What type of test indicates impairment by observing eye movement?

The Horizontal Gaze Nystagmus (HGN) test is a specific type of field sobriety test that focuses on observing eye movement to assess impairment. When an individual is under the influence of alcohol or certain drugs, their ability to control eye movements may be affected, leading to a characteristic jerking motion of the eyes. During the HGN test, law enforcement officers instruct the subject to follow a moving object, such as a pen or flashlight, with their eyes. Officers look for signs of nystagmus—an involuntary jerking of the eye—which can indicate impairment.

This test is scientifically validated and is often used as part of a battery of field sobriety tests to assess a person's level of intoxication. The emphasis on eye movement makes it a key indicator of potential impairment, setting it apart from other tests designed for different purposes, such as measuring blood alcohol content through breath analysis or engaging in verbal or physical assessments in field interviews.