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Unlike the Walk & Turn and the One-Leg Stand, the Horizontal Gaze Nystagmus (HGN) test is not a divided attention assessment. Instead, it’s sole purpose is to measure the tracking of your eyes and their ability to follow (track) an object smoothly at different angles. The HGN test is not admissible in every jurisdiction. Some states require the officer have extra training in order for the test results to be used against you. In other states, the HGN test is not admissible at all. Be sure to check your local laws for more information. Please note that even if the HGN test is not admissible against you in court, that does not necessarily mean it was improper to give it to you on the scene of your arrest.
Here’s an interview describing the test:
Here is a training video used in police officer courses.
What does HGN show? People who are under the influence of alcohol or drugs (as well as people with neurological disorders, brain tumors, head injuries, and those taking certain medications) are said to be unable to smoothly follow an object/stimulus. Such people will display an involuntary jerking movement in their eye. The HGN tests for this jerking in an attempt to prove impairment by showing that such involuntary movement exists in a person suspected of DUI.
What is Nystagmus? The term “Nystagmus” refers to that involuntary jerking I was talking about above. When a person displays nystagmus, their eye will twitch with small, rapid movements.
What’s a stimulus? A stimulus is what the police use to measure your eye movements. A stimulus can be a finger, a pen, or a stick with a light on the top of it. The officer will ask you to follow the stimulus with your eyes as he moves it back and forth at various speeds in a horizontal manner in front of your face.
Proper Administration of the HGN. When an officer administers the HGN test to a DUI suspect in the field, he will have you follow his stimulus using your eyes only, without moving your head, as he moves slowly back and forth horizontally across your face. To produce the most accurate and reliable results, the officer must comply with all the requirements of administration – he must
State and County variations. The HGN and its admissibility against you in court is something that varies drastically from state to state and county to county. In some jurisdictions, your HGN results will not be used against you unless the officer that administered it to you is specially trained as a drug recognition expert (DRE). To become a DRE the officer must take extra courses and training programs specifically for the purpose of administering the HGN. The problem here is that even DRE’s are not doctors, and they are taught how to give the test by other unqualified officers. Additionally, if your officer is a DRE, it is important you make the jury at your trial understand that it does not mean the officer is an actual “expert” – most often he is not any more “qualified” to administer the test because he only got an additional day or so of training then everyone else.
HGN Logs. Officer’s around the country are supposed to maintain special logs keeping track of how many times they have administered the HGN in the field. Most jurisdictions will allow officer’s to testify anyway even if they failed to maintain such a log (as most fail to do so). It is important for impeachment purposes and for defending your case that your attorney ask the officer about how many times he has previously administered the HGN in the field, and how many of those people were ultimately arrested for DUI.
What else can cause Nystagmus? Many other things can cause involuntary jerking that are totally unrelated to impairment. It is important to consult with your attorney and let him or her know if you are taking any medications, have had any head injuries, or suffer from any kind of neurological disorders. You probably wont know ahead of time to tell the officer, and most likely he will fail to ask you.
Important issues to raise. Overall, the HGN is a very scientific and complex exercise that is difficult to grasp let alone defend. It is therefore important for you as well as your jury to understand the following:
Police are not doctors. Even the drug recognition “experts” or DUI task force police officers are not the best at administering and interpreting this test. They did not go to medical school.
Other reasons may explain your failure. Be sure to go over them with your attorney and make sure you point them out to the prosecutor, judge and jury when defending your case.
If you ask the wrong question, you will get the wrong answer. Police often make mistakes administering the test. They may not hold the stimulus the correct number of inches away from your face. They may forget to ask you if you had any head injuries. Invalid testing equals invalid results.