Horizontal Gaze Nystagmus (HGN) is a condition where the eyes make repetitive and involuntary jerking movements while gazing to the side. HGN is involuntary, and the person experiencing the condition is unaware it is happening. Typically the higher a person’s blood alcohol concentration, the sooner their eyes begin jerking as they move to the side.
The Horizontal Gaze Nystagmus test is one of three National Highway Traffic Safety Administration (NHTSA) approved Standard Field Sobriety Tests. Law enforcement uses the HGN test to determine if a driver is driving while under the influence of alcohol.
The other two Standard Field Sobriety Tests approved by the NHTSA are the Walk and Turn and the One-Leg Stand. Of the three tests, the HGN test is the most reliable and scientific.
How Law Enforcement Administers the HGN Test
Before administering the HGN test, the officer should minimize all visual distractions and have the driver face away from any emergency lights or oncoming lights. The officer should also ask the driver to remove any eyeglasses. From there, officers follow a similar script while performing the HGN test.
Usually, an officer says the following or its equivalent:
- I’m going to check your eyes;
- Keep your head still and follow this stimulus with your eyes only; and
- Keep following this with your eyes until I tell you to stop.
The officer then places the stimulus, often a pen, about twelve to fifteen inches from the driver’s nose, slightly above eye level. While watching the driver’s eyes, the officer moves the stimulus slowly across the driver’s field of vision.
Identification of Intoxicated Drivers by the HGN Test
The officer is watching the driver’s eyes for three clues of intoxication, including:
- Lack of smooth pursuit. Whether the eye follows the object smoothly as it moves from the center of the face towards the ear or if it jerks;
- Nystagmus at maximum deviation. If the eye has a distinct jerking motion after holding towards the outer edge for four seconds; and
- Onset of nystagmus prior to forty-five degrees. If the eye jerks before the object is forty-five degrees from the center of the face.
The officer counts one point per clue per eye. There is a maximum score of six points. The officer will likely arrest any driver who scores four or more.
Factors That Can Affect HGN Test Results
Studies show that law enforcement officers can correctly identify intoxicated drivers 77% of the time using the HGN test. However, this means that one in four drivers who fail the HGN test do not fail due to intoxication. Numerous factors can affect the reliability of the HGN test, such as:
- Certain drugs and medications;
- Brain damage;
- Contact lenses;
- Improper positioning of the stimulus;
- Flashing lights on or around the stimulus;
- Unsteady movement of the stimulus;
- Failure to follow NHTSA guidelines for Standard Field Sobriety Tests; and
- Failure of the officer to evaluate the eyes for resting nystagmus before the test.
If you or someone you know failed the HGN test or any other field sobriety test in Arizona, do not despair. An experienced DUI attorney in Scottsdale can help. Call Rosenstein Law Group to discuss the specifics of your case with one of our DUI defense attorneys today.
At Rosenstein Law Group, we know DUIs happen at all hours. That is why we take calls twenty-four hours a day, seven days a week. We are DUI professionals with the credentials and case success to prove it. Call us today to get started on your case.