Horizontal Gaze Nystagmus

Definition - What does Horizontal Gaze Nystagmus mean?

The horizontal gaze nystagmus (HGN) is one of the standard field sobriety tests used by law enforcement to determine whether an individual is under the influence of alcohol or drugs. In the HGN test, the law enforcement officer positions an object (usually a penlight) about one foot from the face of the suspected individual and moves the object from one side to another while observing the individual's eye movement for signs of impairment.

Justipedia explains Horizontal Gaze Nystagmus

There is a biological phenomenon that causes involuntary horizontal jerking of the eye when an individual looks sideways at an angle greater than 45 degree. When a person is intoxicated, the presence of alcohol in the individual’s system hinders the brain’s ability to control eye muscles, resulting in an involuntary jerking of the eyeball at angles less than 45 degrees. While conducting the HGN test, law enforcement officers look for the angle at which the jerking of the individual’s eyeball begins in order to determine if the individual is sober or impaired. The HGN test was developed by the National Highway Traffic Safety and Administration (NHTSA), and it is considered to be scientifically accurate with a reported accuracy of 77% when detecting BAC levels of 0.10 or higher.

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