What can vertical nystagmus signify in a HGN test?

Vertical nystagmus can indicate possible severe intoxication or specific drug effects, making this answer the most accurate. In the context of the Horizontal Gaze Nystagmus (HGN) test, practitioners look for involuntary eye movements as a sign of impairment. While horizontal nystagmus is often associated with alcohol consumption, vertical nystagmus is not typically expected from alcohol alone. Instead, it is more commonly linked to the influence of certain drugs, particularly central nervous system depressants, hallucinogens, or stimulants. Observing vertical nystagmus during the test can thus signal that a subject may be under the influence of such substances, leading to significant impairment in motor coordination and judgment. Researchers and law enforcement agencies take this response very seriously when assessing a driver's capabilities.