When you’re pulled over for suspected drunk driving, it’s scary and abnormal for you but is routine for police officers. This is a regular part of their job, which ideally gives you some comfort in the sense that you are guaranteed certain protections. The police officer must use legally approved methods—called standard field sobriety tests (SFSTs)—to determine if you should be charged with either driving under the influence (DUI) or driving while intoxicated (DWI).
While most people think they will be given a breathalyzer when they’re pulled over, that typically happens after the SFSTs. There are only three SFSTs that have been approved by the National Highway Traffic Safety Administration (NHTSA) and the State of Oklahoma: the Horizontal Gaze Nystagmus, the Walk and Turn, and the One-Leg Stand. These tests are considered the most reliable because studies have proven they produce scientifically validated clues of alcohol impairment.
While the Walk and Turn and One-Leg Stand test are fairly self-explanatory, the Horizontal Gaze Nystagmus test is more of a mystery to people. Because it is the most commonly used SFST in the field, it’s important to know what it is and how you can pass it.
What Is the Horizontal Gaze Nystagmus Test?
The NHTSA considers the Horizontal Gaze Nystagmus test to be the most accurate of the three SFSTs, so there is a very good chance a police officer will give you this test during a DUI stop.
In brief, nystagmus is an involuntary jerking of the eyes. The higher your blood alcohol content (BAC), the more your eyes will jerk. Because it’s not something you can control, or even be aware of, testing your eyes for nystagmus is an objective way that officers can assess whether you’re impaired (in theory).
Here’s how it works: The officer will ask you to stand still with your feet together and, if necessary, remove any glasses you’re wearing. Then, the officer will ask you to watch, without turning your head, as they move a small stimulus (such as a light, index finger, or pen) back and forth horizontally in front of your eyes. The officer is looking for three clues:
Lack of Smooth Pursuit
If your eyes jerk or "bounce" as they follow the moving stimulus. When you’re impaired, your eyes won’t be able to track smoothly and will look more like windshield wipers moving across a dry windshield.
Distinct and Sustained Nystagmus At Maximum Deviation
If your eyes continue to jerk within four seconds when held at maximum deviation—when looking all the way to one side.
Onset of Nystagmus Prior To 45 Degrees
If your eyes begin jerking before hitting 45 degrees, it is a strong indication that your BAC is above .08. The higher the degree of impairment, the sooner the nystagmus is observable.
Each eye will be tested for each clue, so there are a total of six clues the officer will be looking for (three per eye). The officer will make two passes in front of your eyes with the stimulus to check one by one for the clues, and then will total the clues. If four or more clues are evident, the Horizontal Gaze Nystagmus test is allegedly 88% accurate in detecting a person with a blood alcohol level at or above .08.
Vertical Gaze Nystagmus
The officer may also check for vertical nystagmus, which is exactly like it sounds. You’ll need to watch the stimulus as it moves vertically (up and down) instead of horizontally (side to side). If your eyes involuntarily jerk as they move up and down or stare at maximum deviation, the officer will take this as another indication that you are impaired.
Note that your eyes will behave the same whether you’re tested for nystagmus vertically or horizontally. No drug or alcohol will cause your eyes to jerk on one test, and not another. So, if Vertical Gaze Nystagmus is present and Horizontal Gaze Nystagmus is not (or vice versa), it will be a red flag to police officers that there could be another reason you seem to be impaired.
False Positives: Other Causes of Nystagmus
Nystagmus may be caused by factors other than alcohol, such as seizure and other prescription medications, medical disorders or injuries (ranging from brain tumors to inner ear problems), and even close proximity to strobe or rotating lights.
Before starting the Horizontal Gaze Nystagmus, officers will ask about your general and eye health. If you report that you have a relevant medical condition, such as partial blindness or an inner ear disease, the officer may decide not to proceed with that particular test because those conditions can give false positives.
Similarly, the officer will also check you for equal pupil size, resting nystagmus (if your eyes jerk as they look straight ahead), and equal tracking (if your eyes can follow an object together). When the eyes do not track together or the pupils are noticeably unequal in size, it can indicate that you have a medical disorder or injury that causes you to exhibit nystagmus.
Also, if there is a large disparity in your performance on the horizontal versus vertical test, or between the right and left eye, it may indicate a medical condition.
If any of the above applies to you, it could mean you received a false positive on a Horizontal Gaze Nystagmus test and can weaken any case officers have for charging you with a DUI.
What If I Fail a Horizontal Gaze Nystagmus Test?
Was your test administered correctly? Did you receive a false positive? We can help you figure that out in minutes. Tulsa DUI Guy has spent 15 years helping people build effective defense strategies that limit the impact of a DUI on your life. If you failed any SFST and were charged with a DUI or DWI, contact us for a free case evaluation.