Horizontal Gaze Nystagmus (HGN) is an involuntary side-to-side jerking of the eyes. The HGN test is a commonly used roadside sobriety check by law enforcement officers to determine if a driver is impaired by alcohol or drugs. This test is founded on the idea that alcohol and some drugs interfere with the brain’s capacity to control eye movement, resulting in more pronounced and more easily noticeable nystagmus.

In this article, you’ll learn what HGN is, how the test is conducted, and what function it has in DUI investigations and legal proceedings.

Key Takeaways

• HGN is the jerking of the eyes that occurs involuntarily and is made more pronounced by the use of alcohol or drugs.
• The HGN test is a standard field sobriety test law enforcement performs to determine impairment.
• At higher levels of intoxication, the brain is less able to control eye movement, as alcohol impairs this aspect of brain function.
• The accuracy of the test can be affected by officer error, environmental factors, and medical conditions.

What Is Horizontal Gaze Nystagmus (HGN)?

Horizontal Gaze Nystagmus is a natural and involuntary movement of the eyes when you follow a moving object from side to side. Nystagmus is usually so subtle under normal conditions that it is barely noticeable. However, alcohol and some drugs can make this condition worse so that the eye movements are more erratic and more visible.

The HGN test is premised on the fact that being under the influence of drugs and alcohol impairs the brain’s control over muscle function, including control of the small muscles that control eye movement. Law enforcement officers use the fact that a person's eyes jerk more when they are tracking an object as an indication of impairment when they are intoxicated.

How Is the HGN Test Conducted?

The HGN test is one of the standard field sobriety tests law enforcement officers administer to see if a driver is under the influence. The test typically involves the following steps:

1. Positioning: An officer tells the driver to stop, stand still, face forward, and follow a stimulus (e.g., the pen or flashlight) with their eyes.
2. Tracking movement: The eye movements are observed as the officer slowly moves the object horizontally across the driver's field of vision.
3. Observing indicators: The officer looks for three key signs of impairment in each eye:
   • Lack of smooth pursuit: The eyes do not move smoothly while following the object.
   • Distinct nystagmus at maximum deviation: Looking as far to the side as possible causes the eye to jerk significantly.
   • Onset of nystagmus before 45 degrees: Before the eye hits a 45-degree angle from the center, jerking happens.

Each indicator is assessed for both eyes, resulting in a total of six possible clues of impairment. If an officer sees four or more clues, they may decide that there is probable cause for a DUI arrest.

Accuracy and Limitations of the HGN Test

The HGN test is one of the most reliable field sobriety tests but is not foolproof. Various factors can affect its accuracy, including:

• Medical conditions: Inner ear disorders, neurological conditions, or eye problems can cause nystagmus unrelated to alcohol or drug use.
• Environmental factors: Poor lighting, flashing lights, or moving traffic can disturb the officer's observations.
• Improper administration:  The test is very accurate if you strictly follow the standardized procedures. If the officer fails to follow the correct protocol, the results may be unreliable.

Because of these potential limitations, HGN test results can often be challenged in court by experienced DUI defense attorneys.

Bottom Line

The horizontal Gaze Nystagmus (HGN) test is a DUI investigation tool that law enforcement officers use to detect impairment signs through involuntary eye movement. Although the test is considered reliable, it is not without limits and can be challenged for procedural errors or medical conditions.

If you were charged with a DUI after an HGN test, you need to consult with a lawyer. An experienced DUI attorney can help evaluate the circumstances of your arrest and determine the best course of action.