How is the HGN test conducted in the field?

The Horizontal Gaze Nystagmus (HGN) test is a critical component of the Standardized Field Sobriety Tests used by law enforcement officers to assess an individual's potential level of impairment due to alcohol or drugs. This test specifically analyzes the subject's eye movement.

To conduct the HGN test, the officer will typically hold a stimulus, often a pen or a finger, approximately 12 to 15 inches away from the subject’s face and at the subject's eye level. The subject is then instructed to follow the stimulus with their eyes, without moving their head. The officer looks for distinct signs of nystagmus, which are involuntary jerking movements of the eyes.

Key indicators that officers are trained to observe include the lack of smooth pursuit, the presence of nystagmus at maximum deviation (when the eyes are looking as far to the side as possible), and the onset of nystagmus before reaching a 45-degree angle. The more pronounced these signs are, the more likely it is that the subject may have a blood alcohol concentration above the legal limit.

In contrast, monitoring speech patterns, evaluating posture, or determining awareness does not specifically assess eye movement, which is the hallmark of the H