In the never ending quest to squelch drinking and driving, law enforcement officials have gone to great lengths to create ways to investigate whether someone is intoxicated. One of the ways to investigate whether someone has DWI is to use the HGN.
As we left off yesterday, the HGN will indicate whether there is alcohol in a person's system. If there is alcohol in this person's system, then this test will demonstrate a jerking movement of the eyes. The officer will look for six clues (three in each eye) that will indicate impairment.
The three clues are (1) lack of smooth pursuit; (2) distinct nystagmus at maximum deviation; and (3) onset of nystagmus prior to 45 degrees. Lets look at these individually. The lack of smooth pursuit means that the eyes are pushing the instrument (or pen) and instead of a smooth pursuit, the eyes are very jerky and bouncy (as opposed to a marble rolling on a glass surface).
The second clue, distinct nystagmus at maximum deviation, requires the officer to move the instrument over to the corner of the person's vision. The person's eye will follow this movement. The officer will hold the instrument there while he checks to see if there is nystagmus as the eye is located at the farthest corner of the eyeball.
Finally, the last clue examines whether nystagmus is onset prior to a 45 degree angle (this is approximately in front of the suspect's shoulder).
The officer will place a check mark for every clue that they see in each eye. The more clues, the more chance that the officer is going to believe that the person is intoxicated.