Horizontal Gaze Nystagmus (HGN) is one of 3 or 4 tests used by police officer to determine whether the person being investigated is DUI. Nystagmus the involuntary jerking or bouncing of the eyeball. The cause of this is when a disturbance occurs in the inner ear or the nerves that control eye movement. HGN is the jerking of the eyes when the eyes are moving horizontally or vertically.
Police use HGN to determine intoxication. They automatically believe HGN equals intoxication. So, a police officer will mistakenly assume a driver is DUI at the first sight of jerking of the eyes. There is a specific way the police are supposed to administer the HGN test. If it is not administered pursuant to the National Highway Traffic and Safety Administration (NHTSA) standards, then validity is at issue.
HGN is easily challenged. One way to challenge the HGN is by a showing that the police officer did not administer the test pursuant to his training. If he did not do it the way he was trained to do it, how can the conclusion be correct? It can’t. An expert in HGN is a good way to invalidate an officer’s opinion based upon an invalid HGN method.
Even if the officer administers the HGN pursuant to the NHTSA standard, it does not mean that HGN is caused by alcohol. In the case of United States v. Horn (D. Md. 2002) 185 F. Supp. 2d 530, Judge Grimm provided that the court could take judicial notice of many other causes of nystagmus other than alcohol consumption. In fact, it was determined that there is at least 38 different causes of nystagmus other than from alcohol consumption.