What eye movement characteristic is essential to note during the HGN test?

The distinctiveness and duration of nystagmus are critical characteristics to observe during the Horizontal Gaze Nystagmus (HGN) test. This eye movement phenomenon is typically induced during the evaluation of a person's eye tracking capabilities as affected by potential impairment, often due to alcohol or drugs.

In particular, the presence of nystagmus, which is an involuntary eye movement characterized by a jerking motion, provides vital clues about a person's level of intoxication. This includes assessing how distinctly the nystagmus manifests as well as how long it lasts when gazing at maximum deviation. The distinction in the onset of nystagmus when the eyes are moved towards the side is a specific indicator of impairment.

The duration of the nystagmus, especially if it lasts for 4 seconds or more before reaching an angle of 45 degrees, is a significant indicator used by law enforcement in assessing the possibility of alcohol influence. Detecting these features helps in making an accurate evaluation of the subject's impairment levels.