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HGN: Field Sobriety Testing

Updated: Jul 9, 2023 @ 2:23 pm

Less than 1 minute Reading Time: Minutes

Horizontal gaze nystagmus, or HGN, is a medical term for involuntary jerking of the eye. The human eye normally moves smoothly like a marble on glass, however nystagmus would look like a marble on sand paper. Of course, because it is involuntary, the subject exhibiting the nystagmus is unaware that it is happening and it does not affect vision.

There are several types of nystagmus, such as alcohol gaze nystagmus (AGN), which includes HGN, and positional alcohol nystagmus. Although alcohol can cause both types, they are easily distinguishable. Positional alcohol nystagmus is not included in field sobriety testing.

Alcohol gaze nystagmus is a type of jerking wherein the eye gazing upon or following an object lags and has to correct itself; this correction manifests in a jerking movement of the eye. This occurs due to disruptions within the nervous system.

Not surprisingly, AGN is caused by alcohol. Alcohol is a central nervous system depressant affecting motor control systems of the body. The part of the nervous system that controls hand movements also controls posture and eye movements. If an individual is intoxicated, the nervous system will break down the smooth control of eye movements. It may result in the inability to hold the eyes steady and other observable changes in eye functioning.

HGN History

HGN is only one of the tests conducted in a battery of field sobriety testing that assist in the development of probable cause to arrest for an alcohol-related offense.

Most field sobriety tests (FST) test coordination, balance, and dexterity, which diminish as a person’s blood alcohol content increases. FSTs also test a person’s ability to perform simple tasks simultaneously, and generally employ attention-dividing tasks.

The National Highway Traffic and Safety Administration (NHTSA), in response to impaired driving and alcohol-related injuries and fatalities, developed methods to detect impaired drivers using the FSTs. A study in the 1970s validated the relationship between HGN and alcohol consumption, and the HGN test, along with the walk-and-turn and one-leg stand test, were used to administer a roadside detection of impairment. Once these tests were identified as the most accurate, they were implemented throughout the country. Through standardization of the tests, law enforcement officers nationwide could administer the tests quickly and uniformly.

Administering HGN

The test must be administered in a way that ensures the subject’s eyes can be seen clearly. The subject should face away from blinking or flashing lights. The officer first checks to see if there is any physical manifestation of HGN. Eyeglasses are usually removed, and inquiry is made whether the subject wears contact lenses or has an eye impairment. The HGN test requires only an object, such as a pen or finger, for subjects to follow with their eyes. The object is placed 12 to 15 inches from the subject’s face, slightly higher than eye level. The officer instructs the subject to follow the object with their eyes only, and to not move their head.

There are 3 things the officer looks for while performing the test: after the officer decides that the eyes can actually track he equally he moves on the three tests; lack of smooth pursuit, and distinct and sustained nystagmus at maximum deviation, and nystagmus prior to 45 degrees. The officer looks for these in both eyes and notes them as “clues” if they appear. Thus, there are 6 clues (3 in each eye, equal tracking is not a clue, only a disqualifier for the test) the officer looks for that are indicative of impairment, according to NHTSA.

The Courtroom

Apart from the chemical blood and breath tests, the HGN test is the best evidence that the defendant ingested alcohol. However, the HGN test provides the best evidence only if the judge or jury understands that the test results correlate with impairment.

Although the tests are standardized and law enforcement officers undergo training to perform the tests, a defendant can challenge the reliability and validity of the HGN test. The test may not be reliable, or it may be administered incorrectly, in the wrong setting, not according to the standards, or by an officer with little training. Often the officer is so incompetent in the methodology of the HGN the jury doesn’t give it much weight.

It is important to seek professional legal assistance if you have been charged with a DWI. Our attorneys have handled hundreds of DWI cases and understand the intricacies of each case. We understand the science and techniques used in HGN and other field sobriety tests. Both Tad Nelson & Amber Spurlock are certified in the Standardized Field Sobriety Tests and are also certified as instructors in the Standardized Field Sobriety Tests. Contact the legal team at The Law Offices of Tad Nelson & Associates to help with your case today.

Houston DWI Lawyer Tad A Nelson is Board Certified in Criminal Law by the TBLS.

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