Horizontal Gaze Nystagmus (hgn)

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Definition of Horizontal Gaze Nystagmus (hgn)

The Horizontal Gaze Nystamus or HGN test is one of the battery of field sobriety tests which are used by police officers to identity drivers who are intoxicated. The HGN or horizontal gaze nystagmus tests measures the oscillation of the driver's eyes or the "involuntary rapid movement of the eyeball, which may be horizontal, vertical, rotatory, or mixed".

To perform this test the officer will move a small light or pen in front of the driver's eyes and measure the following: lack of smooth pursuit of the stimulus, distinct nystagmus at maximum deviation, and onset of nystagmus prior to reaching a 45 degree angle.

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