Even if you never been stopped before on suspicion of driving under the influence you have undoubtedly seen a drunk driving stop on television at some point in your life. If so, you have also likely seen a law enforcement officer administer what are referred to as “field sobriety tests”, or FSTs. One of the most common FSTs used by law enforcement officers in Omaha involves asking the suspect to follow a pen light with the suspect’s eyes. Whether it is because you have been asked to perform this test or just out of curiosity you may wonder what the officer is looking for when he asks a suspect to follow the pen light.
Omaha Field Sobriety Tests: Horizontal Gaze Nystagmus
The formal name for the FST involving the pen light is the Horizontal Gaze Nystagmus test, or HGN. The HGN test is one of three Omaha field sobriety tests that are among the “standardized” tests approved by the National Highway Transportation Safety Administration, or NHTSA. The word “nystagmus” refers to an involuntary bouncing, twitching, or jerking of the eye that occurs when there is a disturbance of the inner ear. Horizontal gaze nystagmus specifically refers to a jerking of the eye when the eye moves to the side. When an individual has been drinking alcohol, or has ingested certain other central nervous system depressants, the ability to control eye movement is affected. In short, the more intoxicated a person is, the more the person’s eye will twitch or jerk when it moves to the side. It is for this reason that the HGN test is used as a FST. When the officer asks you to follow the pen light the officer is looking for twitching or jerking of your eye as it follows the pen.
The accuracy of the HGN test, although approved by the NHTSA, is questionable. Scientific evidence seems to support the idea that there is a correlation between jerking of the eye and intoxication under ideal circumstances when the test is administered and interpreted correctly. The problem, however, is that there are a number of factors that can affect how an individual performs on the HGN test that have nothing to do with intoxication. More importantly, the results of the HGN test are subjective, meaning that the officer administering the test determines whether or not you passed the test.
These doubts about the efficacy of the HGN test are why the results are often not admitted at trial. Instead, the results are frequently used only to support probable cause for an arrest.