What Is Horizontal Gaze Nystagmus (HGN)?

Even if you haven't been stopped by the police for alleged alcohol or drug impairment while driving, you're likely familiar with the ubiquitous field sobriety test used in all states. One of the three main components of these tests involves horizontal gaze nystagmus (HGN), which is the involuntary jerking of one's eye when it gazes to the side. Since this jerking becomes exaggerated by alcohol consumption, it is used as evidence of impairment in DUI cases. The following provides an overview of the science and law behind the HGN portion of the field sobriety tests.

Horizontal Gaze Nystagmus and Impairment: The Science

The term "nystagmus" generally refers to any bouncing or jerking of the eye. Police officers are concerned only with horizontal gaze nystagmus, a completely involuntary motion that becomes more pronounced when an individual is impaired by alcohol (in addition to some illicit and prescription drugs, primarily depressants). Since alcohol depresses the nervous system, it has a noticeable effect on the ability to control sideways eye movements in a smooth and accurate manner. In fact, those who exhibit these exaggerated eye jerks aren't even aware of it, since HGN has no noticeable effect on one's vision.

Taken as a whole, the three-part field sobriety test has been shown to accurately determine blood-alcohol concentration (BAC) of 0.10 percent or higher 83 percent 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 an accurate indicator of alcohol impairment, and in fact the most accurate portion of the three-part test.

How Police Conduct the HGN Test

Officers conduct the HGN test either in a well-lit area or with the use of a flashlight, but with the suspect facing away from the police cruiser's flashing lights (and the lights from passing cars), which could skew results. The officer then tells the suspect he or she is going to check their eyes and asks whether they wear contact lenses or have any medical issues that would affect the results. The officer then asks the suspect to follow an object (usually the tip of a pen or pen light), placed 12 to 15 inches away, with only their 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 to 45 Degrees- As the officer moves the object towards the edge of the suspect's shoulder, does the eye jerk before the object is 45 degrees from the center of the suspect's face?

It's important to stress that the screening for horizontal gaze nystagmus is not the only tool police employ to establish probable cause for a DUI arrest. Officers typically require suspected impaired drivers to take the three-part sobriety test and then test the individual's BAC using a portable breath alcohol monitor (or "Breathalyzer") before making an arrest. A second, more-conclusive BAC test is taken with a stationary breath alcohol monitor or blood draw at the police station for use at trial.

Admissibility of HGN Test Results at Trial

Most states' courts regard the HGN test as scientific and thus reliable. Still, DUI defense attorneys often attempt to have the evidence thrown out if it can be shown that the test was not conducted according to the Standardized Field Sobriety Test (SFST) guidelines set forth by the NHTSA. It's up to the judge to determine whether HGN test was properly conducted and thus admissible. Generally, the court may rule that HGN evidence is either a physical observation by the officer (in which case the officer would testify to his or her training); a scientific test, either valid on its own or requiring expert testimony; or else inadmissible.

Evidence of nystagmus does not always signify alcohol impairment and can also be caused by the movement of inner ear fluid, eye strain, neural activity, brain damage, or just as a natural occurrence (all of which are quite rare), according to the NHTSA. Defendants with any such conditions may be able to have this evidence thrown out pre-trial.

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

Whether you have questions about the way in which a horizontal gaze nystagmus test was conducted or just need help figuring out your next steps after being arrested, a legal professional can save you the guesswork and improve your outcome. Get started today by speaking with a local DUI attorney.