An arrest for driving while impaired usually begins with the DWI stop. When an officer suspects that a driver is impaired due to alcohol, the officer typically conducts field sobriety tests to determine if he has probable cause for a DWI arrest. The officer uses a variety of tests to look for signs that indicate a driver is impaired. Three standardized field sobriety tests used by officers at a DWI stop are believed to be reliable in detecting alcohol impairment according to the National Highway Safety Administration (NHTSA). One of the standardized tests used at a DWI stop is the Horizontal Nystagmus Test (HGN).
Using a Finger for the Horizontal Gaze Nystagmus Test at a DWI Stop
The HGN test measures the steadiness of your eyes while you follow an object with your eyes. Officers often use their finger or perhaps a pen for this test because they can easily see your eyes while providing an object for you to follow. To understand how the HGN test works, it is important to understand what is meant by “nystagmus.” Nystagmus is the medical term for involuntary jerking of the eye. While nystagmus can occur naturally in a small percentage of the population, it can also be caused by the consumption of alcohol or certain other impairing substances. The HGN test requires a suspect to follow a stimulus (such as a finger) with their eyes, while holding their head steady, to allow the officer to look for involuntary jerking of the eye as it tracks from side to side. Note that the presence of nystagmus does not have any impact on a person’s vision, so it is virtually impossible for somebody to know for themselves if they have nystagmus.
The HGN test, like all standardized field sobriety tests, is divided up into a set of “indicators” or “cues”. This particular test has a total of six possible indicators (three for each eye). The first is the onset of nystagmus prior to 45 degrees. This means that the eye begins to jerk before the eye has tracked the stimulus for more than 45 degrees to the side. The second indicator is “lack of smooth pursuit”, which is another way of saying that the eye did in fact jerk or display nystagmus. The last is “distinct and sustained nystagmus at maximum deviation,” meaning that when the eye got to the edge of its range of movement the jerking was clearly visible and continuous. The police may also check for vertical nystagmus (jerking when the eye moves up and down as opposed to left and right), but that tends to be fairly unusual.
Problems with the HGN Test at DWI Stops and Its Use in Court
There are several problems with the use of the HGN test to detect and prosecute impaired drivers. First, to be valid and accurate the test must be performed fairly precisely. There is a real risk of false positives being recorded when the test is given incorrectly. This can be due to a failure of the officer to follow the instructions given by NHTSA for administering the test. Many officers also have not received formal training in the use of this test but will attempt to administer it anyway along with the other standardized tests.
Second, nystagmus can be attributed to a number of causes besides the consumption of alcohol or other impairing substances. A small but significant portion of the population has naturally occurring nystagmus in their eyes. Also, it can be caused by medical conditions such as a recent concussion or brain injuries in the past. Nystagmus can also be caused by environmental conditions such as bright lights flashing in the eyes such as police emergency lights or passing traffic.
Third, testimony about the presence of nystagmus and its possible cause is ultimately only an opinion. There are limitations on the use of expert opinion testimony, and courts vary widely in the amount of weight and credibility given to this sort of evidence. It is common for the prosecution to offer the opinion testimony of an officer as if he were an expert witness, so a good defense attorney must be prepared to challenge the credentials of any witness who wants to give opinion testimony in court.
Fighting a DWI Arrest
There are several ways to defend a DWI arrest; however, it is important that you contact our office as soon as possible so that we can begin to build a strong defense. Our lawyers will thoroughly investigate your case to determine the defense strategy that will give you the best chance at avoiding a DWI conviction.
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