MA Police Training Manual - OUI Stop Field Sobriety Tests
HORIZONTAL GAZE NYSTAGMUS
Horizontal Gaze Nystagmus (HGN) refers to an
involuntary jerking occurring as the eyes gaze toward the side. In addition to
being involuntary the person experiencing the nystagmus is unaware that the
jerking is happening.
Involuntary jerking of the eyes becomes readily
noticeable when a person is impaired. As a person’s blood alcohol
concentration increases, the eyes will begin to jerk sooner as they move to the
side.
Horizontal Gaze Nystagmus is the most reliable
field sobriety test Especially when used in combination with the divided
attention tests, it will help police officers correctly identify suspects who
are impaired.
In
administering the HGN test, the officer has the suspect follow the motion of a
small stimulus with the eyes only. The stimulus may be the tip of a pen or
penlight, an eraser on a pencil or your finger tip, whichever contrasts with the
background.
When the HGN test is administered always begin
with subject’s left eye. Each eye is examined for three specific clues.
• as the eye moves from side to side, does
it move smoothly or does it jerk noticeably? (As people become impaired by
alcohol, their eyes exhibit a lack of smooth pursuit as they move from side to
side.)
• when the eye moves as far to the side as
possible and is kept at that position for several seconds, does it jerk
distinctly? (Distinct nystagmus at maximum deviation is another clue of
impairment.)
• as the eye moves toward the side, does it
start to jerk prior to a 45-degree angle? (Onset of nystagmus prior to
45-degrees is another clue of impairment.)
As a person’s blood alcohol concentration
increases it is more likely these clues will appear.
The maximum number of clues that may appear in
one eye is three. The maximum total number for any suspect is six. The original
research shows that if four or more clues are evident, it is likely that the
suspect’s blood alcohol concentration is above 0.10. With four or more clues
present, this test is 77% accurate.
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