Horizontal Gaze Nystagmus Test (HGN), How It Is Used In DUI Investigations

The National Highway Transportation Safety Administration, NHTSA, has come up with three “standardized field sobriety tests” for law enforcement to use to determine if someone is under the influence. One of the three tests is the horizontal gaze nystagmus, or HGN, test. (The two others are the walk-and-turn test and the one-leg-stand test.)

Nystagmus is the involuntary jerking or bouncing of the eyeball. HGN is the horizontal bouncing when the eye gazes to the side. As a person drinks alcoholic beverages or consumes central nervous system depressants, the brain loses its ability to control the eye muscles, causing the jerking or bouncing. The greater the impairment, the greater this involuntary movement.

Before the test begins, the officer should have the subject remove their glasses and inquire if the person is wearing contacts. They should be faced away from any blinking lights including the officers flashing lights and passing cars.

The subject is then asked to follow an object that is 12-15 inches from their eyes and slightly higher than eye level, without moving their head. The officer should then check for equal tracking and equal pupil size. This is done by having the subject quickly follow an object through their field of vision. Lack of equal tracking or pupil size can indicate an injury or a medical issue and the test should be discontinued.

The test is then administered and three clues for each eye is checked, for a total of six possible clues. If four or more clues are found, the officer can determine that the subject’s BAC is at or above a .10.

The first is lack of smooth pursuit. The officer starts at the middle of the subject’s face and moves the object towards their left ear. The test is then done on the right eye. The eye being tested should follow the object smoothly. If nystagmus is observed in either eye, the officer notes that as a clue.

Then the officer checks for distinct nystagmus at maximum deviation. Again, the officer starts at the center of the subject’s face, moving the object toward the left ear. However, this time, the object is brought as far as the eye can go, holding it there for at least 4 seconds. Then the test is repeated on the right side. If nystagmus is observed in either eye, the officer notes that as a clue.

The last is angle of onset of nystagmus prior to 45 degrees. For this test, the officer again starts at the center of the subject’s face, moving the object towards the left shoulder at a speed that would take approximately four seconds. It is estimated that the edge of the shoulder is 45 degrees from the center of the subject’s face. The officer notes a clue if nystagmus is seen prior to reaching 45 degrees. This is repeated on the right eye.

There are many problems with the HGN test and how it is used by law enforcement as part of their investigation to determine if someone is under the influence when conducting a DUI investigation. Those issues will be addressed in another future driving under the influence blog article.

The above blog article is by no means all-inclusive and is not legal advice. Laws may change and may not apply to your case. For the latest information or to get legal advice, speak to a DUI attorney in your area.

If you’re charged with driving under the influence of alcohol and/or drugs in San Diego, call the Law Offices of Susan L. Hartman for help. We offer a free, confidential phone consultation, so you can learn about your rights and defenses with no obligation. Call us at 619-260-1122 or use the “Contact Us” form on this page.

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