The above video shows a person with HGN. This is what the officer is looking for.

Out of the three types of field sobriety tests that the National Highway Traffic Safety Administration (NHTSA) has deemed appropriate for law enforcement officials to use for identifying drunk drivers, the horizontal gaze nystagmus (HGN) test is generally considered the most reliable. However, this doesn’t mean that failing the test will automatically seal your fate.

Understanding Nystagmus

Nystagmus is a natural condition that causes a person’s eye to jerk rapidly from one side to the other, which can be enhanced by alcohol. The root cause of nystagmus is an inability to control the eye’s muscles—a symptom that has a number of possible causes. As a result, many officers rely on the horizontal nystagmus test to determine whether a driver should be arrested and charged with driving under the influence (DUI).

To conduct the horizontal nystagmus test, the officer will place a small object, such as a pen or flashlight, about 12 to 25 inches in front of you and slightly above your eye level. Keeping your head still, you will be instructed to follow the object as it is moved in a horizontal direction. As you perform the test, the officer will be watching you closely to check for signs of alcohol-related nystagmus.

How the HGN Test Is Scored

When an officer administers an HGN test, he or she will be looking for the three most common clues of nystagmus due to alcohol impairment:

• an inability to track the object smoothly
• distinct nystagmus at maximum deviation
• onset of nystagmus prior to a 45 degree angle. If two or more of these conditions are observed, you can expect to be arrested for DUI.

Of course, failing an HGN test does not necessarily mean that you will be found guilty of driving under the influence. A number of conditions other than alcohol impairment can cause nystagmus and, depending on the circumstances of your case, there may be another explanation for your test results.