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

•  The Horizontal Gaze Nystagmus Test is not a test of vision but it is imperative that the suspect have vision in both eyes in order to be able to follow the stimulus so that an accurate scoring of this test can be made. The suspect is instructed to keep the head still and straight while following the stimulus until told to stop. In order to prevent a movement of the subject's head during testing, a person is instructed to place both fists together under their chin to act as a brace against this. 

•  The stimulus must be twelve to fifteen inches in front of the suspect's eyes for ease of focus. The subjects eyes must be checked for the ability to track together. The stimulus is moved across the subject's entire field of vision, all the way to the right and back again all the way to the left. It should take two seconds out and two seconds back. At least two passes must be made for each eye and each clue. The officer must check to see if the eyes track the stimulus together or if one lags behind. If the eyes do not track the stimulus together, it could indicate a possible medical disorder, injury or blindness. They must then check to see if both pupils are if equal size. If they are not then there is a probability of a head injury. The stimulus is held in the middle of the face when checking the pupil size. 

•  The officer is trained to receive an acknowledgement from the suspect that the stimulus is at a comfortable distance from the suspect's eyes and to document same. This can be accomplished by having them touch the stimulus with their finger