The Horizontal Gaze Nystagmus test is one of the “standardized” field sobriety tests given to suspected drunk driving offenders. This test suggests that when an individual is intoxicated, “ nystagmus” occurs. Nystagmus refers to involuntary jerks of the eyeball. Under normal circumstances, jerking occurs when eyes are rotated at high peripheral angles; however, when an individual is intoxicated, nystagmus is exaggerated and sometimes occurs at lesser angles. Furthermore, the test suggests that an intoxicated individual will have more difficulty smoothly tracking a moving object compared to one who is sober.
In administering the Horizontal Gaze Nystagmus test, a police officer will ask a California DUI suspect to watch a slowly moving object such as a small flashlight or pen. As the officer moves the object back and forth, the officer will observe the California DUI suspect’s eyes to see if they follow the object. Additionally, the officer will watch for other signs of intoxication that include the angle of onset of jerking within 45 degrees of center, distinct jerking when the eye is at maximum deviation, and whether or not the eye can smoothly follow the moving object.