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HGN: A scientifically reliable indicator?

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Horizontal gaze nystagmus (HGN) is a condition where the eyes involuntarily jerk or twitch as they move laterally, typically observed when an individual’s gaze is directed to the side. During a DUI stop, an officer may conduct the HGN test by moving an object (such as a pen or finger) horizontally across the person’s field of vision while observing the eyes for any signs of nystagmus.

While HGN can indicate impairment, its reliability as a sole indicator is questioned for several reasons:

  1. **Subjectivity**: Interpretation of nystagmus can vary between officers, leading to inconsistent results. Factors such as lighting conditions, the officer’s experience, and the subject’s eye characteristics can influence the observations.
  2. **False Positives**: Nystagmus can be influenced by various factors other than alcohol or drug consumption, such as fatigue, certain medical conditions, prescription medications, or even environmental stimuli like flashing lights.
  3. **Lack of Specificity**: HGN is not specific to alcohol impairment and cannot distinguish between alcohol and drug impairment. Different substances affect the body differently, and HGN alone cannot pinpoint the specific substance causing impairment.

Therefore, while HGN can be a useful tool in conjunction with other field sobriety tests and evidence, its limitations make it unreliable as a standalone indicator of impairment in DUI cases.

HGN and Head Trauma

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If you have been pulled over in Georgia on suspicion of DUI, the investigating officer will probably ask you to perform a battery of standardized field sobriety tests. This request may take the form of any number of questions, such as “can we just check to see if you are safe to drive?” or “we want to perform some tests before we let you on your way, is that alright?”. These tests are not required, and declining to perform these tests cannot be used against you in a prosecution of DUI. For this reason, it is better to decline to perform any tests, no matter how much reassurance the police officer gives you that they are “just to make sure you are safe on the roads.”

Still, many people opt to perform the tests, either because they don’t see the harm, they wish to be congenial with the officer, or because they don’t know that they can decline to perform the tests. If you choose to perform the tests, the officer may ask you if he can “take a look at your eyes.” This is an indication that he is about to perform the first of three standardized field sobriety tests, the horizontal gaze nystagmus.

This test is considered to be a “scientific” test, and because of this, it is important that the officer comply with his training as exactly as possible. The test must begin with a number of questions designed to medically qualify the participant. The officer is trained that he must ask you whether or not you have recently had any head, neck, or brain injuries, as these kinds of trauma can affect whether someone exhibits nystagmus, even if not under the influence of alcohol. It is common practice to ask whether or not the subject has “any eye problems” or vision issues, but this is not enough. The officer must also determine that it is appropriate to use this test. If the subject has been in a recent accident, suffering from whiplash, a concussion, vertigo, or some other balance and coordination related condition, the HGN test may not be accurate or reliable.

If you have been in an accident at the time of your DUI investigation, the officer may have overlooked potential head trauma before administering this test. As a result, the “clues” of the test may be unreliable, and could be subject to suppression before trial.

It is important to understand your rights and protections when you are charged with DUI. If you want an attorney that is knowledgeable about DUI police training and procedure, call our office for a free consultation at 404-581-0999. Written by Attorney Katherine A. Edmonds.