Understanding Horizontal Gaze Nystagmus Testing in a Texas DWI Case

Justin Sparks
5 days ago
5 min read

If you were stopped for a suspected DWI in Fort Worth, and you were asked to take standard field sobriety tests, the police officer may have asked you to follow a pen or light with your eyes. This is called the horizontal gaze nystagmus (HGN) test. It is one of the most commonly used field sobriety tests in Texas.

Most drivers don't fully understand what the officer is looking for when the HGN test is done, and how it relates to having a high blood alcohol concentration and, ultimately, an arrest for DWI.

If you are facing a DWI charge, understanding how HGN testing works, how it can be challenged, and how it is used in court is critical.

What Is Horizontal Gaze Nystagmus (HGN)?

“Nystagmus” refers to an involuntary jerking of the eyes. When a person drinks alcohol, this movement becomes exaggerated.

During the HGN test, with is part of standardized field sobriety tests, an officer does the following:

Holds a stimulus (usually a pen or small flashlight) about 12–15 inches from your face.
Instructs you to keep your head still.
Moves the object slowly from side to side.

Then, the officer looks at your eyes for the "three major clues" of intoxication. (Really six...one for each eye.)They are:

Lack of smooth pursuit (the eye does not move smoothly)
Distinct and sustained nystagmus at maximum deviation
Onset of nystagmus prior to 45 degrees

If four or more of these clues are observed, officers are trained to conclude that the person is likely intoxicated; driving over the legal limit.

However, the officer conducting the test must do it correctly, and there are a number of things that can create reasonable doubt in these tests.

How HGN Is Used in Texas DWI Cases

Here in Texas, when an HGN test performed during a DWI stop is used in court, it can be part of the prosecution's case for DWI charges.

Unlike other field sobriety tests, like the "walk-and-turn" or "one leg stand," this test is seen as more "scientific," because it's based on the actions of the eye muscles.

Texas courts allow police officers to testify about these results, as long as they are trained and certified, and this evidence often plays a central role in DWI cases.

Is HGN Testing Accurate?

There are several factors when considering the accuracy of HGN tests. These include:

Proper administration, training, and certification
Strict adherence to NHTSA protocols
Correct positioning of the stimulus (the pen or light)
Accurate timing of eye movements (distinct nystagmus movements)
Environmental conditions or medical conditions that could affect the test results

Even small deviations from the usual procedures can cause the test to be seen as invalid. If the test is rushed or performed incorrectly, the results may be unreliable.

Medical and Environmental Factors That Affect HGN

Medical impairments were mentioned, earlier, and this could affect the eye test and cause false positives.

Factors that can produce similar eye movements include:

Inner ear disorders
Neurological conditions or brain damage
Other health conditions
Fatigue
Certain medications
Head injuries/head trauma
Bright or flashing lights

If a person was tested on the side of a busy highway at night—with patrol lights flashing into the driver's eyes or moving objects, like cars going by—environmental interference or external factors may be an issue, too. Officers are trained to account for these possibilities, but errors occur. A DWI attorney will certainly be paying attention to this and explain how retrograde extrapolation is used in DWI cases in Texas.

Can You Refuse the HGN Test?

Yes. You can. Field sobriety tests, including HGN, are voluntary in Texas. They are not covered by implied consent laws, which apply to chemical testing after arrest.

However, refusing field sobriety tests may still result in arrest if the officer believes other evidence supports probable cause. These might include slurred speech, weaving, falling, or the odor of alcohol, but none of it is fool proof.

It is important to remain calm and respectful during any traffic stop and follow the officer's instructions, regardless of your decision.

HGN vs. Breath or Blood Tests

The HGN test is not a chemical test. It does not directly measure blood alcohol concentration (BAC).

Instead, it is used to:

Establish probable cause for arrest
Support officer observations
Corroborate chemical test results

If a breath or blood test later shows a BAC above 0.08%, prosecutors often combine that evidence with HGN results to strengthen their case.

However, if chemical testing is unavailable or disputed, HGN testimony may carry increased importance.

Challenging HGN Evidence in Court

An experienced DWI defense attorney may challenge HGN evidence by examining:

The officer’s training and certification
Whether NHTSA guidelines were strictly followed
Body cam footage showing test administration
Timing and pacing of the stimulus
Lighting conditions
Medical explanations for eye movement

Cross-examination can reveal inconsistencies between the officer’s report and the video evidence.

Because HGN involves subjective interpretation, credibility matters.

The Importance of Body Camera Footage

Modern DWI stops often involve body camera and dash cam recordings. These videos are critical in reviewing HGN administration.

An attorney may analyze footage to determine:

Whether instructions were clear
Whether your head moved
Whether the officer moved the stimulus too quickly
Whether distractions were present

Video evidence can significantly impact the weight given to HGN testimony.

DWI Penalties in Texas

A DWI conviction in Texas can result in:

Jail time
Fines
Driver’s license suspension
Ignition interlock requirements
Increased insurance premiums
A permanent criminal record

Because HGN testing is often used to justify arrest and prosecution, challenging flawed administration may be a key component of your defense strategy.

Timing Matters After a DWI Arrest

If you were arrested:

You typically have 15 days to request an Administrative License Revocation (ALR) hearing.
Evidence such as body cam footage should be reviewed promptly.
Early investigation allows your attorney to identify weaknesses in the state’s case.

Delaying action can limit available defenses. For more information, a top criminal defense attorney in Fort Worth TX may be able to help.

Contact Sparks Law Firm in Fort Worth for a Free Consultation

Horizontal Gaze Nystagmus testing is frequently presented as objective science—but its reliability depends on proper procedure and accurate interpretation.

If you were arrested for DWI in Fort Worth or anywhere in Tarrant County, Sparks Law Firm can carefully examine the evidence, review HGN administration, and identify potential constitutional or procedural violations.

DWI cases often turn on technical details. A thorough legal review may uncover defenses that are not immediately obvious.

If you have questions about HGN testing or a pending DWI charge, contact Sparks Law Firm to discuss your options and protect your rights.