HGN Test: Horizontal Gaze Nystagmus

The HGN test is one of the main components of the standard field sobriety test that is used by police nationwide. The HGN test looks at the eye’s involuntary jerking when looking to the side. When someone is intoxicated, this jerking becomes more pronounced and therefore is a good measure of if someone has consumed alcohol or not.

In this article, we will examine the HGN test and the science behind its use in sobriety tests.

The Science Behind the HGN Test

Horizontal gaze nystagmus test or the HGN test looks at the involuntary movement of the eye. When police officers are conducting field sobriety tests, they examine the horizontal movement of the eye that occurs when the eye is gazing to the side. Alcohol and other depressants will affect the body’s ability to control sideways eye movements, and therefore gazing to the side will look more jerky than when sober. Most people do not even notice that their eyes do this as it does not affect the vision at all.

The HGN test is believed to be the most accurate test in the three-part field sobriety test. NHTSA has funded multiple studies on the HGN test and the field sobriety test since 1977. The current three-part sobriety field test will identify BAC of 0.10 or above with 83% accuracy. Currently, the three-part field sobriety test includes:

The HGN test
One leg stand
Walk and turn

How Is the HGN Test Conducted By Police?

The HGN test must be conducted in a well-lit location if possible. At night officers will often use a flashlight so they can see the eye during the HGN test. If the driver is facing towards passing cars or the flashing lights of the police vehicle, then the results may be skewed.

Before starting the HGN test, the police officer will ask the driver if they have medical issues affecting their eyes or wear contact lenses. They will then ask the driver to follow an object with their eyes. This is usually a small light or a pen. The object must be 12 to 15 inches away from the driver’s eyes.

When doing the HGN test, the police are looking for 3 signs which indicate alcohol consumption:

The eye does not move smoothly when it moves to the side. It will often jerk or bounce.
When the eye is held at the outer edge for four second or longer, it will often jerk or move.
As the eye begins to move the object, does the eye start to jerk before it is rotated 45 degrees from center?

The HGN test is part of a three-part sobriety test as it cannot be used as the sole indicator of alcohol consumption. For police to have probable cause to arrest a driver for DUI, they cannot rely on the HGN test alone.

If a driver fails the three-part field sobriety test, they will often perform a breathalyzer test in the field to get a reading of the driver’s blood-alcohol level before they perform an arrest. Another breathalyzer test will be performed at the station on a more accurate machine, or the police may even draw blood for a test.

Are the HGN Test Results Admissible In Trial?

The HGN test is backed by science, and therefore, most states view it as an accurate measure of sobriety. However, if the police did not perform the HGN test correctly, the defendant’s lawyer will often have the results thrown out. The HGN test must be performed as per the Standardized Field Sobriety Test guidelines to be admissible.

If there is doubt over how the test was performed, the judge will need to decide if they want to admit the HGN test results. The judge may ask the arresting officer about their HGN test training or rely on expert testimony to make this decision.

HGN test results may be skewed by a number of factors such as:

Eye strain
Brain damage
Neural activity
Inner ear fluid movement
A natural occurrence

Therefore the HGN test alone is not enough to determine the blood alcohol levels of a driver. If a defendant has a condition that may affect the HGN test, then they may have the HGN test evidence thrown out before trial.

Can The HGN Test Affect My Case?

Seek legal counsel if you want to know if the HGN test impacted your arrest. Our DUI attorneys can look at your case and help you to determine your next steps after arrest. With legal help, you can improve the outcome of your case and therefore reduce any penalties you may receive. If you were wrongfully arrested, we can help you to prepare a defense and throw out key evidence against you, such as the HGN test.