During the Horizontal Gaze Nystagmus test, when does the nystagmus onset occur?

During the Horizontal Gaze Nystagmus (HGN) test, the onset of nystagmus is observed prior to reaching 45 degrees of lateral eye movement. This is a key indicator used by law enforcement to assess potential impairment. When a person's eye begins to demonstrate nystagmus before reaching that 45-degree mark, it suggests that the individual's ability to track smoothly is affected, which is consistent with certain levels of intoxication, particularly from alcohol.

Understanding that nystagmus occurring before this angle signifies impairment helps officers in making informed decisions during field sobriety evaluations. The HGN test relies on the fact that alcohol and certain drugs impact the central nervous system, causing noticeable eye movement patterns that are indicative of impaired function.