Horizontal Gaze Nystagmus (hgn)

The Horizontal Gaze Nystamus or HGN test is frequently used by United States law enforcement to evaluate whether a driver is intoxicated by gauging the involuntary movement or oscillation of the driver's eyes or the "involuntary rapid movement of the eyeball, which may be horizontal, vertical, rotatory, or mixed".

The HGN test is most frequently used in conjunction with other field sobriety tests and may help law enforcement officers determine if a driver is under the influence of alcohol.

For the horizontal gaze nystagmus test the officer will evaluate the nystagmus of the intoxicated driver by analyzing the involuntarily "jerk" of the eyes which occurs at earlier points in the test if the driver is intoxicated. To perform the test the police officer will first check the eyes to ensure that the pupils are equal in size and have equal ability to track, thus determining that other disorders or injuries are not present. The officer will then hold a stimulus in front of the driver's nose, with the tip elevated slighted above the driver's eyes. All movements must be smooth as the stimulus moves from side to side. The officer is evaluating the following:

1. Lack of smooth pursuit of the stimulus
2. Distinct nystagmus at maximum deviation
3. Onset of nystagmus prior to reaching a 45 degree angle

HGN can not be used to estimate blood alcohol concentration, and the NHTSA does not recognize any other clues as valid indications of HGN.