The term "nystagmus" refers to involuntary movement of the eyes. A nystagmus test measures the involuntary bouncing or jerking of the eyeball. It is conducted by asking the person being tested to follow with his or her eyes the path of an object, such as a pen or a small flashlight, back and forth about 12-15 inches (about 30-38 cm) in front of his or her face. The most common use of this test is as part of an investigation to determine whether a person was driving while intoxicated (DWI) or driving under the influence (DUI). If the subject of the nystagmus test fails the test, he or she might be arrested for DWI or DUI.
If a driver commits a traffic violation, a police officer can legally stop that driver to begin an investigation. While talking to the driver, the police officer will be making observations about the driver. If the driver has trouble producing his or her driver's license and vehicle registration, smells of alcohol or has bloodshot eyes, the police officer might ask the driver to step out of the vehicle and submit to field sobriety tests such as a nystagmus test.
The nystagmus test, in addition to the one-leg-stand test and the walk-and-turn test, make up the battery of standardized field sobriety tests. If a driver has consumed alcohol recently, his or her brain will have trouble controlling the muscles of the eyes, resulting in a bouncing or jerking of the eyeball. The greater the degree of the driver’s impairment, the more pronounced the bouncing or jerking.
When conducting a nystagmus test, the police officer must have a clear view of the driver’s eyes in order to examine the driver’s eyes for involuntary motions. To conduct a nystagmus test, the police officer will hold an ink pen or a pen light approximately 12-15 inches (about 30-38 cm) in front of the driver's face. The officer will instruct the driver to follow the movement of the object with his or her eyes. While conducting the test, the police officer will make note of any bouncing or jerking of the driver’s eyeballs. If the driver fails the test, the police officer will have probable cause to believe that the driver has been driving while intoxicated and could make an arrest.