Smooth Pursuit

Smooth pursuit refers to the DUI suspect's inability to pursue a moving stimulus during a horizontal gaze nystagmus test. The suspect must be able to track the stimulus and focus on it as it is moved horizontally from side to side. Drivers who are highly intoxicated may be unable to smoothly pursue or track the object and their eyes will bounce or jerk as they move.

Smooth pursuit is one factor a law enforcement officer will evaluate in a standardized field sobriety test as they perform the horizontal gaze nystagmus or HGN test. During the HGN test the officer will evaluate the left eye as he moves the stimulus to the right. The stimulus should be moved slowly and moved as far to the right as the eye can go. Fast movements or jerky hands on the part of the police officer can invalidate the testing and the scoring of the HGN test. Additional passes are made in front of the driver's eyes to help the officer completely validate the results. The officer must then complete the same process in the driver's right eye.

All individuals have a small degree of nystagmus, but under normal conditions it is not visible to the naked eye. Individuals who are highly intoxicated will have obvious and distinct nystagmus which has been exacerbated by the alcohol and will be unable to smoothly pursue the stimulus as the officer moves it from left to right in front of the driver's eyes.