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Horizontal Gaze Nystagmus (HGN) - The Eye Test

This test refers to the involuntary jerking of the eye as it gazes to the side. When this occurs, the person is unaware of the jerking, and cannot control it. This involuntary jerking becomes noticeable as persons' blood alcohol increases.

When the HGN test is administered, the officer holds a stimulus 12-15 inches in front of your eyes and asks you to follow the stimulus without moving your head. The officer should start with the left eye, looking for 6 total clues.   Most importantly, there is a standardized pace the officer is to have the eyes move side to side - if the standard is not followed by the book, the HGN results must be thrown out!

1. Lack of Smooth Pursuit in each eye: Law enforcement will attempt to say that nystagmus is visible as they tract the movement of your eyes from side to side. From the center of the face, they are to move the eye all the way out to the side in approximately 2 seconds, then 2 seconds back to center, approximately 2 seconds to the other side, and 2 seconds back to center, repeating the test. If they  move too fast, they are doing the test wrong and in fact creating nystagmus!
    
2. Distinct Nystagmus at Maximum Deviation in each eye: Next, law enforcement will take their stimulus (the pen) and move your eyes as far to the side as possible.  They must hold that position for a minimum of 4 seconds.  Did you know that some people have naturally occurring nystagmus in this position - without a drop of alcohol in their system?  And once again, if law enforcement does the test too fast, they're doing it wrong!
    
3. Onset of Nystagmus Prior To 45 Degrees in each eye:   Finally, law enforcement will take and hold the stimulus to a 45 degree angle (approximately your shoulder), and must once again hold it for 4 seconds per eye, per pass.

With all this, the HGN test has a failure rate of 23 percent!