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
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