The Horizontal Gaze Nystagmus (HGN) test is a common field sobriety test used to detect impairment due to alcohol or drugs.
It assesses the involuntary jerking of the eyes, known as nystagmus, which can be exaggerated under the influence of certain substances.
During the HGN test, the individual is asked you to follow a stimulus, such as a pen or flashlight, with their eyes while keeping your head still.
The stimulus is horizontally moved across the field of vision, observing the eyes for any signs of nystagmus.
There are three specific clues:
1. Lack of smooth pursuit: The eyes are able to smoothly track the stimulus or if they exhibit jerking movements as they follow it.
2. Distinct and sustained nystagmus at maximum deviation, with distinct and sustained jerking of the eyes when they are moved as far to the side as possible and held for a few seconds.
3. Onset of nystagmus prior to 45 degrees, indicating an earlier onset of the involuntary eye movement.
The HGN test is just one component of a field sobriety test battery and is not considered definitive evidence of impairment.