What is Horizontal Gaze Nystagmus (HGN)?

Horizontal Gaze Nystagmus (HGN) refers to a specific involuntary eye movement that can occur when a person is under the influence of alcohol or certain drugs. When conducting the HGN test, law enforcement personnel observe the way the eyes move as they follow a stimulus, typically a pen or finger, horizontally from side to side. The presence of HGN is a sign that can indicate impairment, as it differs from normal eye movement patterns.

In contrast to voluntary eye movements that are under conscious control, HGN is involuntary and often results from disruptions in the brain's ability to control eye muscles due to the effects of intoxicants. This form of eye movement can be indicative of impairment, making it a valuable tool in the field for officers assessing potential driving under the influence situations. Understanding this distinction is crucial for recognizing how HGN is used in sobriety assessments.