Horizontal Nystagmus is one of three common field sobriety tests currently administered by law enforcement officers across Washington State. Nystagmus is an involuntary rapid movement of the eyeball, which may he horizontal, vertical, or rotatory. An inability of the eyes to maintain visual fixation as they are turned from side to side (in other words, jerking or bouncing) is known as horizontal gaze nystagmus, or HGN. Some investigators believe alcohol intoxication increases the frequency and amplitude of HGN and causes HGN to occur at a smaller angle of deviation from the forward direction.
In administering the horizontal gaze nystagmus test, an officer will look for the three following clues in each eye: (1) lack of smooth pursuit, (2) distinct nystagmus at maximum deviation, and (3) onset of nystagmus prior to 45 degrees. Continue reading