Understanding HGN

The Standard Field Sobriety Tests (SFSTs) play an important in most St. Charles DWI investigations. There are three SFSTs: Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn, and the One-Leg stand. Each of these tests was developed through research sponsored by the National Highway Traffic Safety Administration (NHTSA). The research began back in the 1970s, and by 1981, the tests were being adopted by law enforcement agencies across the country. NHTSA reports the HGN test as being the most accurate predictor of intoxication.

NHTSA analyzed the original SCRI research laboratory test data and found: HGN, by itself, was 77% accurate.

In 1975, NHTSA commissioned the Southern California Research Institute (SCRI) to determine the most accurate field sobriety tests. The research found that the most reliable tests were: the HGN, the Walk-and-Turn, and the One-Leg Stand; it indicated the HGN test was 77% accurate in determining BAC over .10%.

Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. Horizontal Gaze Nystagmus (HGN) refers to a lateral or horizontal jerking when the eye gazes to the side. In the impaired driving context, alcohol consumption — or consumption of certain other central nervous system depressants, inhalants, or phencyclidine — hinders the ability of the brain to correctly control eye muscles, therefore causing the jerk or bounce associated with HGN. As the degree of impairment becomes greater, the jerking or bouncing, a.k.a. the nystagmus, becomes more pronounced, determining the result of the test
Horizontal Gaze Nystagmus: The Science and the Law, APRI

The HGN test is administered using a stimulus placed 12-15 inches from the driver’s face, which is then moved across the entire field of the driver’s vision. The stimulus is passed once across each eye to make sure the driver’s eyes track equally. After this initial pass, the officer will start with the driver’s left eye and move the stimulus as far to the side as the eye can go; the driver is to follow the stimulus with her eyes while keeping her head still. The officer performs the same procedure for the right eye, and then repeats it once more for both. During these passes, the officer is checking that the driver’s eyes smoothly pursue the stimulus.

After checking for smooth pursuit, the officer will then check for distinct and sustained nystagmus at maximum deviation. The officer will start with the left eye and will move the stimulus to the driver’s left until her eye has gone as far as possible. The stimulus is held out for a minimum of four seconds, and then it is moved all the way to the right and held for a minimum of four seconds. The officer repeats this procedure for both eyes.

The last phase of the HGN test analyzes the onset of nystagmus prior to 45 degrees. In this phase, the officer will start with the driver’s left eye and move the stimulus to the left at a speed that would take about four seconds to reach the edge of the driver’s shoulder. The stimulus is stopped and held as soon as the officer observes nystagmus. The officer repeats the procedure for the right eye and tests each eye twice.

If the officer observes nystagmus as part of the test, he will mark the Alcohol Influence Report (AIR). NHTSA refers to eachobservation of nystagmus during this test as a “clue.” Three clues per eye, a total of six, are possible: 1) no smooth pursuit; 2) distinct nystagmus at maximum deviation; 3) onset of nystagmus before 45 degrees with some white showing. Four or more clues indicate the driver’s BAC is likely at or above 0.08 (the legal limit in Missouri).

The results of the HGN test can be used against you if the state shows: 1) the officer was adequately trained to administer the test and render an opinion; and (2) the test was properly administered. State v. Burks, 373 S.W.3d 1,6 (Mo. App. S.D. 2012)( SD31023), Results of the HGN test are admissible as circumstantial evidence of intoxication only if the state lays a proper foundation. In St. Charles DWI cases, HGN test results are inadmissible to show a driver’s BAC was at a particular level. HGN tests are inadmissible to show that a driver’s BAC exceeded a particular level. State v. Hill, 865 S.W.2d 702, 704 (Mo. App. 1993). It is important for drivers to remember that an HGN test that was improperly administered is inadmissible to show intoxication.

The Ryan Law Firm

415 N. Second St.
St. Charles, Missouri 63301

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