If you are suspected of drunk driving, the officer will usually administer a field sobriety test. The horizontal gaze nystagmus test tracks a person’s eye movement while following an object, such as a pen. Certain involuntary jerking movements of the eyes during this test are thought to indicate intoxication, especially when other test results show the same. Here are a few cues officers look for when performing this field sobriety test. 

The officer will begin by positioning the object in front of the driver’s line of vision from a certain distance. The driver will be instructed to follow the object visually as the officer moves it front of their face. Most sober people can track movement with their eyes in one smooth motion. Along with inebriation, certain medical conditions can impact smooth tracking, so officers are encouraged to ask drivers about their medical history before administering the test. 

Officers move the object as far as possible to get a maximum extension of the person’s line of vision. This is known as maximum deviation, and when administering this portion of the test it is recommended that the object is held in place for at least four seconds. This accounts for any involuntary jerking caused by the rapid movement of the object. 

Finally, the eyes should not exhibit involuntary movement before the object is at least forty-five degrees from the center of the person’s face. It is ultimately up to the officer to determine the proper angle, but even without precise measurement, it is considered a reliable factor when determining inebriation. In court, horizontal gaze nystagmus testing can be presented as proof of the established physical characteristics of drunkenness, or it can be considered scientific evidence.