The HGN supposedly detects an
involuntary jerking of the eye (nystagmus) while the eyes attempt to follow a
stimulus. In other words, it involves the eye following an object to determine
characteristic eye movement. Police officers across the county have been
trained to use this as a means of determining if a driver is inebriated even
though there are more than three dozen potential causes of nystagmus other than
intoxication. This is among the longest and most complex FST. Here's how it
goes.
First, the officer must check the
eyes by holding a stimulus (usually a pen) 12 to 15 inches from the nose
slightly above eye level and then move it smoothly across the field of vision
checking for resting nystagmus, equal pupil size, and equal tracking. The test
must begin with the left eye and then right at a rate of two seconds per each
eye per pass. Then, the officer must check for distinct and sustained nystagmus
at maximum deviation, tracking each eye separately starting with the left eye.
He must hold the stimulus at least four seconds once the stimulus is at the
farthest point and the eye is at maximum deviation. This is repeated to check
for heavy or distinct, sustained nystagmus.
The next step
is to check for an onset of nystagmus prior to 45 degrees for the stimulus to
reach the edge of the driver's shoulder. The officer must stop if he sees any
signs of jerking to see if it continues. This is repeated so that each eye is
checked twice. The full four seconds must be used because if the stimulus moves
too fast, the officer may go past the point of onset or miss it altogether.
Officers are
trained to look for three clues when evaluating the nystagmus in each eye:
1.
An inability to follow a
moving object smoothly
2.
A distinct and sustained
nystagmus at maximum deviation
3.
An onset of nystagmus at
prior to 45 degrees
There must be a
total of 14 passes for approximately 84 seconds. If the officer does a
different number of passes than this or the time if significantly above or
below 84 seconds, you know they've done it wrong. It is very important that the
police report and cruiser cam videos be
carefully examined to scrutinize the administration of this test.
