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The Horizontal Gaze Nystagmus

The Horizontal Gaze Nystagmus test, and how it is performed.

HGN Information

Horizontal Gaze Nystagmus (more accurately lateral gaze nystagmus) is a medical term describing the involuntary jerking of the eyes. Officers perform this alleged “scientific” field sobriety test on your eyes. With your head still, he instructs you to follow his pen with your eyes as he moves his pen (he will call it a “stimulus” to make the test sound more scientific) back-and-forth across your field of vision. As he moves his pen from side to side, he watches your eyes to see whether they jerk involuntarily; he is trained that central nervous system depressants (like alcohol or antidepressants) cause this involuntary jerking.

He is trained to perform three different tests to detect this involuntary jerking, and assess a “clue” for each time he sees the jerking. The first test is called Lack of Smooth Pursuit. The officer holds the stimulus 12-14 inches from your face and moves it from right to left. He watches to see whether your eyes move smoothly or if they move like a windshield wiper moving across a dry windshield. Lack of smooth pursuit is present when the eyes move in a jagged motion like a dry windshield wiper moving over a dry windshield. The officer assesses two clues if he sees lack of smooth pursuit in each eye.

The second test to detect nystagmus is called Distinct Nystagmus at Maximum Deviation. Here, the officer moves the stimulus to the far end of your peripheral vision. Once your eyes reach the maximum deviation, the officer holds his pen in that position for four seconds (at least). He looks to see whether your eyes remain still or whether they exhibit a pronounced jerking motion. If your eyes exhibit this pronounced jerking motion, then distinct nystagmus at maximum deviation is present. The officer assesses two clues if he sees distinct nystagmus at maximum deviation in each eye.

The third and final test the officer performs to detect nystagmus is called Onset of Nystagmus Prior to Forty-Five Degrees. The officer moves his pen slowly from the center of your face to the side of your face. If your eyes begin to jerk prior the officer reaching a forty-five degree angle from the center, then the officer will say you have onset of nystagmus prior to forty-five degrees.

The HGN a progressive evaluation. As your level of intoxication increases you begin to exhibit lack of smooth pursuit first, then distinct nystagmus at maximum deviation, and finally onset of nystagmus prior to 45 °. Officers testify that your alcohol content exceeds .08% when your eyes exhibit four or more clues on the HGN evaluation. To support their position, they cite studies conducted by NHTSA. However, the NHTSA studies are flawed, and provide no scientific basis for this claim.

Why the HGN test is not a good test.

First the NHTSA studies concluded the HGN test is only 77% accurate; this assumes that the officers conducted the test correctly. We all have nystagmus, however most of the time it is not detectable by the human eye; alcohol and drugs merely exaggerate this natural nystagmus to make it observable. Additionally, many people have an observable natural nystagmus. These factors contribute to false positive results on the HGN test.

Many officers perform this test incorrectly. Since the HGN test is considered a scientific test, the officer must administer the test the same way that it was validated. The National Transportation Safety Administration (NHTSA) validated the HGN test by performing it on subjects in a controlled environment. NHTSA mandates that the officer must perform the test according to their strict guidelines or the results are invalid. I have had a great deal of success convincing judges to suppress HGN results when officer administered the test incorrectly. If the HGN test is excluded the jurors in your case will never know that the officer administered the HGN test on you.

Furthermore, NHTSA's study validating the HGN test is flawed in two ways. First, NHTSA used a flawed scientific method in analyzing their data. Second their data are inconsistent. The scientific method is erroneous because they grouped four clues (lack of smooth pursuit and distinct nystagmus at maximum deviation in both eyes) together with six clues on the HGN (add onset prior to 45) in the validation study. NHTSA wanted to know how many clues indicates a BAC exceeding .08 percent. NHTSA should have studied DUI subjects with four HGN clues to determine whether their BACs exceeded .08 percent, then study DUI subjects with six HGN clues asking the same question. Instead, NHTSA studied DUI subjects with four HGN clues with those having six clues. Six HGN clues indicates .08 or greater while four HGN clues does not, however NHTSA's study did not seek to address this issue. NHTSA's study is also flawed due to inconsistent data. Some NHTSA studies conclude that four HGN clues indicates a BAC exceeding .08 percent while Drug Recognition Officer (DRE's) are trained in advanced DUI courses that four HGN clues means that the DUI subject's BAC is lower than .05%. Under Georgia law the DUI suspect is presumed not to be impaired if his BAC is .05% or lower.

Your lawyer must be well versed in the NHTSA studies in order to either exclude the test from your trial, or show the jurors that the HGN test is meaningless.

Sobriety tests are identical in Gwinnett County, Atlanta, Fulton County, Barrow County, Hall County. Call us anytime at 678 376 5541 for a better understanding of DUI Sobriety test.