Which aspect of field sobriety tests is primarily measured by the Horizontal Gaze Nystagmus?

The Horizontal Gaze Nystagmus (HGN) test primarily measures vision tracking. This test assesses an individual's ability to follow a moving object with their eyes, specifically focusing on the involuntary jerking or "nystagmus" of the eyes that occurs when a person is impaired by alcohol or drugs. By observing how smoothly and accurately a subject's eyes can track a stimulus, officers can evaluate potential impairment.

HGN is particularly effective because it provides insights into how well an individual can maintain focus and exhibit appropriate eye movement, which may be compromised due to intoxication. This visual tracking component is essential in determining sobriety, as impaired vision tracking can indicate a level of impairment that affects a driver's ability to operate a vehicle safely.