HGN (Horizontal Gaze Nystagmus)
Nystagmus is simply the involuntary jerking or bouncing of the eyeball. It occurs when there is a disturbance of the inner ear or oculomotor control of the eye.
Horizontal Gaze Nystagmus (HGN) refers to the horizontal jerking of the eye when you look to the side. In the DUI context, alcohol consumption impedes the ability of the brain to correctly control eye muscles causing the "jerk" or "bounce" associated with HGN. As the degree of impairment becomes greater, the jerking or bouncing, becomes more pronounced.
When a person is stopped or contact and the officer begins to question you with regard to potentially being DUI usually the first thing he will do is perform the HGN or "follow the pen" test.
The officer is supposed to have you out of your vehicle standing upright before beginning this test and while conducting the test the officer is supposed to be looking for six "clues" (3 in each eye). The clues, according to NHTSA are:
LACK OF SMOOTH PURSUIT:
The officer moves the object (usually a pen or flashlight) slowly and steadily from the center of your face towards the left ear. Your left eye should "smoothly" follow the pen.
DISTINCT NYSTAGMUS AT MAXIMUM DEVIATION:
Starting again from the center of the your face, the officer should move the pen toward the left ear, bringing your eye as far over as possible, and then holding the pen there for four seconds. The Officer is supposed to do this for at least four seconds to ensure that quick movement of the object did not possibly cause the nystagmus.
ANGLE OF ONSET OF NYSTAGMUS PRIOR TO FORTY-FIVE DEGREES:
The officer moves the pen or other object at a speed that should take about four seconds to reach the edge of your left shoulder. The officer should note this clue if the point or angle at which the eye begins to display nystagmus is before forty-five degrees from your nose. The officer then should move the object toward your right shoulder.
The officer is supposed to also check for vertical nystagmus. The officer should check for vertical nystagmus by raising the pen several inches above the your eyes.