What is the angle at which the onset of nystagmus is observed in the Horizontal Gaze Nystagmus (HGN) test?

In the Horizontal Gaze Nystagmus (HGN) test, the onset of nystagmus is typically observed as the eye deviates from the center at an angle of 45 degrees. This is a critical point in the evaluation because the presence of nystagmus at or before this angle is one of the indicators used to assess impairment due to alcohol consumption.

During the HGN test, the officer will check for nystagmus, which is an involuntary jerking of the eyes, as the subject tracks a moving stimulus, usually a pen or a finger. When an individual is sober, the eyes generally do not exhibit nystagmus until they reach about 45 degrees from the center. If nystagmus is observed before this angle, it suggests that the subject may be under the influence of alcohol or other substances, as alcohol can impair the functioning of the vestibular system responsible for eye movement control.

This understanding is crucial for law enforcement as they conduct field sobriety tests, ensuring that they accurately interpret the results of the HGN test in the context of potential DUI offenses.