Police officers use a battery of tests called field sobriety tests (FSTs) to determine whether the driver of a vehicle is impaired and to develop probable cause to arrest the driver for drunk driving. Most of these FSTs test balance, coordination and dexterity. The most common field sobriety test, however, tests none of these; it is called the HGN test, or Horizontal Gaze Nystagmus. It is one of the three standardized FSTs under the National Highway Traffic Safety Administration.
What is Nystagmus?
Nystagmus is a term used to describe a “bouncing” or “jerking” of the eye when tracking an object with only your eyes. The normal eye tracks an object much like a marble would roll on a piece of glass: smoothly. When nystagmus is present the eye moves in a jerking motion tracking the object. Nystagmus is an involuntary muscle movement; you can’t feel nystagmus and vision is not affected by the jerking movement. Horizontal gaze nystagmus simply means the presence of the jerking movement while tracking an object side to side on a horizontal plane.
How does alcohol affect nystagmus? Alcohol is a central nervous depressant affecting both higher and lower functioning motor skills. The parts of the nervous system that controls eye movements is affected by alcohol. When a person becomes increasingly intoxicated, the nervous system breaks down the smooth and accurate control of eye movements. The break down in the smooth control results in the inability to hold the eyes steady. A person specifically looking for these changes can observe the lack of smooth eye movements.
How is the Horizontal Gaze Nystagmus Test Administered?
If you were suspected of drunken driving, you may have been administered the Horizontal Gaze Nystagmus test and not known it at the time. It is very common and often the first test administered. You may have had it done while you were sitting in your own vehicle. The Horizontal Gaze Nystagmus test only requires an object, such as a pen, small flashlight, or even a finger tip. The subject is instructed to follow the object only with their eyes and keep their head and body still. The object is placed about a foot to a foot and a half from the subject’s face, just above eye level. The officer will then move the object from the front of the subject’s face to the side ending near the subject’s ear and observe the subject’s eye movement. Officers are looking for three things:
Lack of Smooth Pursuit
Here, the officer is looking to see if the eye jerks or bounces while the object is being moved and the eye is tracking the object.
Nystagmus at Maximum Deviation
Here, the officer moves the object towards the ear as over as possible where the eye continues to track the object. The officer holds the object at that point for several seconds. The officer looks to see if there is jerking or bouncing while the object is being held at the end point.
The Angle of Onset of Nystagmus
While moving the object to the side, the officer notes where the angle of nystagmus begins. The officer is specifically looking for whether the nystagmus begins prior to a 45 degree angle.
The presence of some or all of these indicators or “clues” may lead the officer to believe that the driver is under the influence of alcohol while driving.
Horizontal Gaze Nystagmus in the Courtroom
Prosecutors, law enforcement, and NHTSA argue that the presence of horizontal gaze nystagmus are reliable indicators of whether a person is driving under the influence of alcohol or DUI. Experienced DUI defense attorneys argue that nystagmus is not reliable. For example, there may be other medical reasons a person exhibits nystagmus. Further, different states have different views on the reliability of Horizontal Gaze Nystagmus. Obtain an experienced DUI attorney to help prevent the use of HGN in your case.