October 29, 2015

WI Detection and Standardized Field Sobriety Testing • Positional Alcohol Nystagmus Vestibular Nystagmus (Cont.) 8-21

Session 8 - Concepts and Principles of the Standardized Field Sobriety Tests (SFST)
DWI Detection and Standardized Field Sobriety Testing
Positional Alcohol Nystagmus
Vestibular Nystagmus (Cont.)
8-21
Positional Alcohol Nystagmus (PAN) occurs when a foreign fluid, such as alcohol, that
alters the specific gravity of the blood is in unequal concentrations in the blood and the
vestibular system. This causes the vestibular system to respond to gravity in certain
head positions, resulting in nystagmus.
In the original HGN study, research was not conducted for performing HGN on people
lying down. Current research demonstrates that HGN can be performed on someone in
this position.
Session 8 - Concepts and Principles of the Standardized Field Sobriety Tests (SFST)
DWI Detection and Standardized Field Sobriety Testing
Optokinetic
Neural Nystagmus
8-22
Nystagmus can also result directly from neural activity:
Optokinetic Nystagmus occurs when the eyes fixate on an object that suddenly moves
out of sight, or when the eyes watch sharply contrasting moving images.
Examples of optokinetic nystagmus include watching strobe lights, rotating lights, or
rapidly moving traffic in close proximity. The Horizontal Gaze Nystagmus test will not be
influenced by optokinetic nystagmus when administered properly. During the Horizontal
Gaze Nystagmus test, the suspect is required to fixate the eyes on a penlight, pencil or
similar object that moves in accordance with the HGN testing procedures, thus
optokinetic nystagmus will not occur. The movement of the stimulus and the fixation on
the stimulus by the subject precludes this form of nystagmus from being observed by
the officer.
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HS 178 R5/13 14 of 62

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