-counter medications, caffeine and nicotine, inner-ear problems, neurological conditions, and even the flashing lights and passing traffic at a roadside can all produce or mimic it. The officer at the scene is not a doctor, did not take a medical history, and cannot tell alcohol nystagmus from the other kinds by looking. That gap is a large part of the defense.
HGN, drugs, and the scientific fight
If your case involves drugs rather than alcohol, the eye findings get more complicated, and that analysis lives in the drugged driving section. The deeper legal fight over whether the HGN evidence should reach the jury at all, the Meador predicate, and the false positive rate, is covered on the HGN as scientific evidence page.
HGN is the one exercise the State dresses up as science, so I hold it to the science. I go through the video with you and check the plain things the manual demands, the screen, the pacing, and whether the onset was measured or only guessed. There is also a natural jerking called end-position nystagmus, documented in Booker, End Position Nystagmus as an Indicator of Ethanol Intoxication, 41 Science and Justice 113 (2001), that shows up in sober people right at maximum deviation, the exact spot being graded. In the government’s own San Diego data, two out of every three people wrongly arrested under the limit had all six of these clues, so six clues has never meant what a jury is told it means. I teach this test, and I make the State earn every part of the reading before anyone uses it against you.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida and an NHTSA-qualified field sobriety instructor. I was trained to administer this battery and to teach the officers who give it, so I can stand in front of a jury and show the exact point where the roadside exercises in your case left the standard. Learn more about my background.
What the Validation Numbers Really Say About HGN
NHTSA’s own materials put numbers on this test: the original research classified about 77 percent of suspects correctly against a 0.10 standard, and the 1998 San Diego study published an 88 percent figure for HGN at 0.08. Even taken at face value, that is one wrong call in every four to eight. The same materials list seizure medications, inhalants, barbiturates, and other depressants as additional causes of nystagmus, which is why the medical history in your case matters.
The face value deserves scrutiny. The San Diego study was never peer reviewed, it was conducted by the developer of the tests with seven officers, and its subjects averaged a 0.122 BAC, so the test was graded largely on easy calls. Run the arithmetic on the study’s own tables and, among drivers who were under 0.08, HGN flagged 30 of 81 as over. That is a 37 percent false positive rate for the sober-side drivers the test exists to protect, and it was validated only to predict a number, never to measure impairment.
Questions About the HGN Eye Test
What is horizontal gaze nystagmus?
It is an involuntary jerking of the eye as it moves to the side. The officer watches for it while you follow a pen or fingertip. The idea is that alcohol and certain other drugs make the jerking start earlier and look stronger, though many ordinary things cause it too.
How many HGN clues are there?
Six, three in each eye. The officer looks for a lack of smooth pursuit, a distinct and sustained jerk at maximum deviation, and the jerking starting before the eye reaches 45 degrees. Four or more across both eyes is the training's decision point.
Can the HGN test be wrong?
Yes. Fatigue, some medications, caffeine and nicotine, inner-ear and neurological conditions, and even roadside lights can produce or mimic nystagmus. The officer cannot tell alcohol nystagmus from the other kinds by looking, and a stimulus moved too fast creates false clues.
Is HGN admissible in Florida?
It is treated as scientific evidence. Under State v. Meador the State must lay a predicate before the jury hears it, which is a higher bar than the officer's plain observations of the walk and turn or one leg stand. That predicate is something we can challenge.
What is the most common HGN error?
Speed. The stimulus is supposed to move at a set pace and hold at maximum deviation for at least four seconds. An officer who sweeps it across in one or two seconds will see false clues in almost anyone, and bodycam timing can prove it.
How do you use the credential here?
As an NHTSA-qualified instructor in this battery, I review the bodycam against the exact procedure the officer was trained on, time the stimulus movements, and show the jury where the eye exercise left the standard.
Related: the walk and turn, the NHTSA standard and accuracy, HGN as scientific evidence, medical conditions that affect the exercises, and HGN and drugs.
What has to happen before the officer scores my eyes?
A short medical screen. The officer is supposed to check that your pupils are equal, that your eyes track together, and that there is no nystagmus at rest, because those can point to a medical cause rather than alcohol. When that screen is skipped, the reading rests on nothing.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Field sobriety exercises in Florida are governed by case law and by section 316.193, Florida Statutes. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
