An In-Depth Look at Field Sobriety Tests, Issue 1: Horizontal Gaze Nystagmus

The Horizontal Gaze Nystagmus, or HGN, is one of three tests administered in the Standardized Field Sobriety Test (SFST), which is administered in the event that a driver is pulled over for suspected drunk driving. The SFST is used to establish probable cause that a driver is under the influence of alcohol and should be arrested for DUI (driving under the influence). SFST results may also be used against a driver in criminal court to prove his or her abilities were impaired by alcohol – which may enable the prosecution to secure a conviction even if a breath test yielded a result below .08%.

Understanding each of the three tests administered as part of the SFST is important. Field sobriety tests have faults and are not 100% accurate, particularly if they are administered or graded improperly. Although "failed" field sobriety tests may seem difficult to disprove, there are ways to challenge results.

Here we will take a closer look at the HGN test, which is generally considered the most accurate of the three tests included in the SFST. Merriam-Webster defines nystagmus as:

Involuntary usually rapid movement of the eyeballs (as from side to side) occurring normally with dizziness during and after bodily rotation or abnormally following head injury or as a symptom of a disease.

Nystagmus has also been associated with alcohol consumption. According to the National Highway Traffic Safety Administration (NHTSA), which operates under the U.S. Department of Transportation (DOT), nystagmus occurs naturally when the eyes are rotated at extreme angles, but "when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles." This theory provides the foundation for the HGN test.

How the HGN Test Is Administered

During the HGN test, the officer administering the test has the subject follow a slowly moving object, such as a pen or a small flashlight, with his or her eyes. The officer holds the object about 12 to 15 inches from the face and slightly higher than eye level and then moves it slowly and steadily from side to side while observing the subject's eyes. The officer looks for the following signs: the subject's eye cannot follow the object smoothly, there is a distinct jerking movement in the eye at maximum deviation, and the jerking of the eye begins within 45 degrees of the center. If four or more signs are noted between the two eyes, it is considered likely that the subject has a blood alcohol concentration(BAC) of .08% or greater.

According to the most recent NHTSA-sponsored study on the tests included in the SFST, the HGN test was found to be accurate in approximately 88% of subjects included in the study – when properly conducted and administered (Stuster and Burns, 1998).

False Positives, Errors in Test Administration and Other Possible Defenses

It is essential to look at possible errors and other factors that could influence a driver's performance on the HGN test. The HGN test must be properly administered if it is to yield the most accurate result, and this is actually the most complex field sobriety test to administer. If the officer holds the object too far away from the subject's face, inaccurately records the degree at which the subject's eye began to jerk, starts on the wrong side or does not hold the object to the side for a long enough period of time (four seconds is the requirement, which would give time for normal nystagmus to cease), the test results may be skewed.

The HGN test must also be administered in ideal conditions to be as accurate as possible. The subject should not be facing bright lights or moving objects, such as traffic or a police cruiser with its lights on. The subject's eyes should be well-lit to allow the officer to accurately track any jerking movements and to note when they occur. Problems with testing conditions could affect HGN results.

Nystagmus can also be caused by conditions or medications that have nothing to do with alcohol consumption. Inner ear problems, brain damage, brain tumors and seizure medications may cause nystagmus in people who are not impaired by alcohol. It is important to consider any medical conditions or eye problems that could affect the results of the HGN test.

Interested in learning more about field sobriety tests and the ways an Orange County DUI attorney can challenge test results to help you avoid a conviction or even formal charges in the first place? Contact the Law Offices of Virginia L. Landry today. We represent drivers arrested for and charged with DUI in and around Orange County, California.

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