In Oakland County, nearly every officer uses the horizontal gaze nystagmus test, better known as the HGN test, as part of their investigations for drinking and driving. The HGN test was devised from academics working on behalf of the National Highway Traffic Safety Administration. What they came up with was a test that officers could supposedly detect intoxication simply from looking at somebody's eyes.
The test itself is performed by an officer taking a pen or their finger and waiving it back and forth in front of the suspect's face. The theory is that the eye will "jerk" as the pen goes back, much like a yard sprinkler. In order to do this, the officer must first check the suspect's eyes for equal tracking, which means that the eyes follow the pen simultaneously. Next, the officer checks to make sure the pupils are the same size. Finally, the actual test begins.
There are three categories in the test that the officer is grading the suspect on: (1) the lack of smooth pursuit, in which the officer moves the pen slowly from the center of the face to each ear, looking for nystagmus, (2) distinct nystagmus at maximum deviation, where the officer will start at the center of the face and move the pen towards the ear until the eye cannot go anymore, then they watch to see if nystagmus occurs, (3) and angle of onset of nystagmus prior to forty-five degree, in which the officer moves the pen faster from the center of the face and attempts to see whether nystagmus occurs less than forty-five degrees from the center of the face.