Understanding the Horizontal Gaze Nystagmus Test

The officer begins by asking you to stand with your feet together and your arms at your sides. They will then tell you to look at a pen or object about 12-15 inches in front of you, instructing you to look forward and keep your head still. Per their training, the officer should ask if you can see the object comfortably and document your answer. The stimulus should be held at eye level, and you must look into a quiet background—in other words, away from oncoming motorists. This is to prevent the eye from trying to focus on numerous sharply contrasting moving objects at once or objects that are gradually moving further away from you, as either of these can lead to opto-kinectic nystagmus. 

The officer then moves the object from side to side, and you are instructed to keep your eyes on it while the officer examines any indications of impairment. The object must make a total of 14 passes in front of you, and the test must take at least 82 seconds to perform. The test can be considered improperly administered if these conditions are not satisfied.

If you wear glasses, they must be removed before taking the test unless they are required to see the object. The test cannot be administered to people wearing hard contacts, as they may become dislodged during the test and damage the eye. You may lack vision in one eye. If this is the case, you cannot be given the HGN test. Some officers have tried to get away with testing only one eye and doubling the results to simulate the results of testing both eyes; however, this is not allowed and produces results that should not be considered legitimate. 

If you have a "lazy eye condition," the officer will instruct you to cover one eye and then perform the test, which is repeated for the other eye.

Some people exhibit pathological nystagmus, which should not be interpreted as a failure to pass the HGN test.

Many conditions can lead to pathological nystagmuses, such as: 

• Colorblindness
• Concussion
• Epilepsy
• Some neurological disorders

Some clues can tip off an officer as to whether or not you exhibit pathological nystagmus, but they are not always considered when performing the test.

If there is any doubt about whether you passed the test, the benefit of the doubt is in your favor, and you are considered to have passed the test. 

What Are Officers Looking for When They Administer the HGN Test? 

The test examines each eye for the presence of three signs of intoxication. Officers look for these clues in the order listed below, as people exhibit the following indications in order as they become more intoxicated.

In an HGN test, officers are looking for the following clues: 

• Lack of Smooth Pursuit: Here, officers are trying to determine if you can follow an object's movement without any unnecessary jerking of the eye. For example, your eye should move like a ball over a flat and even plane—smooth without any disruption. However, if you are under the influence, your eye will move the way a ball rolls across a rough surface, like a sidewalk—movements will be jerky. To properly administer the test, the officer must not make any sudden or shaky hand movements and spend two seconds on each pass. They must make two passes to be sure that any suspected nystagmus is present for the clue to be valid.
• Distinct Jerkiness at Maximum Deviation: When testing for this clue, an officer will have you follow the object to the side until you are looking as far to the side as possible. For the test results to be considered valid, there should be no visible sclera in the corner of the eye. You will hold your eye in that position for two to three seconds while the officer examines your eye for distinct jerkiness. If the jerkiness is not distinctive and unmistakable, the benefit of the doubt goes to you. If the officer moves the object too quickly, they may be administering and scoring your test incorrectly.
• Onset of Nystagmus Prior to Forty-Five Degrees: The stimulus is placed halfway between the suspect's ear and nose. The object is then moved to the left or right over a period of four seconds. If the eye begins to jerk before forty-five degrees of angle, the officer will record this as a positive clue. When assessing for nystagmus, officers must be diligent in their observations. If they spot any signs of jerking, they must halt the stimulus and carefully evaluate the situation. This means checking for the presence of white at the corneal margins and estimating the angle of deviation. If no white can be seen, it may be that the eye has reached its maximum deviation or that the person has unique eyes that don't deviate as far. Ultimately, the criterion of onset before 45 degrees can only be used in cases where white is visible on the exterior of the eye. 

Several other factors, such as fatigue, medical conditions, and certain medications, can affect the eyes. For example, people with certain medical conditions, such as Parkinson's, may have nystagmus even when not under the influence of alcohol or drugs.