October 29, 2015

WI Detection and Standardized Field Sobriety Testing Session 7 – Phase Three: Pre-Arrest Screening Involuntary Jerking of the Eyes Nystagmus

DWI Detection and Standardized Field Sobriety Testing
Session 7 – Phase Three: Pre-Arrest Screening
Involuntary Jerking of the Eyes
Nystagmus
7-7
B. Gaze Nystagmus – Definition
"Nystagmus" means an involuntary jerking of the eyes.
Alcohol and certain other drugs cause Horizontal Gaze Nystagmus.
DWI Detection and Standardized Field Sobriety Testing
Session 7 – Phase Three: Pre-Arrest Screening
Most reliable field sobriety test
Involuntary jerking occurring as eyes
gaze toward the sides
Readily noticeable when a person is
impaired
As BAC increases eyes begin to jerk
sooner as they move to the side
CNS depressants, inhalants, and
dissociative anesthetics also cause
HGN
Horizontal Gaze Nystagmus (HGN)
7-8
C. Horizontal Gaze Nystagmus – Definition, Concepts, Demonstration
Horizontal Gaze Nystagmus (HGN) is the most reliable field sobriety test. Especially
when used in combination with the divided attention tests, it will help law enforcement
officers correctly identify subjects who are impaired.
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 refers to an involuntary jerking occurring as the eyes gaze
toward the side. In addition to being involuntary the person experiencing the nystagmus
is usually unaware that the jerking is happening.
In administering the HGN test, the officer has the subject follow the motion of a small
stimulus with the eyes only. The stimulus may be the tip of a pen or penlight, or an
eraser on a pencil, whichever contrasts with the background.
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HS 178 R5/13 5 of 26

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