The Horizontal Gaze Nystagmus (HGN) is a test where the officer will have the driver look straight ahead, then use only his or her eyes to follow the movement of an object the officer is moving from side to side in front of the driver's face. If the driver's eyes begin to spasm or twitch involuntarily when he or she is looking all the way to one side, it can be a condition known as nystagmus. Nystagmus is often exaggerated or more predominant when an individual is impaired by drugs or alcohol.