This description of the Horizontal Gaze Nystagmus test posted by California DUI Lawyer.

The officer is also trained at the conclusion of HGN, to check for Vertical Gaze Nystagmus or VGN. This is commonly used by the Drug Recognition Technician or Expert to determine impairment due to certain drugs. The VGN test is simple to administer. The stimulus is once again positioned 12 to 15 inches in front of the person's face. It is positioned horizontally in front of the face. The person must hold the head still and is instructed in same. They must follow the stimulus with their eyes as the stimulus or object is raised until the person's eyes are elevated as far as they can go. The object is held for four seconds and nystagmus or evidence of jerking is watched for. 

The ADMINISTRATIVE PROCEDURES of the HGN test are:  

1) Eyeglasses/Contacts

2) Verbal Instructions

3) Positioning of the Object (12-15 inches)

4) Tracking Ability

5) Check for Pupil Size

6) Lack of Smooth Pursuit

7) Distinct Nystagmus at Maximum Deviation

8) Angle of Onset of Nystagmus Prior to 45 Degrees

9) Total The Clues-Decision Point 4

10)Vertical Gaze Nystagmus