The horizontal gaze nystagmus test is often the first roadside exercise in a Florida DUI investigation, and the hardest for a driver to judge. Challenges usually focus on how it was given, how it was scored, and what else could explain the eye movement.
What nystagmus is and what officers look for
Nystagmus is an involuntary jerking of the eye. During the HGN test, the officer moves a stimulus horizontally while the person follows it with the eyes only, keeping the head still. The officer watches each eye for specific signs, such as whether it tracks smoothly and at what point the jerking begins.
The test is meant to be given and scored under national NHTSA standards. Those standards address how far the stimulus is held from the face, how fast it moves and how long it is held at certain positions.
Why HGN is hard to verify after the fact
Unlike the walk and turn, the HGN test happens at close range and depends on small movements of the eyes. Dashcam or body camera video often cannot show the eyes clearly, so the officer's report may be the main record of what happened.
That makes the officer's training and the detail in the report especially important. Through discovery and depositions, the defense can ask how the officer was trained on HGN, how the test was positioned and timed, and how each sign was scored. In misdemeanor cases, which include most first and second DUIs, depositions are more limited and generally need a showing of good cause.
Lighting, distractions and other causes of nystagmus
Flashing emergency lights, passing headlights, wind and the person's position relative to traffic can distract the eyes or make tracking difficult. A test given while facing strobing lights is far from a controlled setting.
Nystagmus also has causes besides alcohol. Some medical and neurological conditions, certain medications, eye conditions and fatigue can produce eye movement that looks similar. A person's medical history can therefore be relevant to what the officer saw.
Putting HGN in context with the rest of the case
HGN is one piece of a larger set of observations. When it is the strongest indicator in a report and the driving, speech and other exercises are unremarkable, how much weight it deserves becomes a fair question for the court.
A challenge to HGN may take the form of questioning its administration at a deposition, arguing about its weight at trial, or raising it in a motion when the evidence supports one. DUI Lawyers Shield Firm reviews HGN and other field evidence in a free case review.
Keep reading
- Florida Field Sobriety Tests: What They Are and How They Fail
- What Happens at a Florida DUI Arraignment
- How Long a Florida DUI Stays on Your Record
General information only, not legal advice. Every case depends on its own facts, and prior results do not guarantee a similar outcome.
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