The eye examination carries a lot of weight in a drug evaluation, and horizontal gaze nystagmus, the involuntary jerking of the eyes as they follow an object to the side, sits at the center of it. What many people do not realize is that, in the drug recognition model, eye bounce is expected for only some categories of drug. That makes it a discriminator, a sign that is supposed to point toward certain drugs and away from others, and a discriminator is something the defense can use as readily as the State.
In the DRE model, horizontal gaze nystagmus is expected for only some categories. That means the presence or absence of eye bounce is a tell, and when it does not fit the alleged drug, the opinion is in trouble.
The Categories That Should and Should Not Show It
In the model the officer is taught, horizontal gaze nystagmus is expected with central nervous system depressants, inhalants, and dissociative anesthetics. It is not expected with stimulants, hallucinogens, narcotic analgesics, or cannabis. So the presence or absence of eye bounce is supposed to help sort one category from another. The moment the eye findings and the alleged drug do not line up, the evaluation is at war with itself.
| Drug category | HGN expected? |
|---|---|
| CNS depressants | Yes |
| Inhalants | Yes |
| Dissociative anesthetics | Yes |
| CNS stimulants | No |
| Hallucinogens | No |
| Narcotic analgesics | No |
| Cannabis | No |
If the officer marked the eye test as failed but the lab found cannabis, a stimulant, a narcotic, or a hallucinogen, the officer’s own chart says that should not have happened.
When the eyes and the lab disagree
This is one of the cleanest contradictions in a drug case. If a DRE writes down that your eyes showed nystagmus and then the blood comes back positive for marijuana, the officer’s own training says that combination should not exist, because cannabis does not cause the eyes to do that. The same is true for stimulants, narcotics, and hallucinogens. So either the officer misread the eye test, which is easy to do on a dark shoulder with a moving stimulus, or the category opinion was wrong, and both of those help you. Nystagmus is not even unique to drugs in the first place, since plain fatigue can cause it, documented in Booker, End Position Nystagmus as an Indicator of Ethanol Intoxication, 41 Science and Justice 113 (2001), which means a tired, frightened person can show the very sign the officer treats as proof.
When the Eye Test Cuts for the Defense
Two patterns come up often. In the first, the officer records pronounced eye bounce, but the toxicology names a drug from a category that should not produce it, such as cannabis or a stimulant. In the second, the chart expects eye bounce for the alleged category and the officer found none. Either way the internal logic of the evaluation has failed. Add the ordinary, non-drug causes of nystagmus and the common errors in giving the test, and a sign the State counts on can become one of the better tools the defense has.
The Instructor’s View of the Eye Test
Horizontal gaze nystagmus is one of the exercises I teach, and teaching it means knowing every way it goes wrong. The stimulus has to be held at the right distance and moved at the right speed, the passes have to be timed, and the officer has to watch for distinct jerking at the correct angles rather than the slight bounce many sober eyes show at the extreme edge of vision. Nystagmus also has causes that have nothing to do with drugs, including certain medical and neurological conditions and even some people’s normal physiology. When the recorded eye findings do not match the category the officer named, or when the test was administered loosely, the eye evidence stops supporting the opinion and starts undercutting it.
How the Eye Test Is Supposed to Be Given
The eye test only means something when it is given correctly, and the procedure is exacting. The stimulus is held a set distance from the face and moved at a controlled speed, the eyes are checked in distinct passes, and the officer is looking for sustained jerking at maximum deviation and for onset of the bounce before the eye reaches a certain angle, not the slight flutter many sober eyes show at the far edge. Both eyes should track together. When the stimulus moves too fast, the passes are rushed, the angles are eyeballed, or equal tracking is never checked, the result is unreliable before any question of drugs comes up. Because I teach this exercise, I can read the report and the video against the standard and show a court exactly where the administration fell short.
The eye test is where I spend a lot of time, because I teach it and I know exactly how it is supposed to be done and what it can and cannot show. When the officer’s eye findings and the lab result point in different directions, I put the officer’s own chart in front of the jury and let the contradiction speak. A finding that fights the toxicology is a crack in the case rather than strong evidence, and I open it up.
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. I work both the science and the procedure in your case the way the State’s own analysts and officers are trained to, and I show a jury the exact point where the evidence does not hold up. Learn more about my background.
Questions About HGN and Drugs
What is HGN?
Horizontal gaze nystagmus is an involuntary jerking of the eyes as they track an object to the side. Officers look for it during the eye examination because some impairing substances make it more pronounced.
Do all drugs cause HGN?
No, and that is the key point. In the DRE model, eye bounce is expected for central nervous system depressants, inhalants, and dissociative anesthetics, and it is not expected for stimulants, hallucinogens, narcotic analgesics, or cannabis.
How does that help the defense?
It cuts both ways. If the officer reports strong eye bounce but the alleged drug is one that should not produce it, the evaluation contradicts itself. The absence of HGN where the chart expects it is just as telling.
Can things other than drugs cause nystagmus?
Yes. Some people have natural nystagmus, and certain medical and neurological conditions can cause it. Bright lights and a poorly administered test can also produce what looks like a positive result.
Why does it matter that you teach this test?
Because most of the value is in how the test was given. I teach the eye test and know the distance, speed, and timing it requires, so I can show when it was administered loosely and when the eye findings do not fit the drug the State alleges.
The officer said my eyes failed, but I only had marijuana. Does that add up?
By the DRE program's own matrix, no. Horizontal gaze nystagmus is expected only for CNS depressants, inhalants, and dissociative anesthetics, and cannabis does not cause it. So an HGN finding paired with a cannabis result is a contradiction, meaning the eye test was misread or the category opinion was wrong.
Related pages: drug recognition expert overview, the categories and matrix, field sobriety tests in a drug case, and marijuana and THC DUI.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Drug DUI in Florida is governed by Fla. Stat. 316.193, section 877.111, and chapter 893, and expert testimony is governed by Fla. Stat. 90.702. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
