What Is Horizontal Gaze Nystagmus (HGN)?

Drugged and drunk driving is a serious and common problem. DUI laws are strict in order to combat the issue. Law enforcement officers receive specialized training to look for signs of impairment.

When law enforcement suspects you of driving under the influence of alcohol or drugs, they will likely ask you to perform a series of field sobriety tests (FSTs). There are three accepted standardized field sobriety tests: the walk-and-turn test, the one-leg stand test, and the horizontal gaze nystagmus test.

Horizontal gaze nystagmus (HGN) is the involuntary jerking of one’s eye when it gazes to the side. Alcohol use exaggerates this jerking motion. The HGN test is evidence of impairment in DUI cases. The following provides an overview of the science and law behind the HGN field sobriety test.

Horizontal Gaze Nystagmus and Impairment: The Science

“Nystagmus" refers to the bouncing or involuntary jerking of the eye. Police officers want to observe horizontal gaze nystagmus, a completely involuntary motion that becomes more pronounced when impaired by alcohol and some illicit and prescription drugs, primarily depressants.

Alcohol depresses the central nervous system. It slows our reflexes and reduces coordination. As a result, it has a noticeable effect on our ability to control eye movements smoothly and accurately. When a law enforcement officer asks you to keep your head still and follow a pen or finger with your eyes, this triggers nystagmus or eye jerking when you are under the influence.

HGN has no noticeable effect on your vision. You cannot control or stop these movements from happening. If the police officer sees sustained nystagmus when your eye is at maximum deviation from the center, this is strong evidence of intoxication.

Taken as a whole, the three-part field sobriety test can accurately determine a blood-alcohol concentration (BAC) of 0.10% or higher 83% of the time, according to a study cited by the National Highway Traffic Safety Administration (NHTSA). The other two components are the one-leg stand (OLS) and walk-and-turn (WAT) tests.

Multiple NHTSA-funded studies since 1977 have shown the HGN screening to be a valid indicator of alcohol impairment. It’s the most accurate part of the three-part FSTs. The precision of the test increases to 92% as an officer gains additional training and experience conducting the HGN test.

How Police Conduct the HGN Test

Officers conduct the HGN test in a well-lit area or with a flashlight. The suspect’s eyes will face away from the police cruiser’s flashing lights and the lights of passing cars. These extra lights can skew results. The officer then tells you they are going to check your eyes. They will ask whether you wear contact lenses or have any medical condition that would affect the results. You will need to remove any eyeglasses, as they can make it difficult for the officer to observe your eyes.

The police officer will look for equal pupil size and equal tracking in your eyes. If your pupils are different sizes or your eyes move at different rates, this may indicate a head injury or other serious condition. They will observe your resting nystagmus, which could mean a medical condition.

The officer then asks you to follow an object. This is usually the tip of a pen or pen light, placed 12 to 15 inches away and slightly above eye level with only your eyes.

The officer is looking for the following three clues in each eye:

1. Lack of smooth pursuit: Does the eye follow the object smoothly as it moves from the center of the face toward the ear, or does it jerk?
2. Distinct nystagmus at maximum deviation: Does the eye have a distinct jerking motion after being held toward the outer edge for four seconds?
3. Onset of nystagmus prior at 45 degrees: As the officer moves the object towards the edge of the suspect’s shoulder, does the eye jerk before the object is at a 45-degree angle from the center of the suspect’s face?

Screening for horizontal gaze nystagmus is not the only tool police use to establish probable cause for a DUI arrest. Law enforcement officers typically require more chemical tests to determine your BAC. Chemical tests include using a portable breath alcohol monitor (or breathalyzer) and blood or urine analysis.

You can be charged even if only your BAC is at or above your state’s legal per se limit. All states have a per se legal limit. At this level, the law sees you as legally intoxicated. This limit is 0.08% for all states except Utah, which recognizes a per se limit of 0.05%.

Law enforcement will take a second, more conclusive BAC test at the police station for use at trial. They may transport you to a hospital for blood or urine tests, which will look for intoxicating substances.

Admissibility of HGN Test Results at Trial

Most state courts regard the HGN test as scientifically reliable and valid. Part of a defense attorney’s job is to explore if the police conducted testing according to the Standardized Field Sobriety Test (SFST) guidelines set forth by the NHTSA. Should your DUI lawyer challenge the tests, the judge will determine if this evidence is admissible. The court may rule that HGN evidence is:

• A physical observation by the officer
• Valid on its own
• Requiring expert testimony
• Inadmissible

Evidence of nystagmus does not always signify drug or alcohol impairment. Movement of inner ear fluid, eye strain, neural activity, or brain damage, among other issues, can cause abnormal nystagmus, according to the NHTSA.

How Can a Horizontal Gaze Nystagmus Test Affect Your Case? Get Legal Help Today

If you have questions about a horizontal gaze nystagmus test or need help figuring out your next steps after a DUI arrest, consult a DUI defense attorney. A defense lawyer can save you the guesswork and provide valuable legal advice. Get started today by speaking with a local DUI defense attorney.