Horizontal Gaze Nystagmus is the involuntary jerking of the eye horizontally when the gaze is brought out to the side. Nystagmus can be caused by a few factors, one of them being the consumption of alcohol. While general nystagmus is natural, under the influence of alcohol the results can be exaggerated and occur earlier than they normally would. Michigan requires that Standardized Field Sobriety Tests be used and the HGN test is considered to be one of these, while other forms of testing for nystagmus are not considered evidence for a charge. When an officer is conducting the test they will first need to make sure that you are not wearing glasses or they should ask if you have contacts in. The officer should be aware of the presence of contacts but they are not believed to influence the test results in any way. They should also check for any medical concerns that could influence the final results.
They will ask you to follow an object (typically a pen or flashlight) with your eyes while keeping your head still. First they will be looking at whether or not you are able to follow the object with a smooth transition or if your eyes seem to show signs of nystagmus. Second they will start with the object at the center of your face and bring it out to the side where they will hold if for a few seconds. If they see signs of nystagmus then they should hold the object out at maximum deviation for a few seconds for a more thorough assessment. Third they will look at the angle at which the nystagmus begins by taking the object out to your shoulder slow enough that it takes four seconds. They are looking to see if the nystagmus occurs prior to reaching forty five degrees from the center.
All three of these categories will be tested in both eyes meaning there will be six tests total. If the above indicators of nystagmus are found four or more times during these tests, research from the National Highway Traffic Safety Administration has stated that around 88 percent of suspects can be classified as having a blood alcohol level of .08 or higher. In spite of these statistics from the NHTSA many cases have come up where the officer administered the test incorrectly, causing the results to be skewed. The presence of Horizontal Gaze Nystagmus can also be caused by other factors so if it is found in a suspect it does not always mean that they are guilty. If you have allegedly failed an HGN test, talk with an attorney about your options.