If you are stopped by the police and asked to perform Field Sobriety Tests, one of the test will likely be the HGN Test. This test is to check for movement of your eye. It is based upon research that shows that if you consume alcohol you eye will jerk back and forth if you follow an object closely.
The Horizontal Nystagmus Test (HGN) was first adopted in the early 1980’s. The test is a superficial test to determine blood alcohol content. The HGN test is essentially a measurement of the eye. Simply stated, nystagmus means a jerking of the eyes. There are many different types of nystagmus, however, the type involved in a field sobriety test (DUI Test) involves the pendular (back and forth) movement of the eye.
While administering the test, the officer will ask the individual to “keep his head straight and to follow the object with his eyes only.” The officer will then move his finger, or any object in his possession from the center of the device steadily toward one side. The object is usually held 12 to 15 inched directly in front, 2 to 3 inches above the eye being tested. The object is moved slowly (three to four seconds to complete the arc) in a level, even arc maintaining the 12 to 15 inch distance. The jerking of the eye should continue as long as the individual stares at the object, even though it is no longer being moved. The officer then repeats the test with the other eye.
Typically the eyes of a suspect under the influence of alcohol will begin to jerk sooner than those of a person who is not intoxicated. Further, the more intoxicated the person the more sooner the person’s eye will begin to show jerking. You might be asking yourself, okay so what does the jerking have to do with my blood alcohol content? Well, in a study for the National Highway Traffic and Safety Administration, researchers concluded that the onset of nystagmus (jerking) at about 40 degrees would correlate with a blood alcohol level of .10 percent; a level of .15 percent would be indicated where the involuntary movement began at about 35 degrees; onset at 30 degrees indicates a level of about .20 percent. Individuals with a blood-alcohol level above .20 percent usually cannot follow a moving object with their eyes.
However, current Illinois law only allows the HGN to be used to show the consumption of alcohol and it cannot be used to establish a certain level of intoxication. The HGN is usually the first of three tests that are administered prior to a person being arrested for DUI.