What does the HGN test assess?

The HGN test, which stands for Horizontal Gaze Nystagmus, is a field sobriety test used by law enforcement to assess a person's level of impairment due to alcohol or drugs. During this test, an officer observes the eyes of the subject while they follow a slowly moving object, typically a pen or flashlight, with their gaze. The test specifically looks for nystagmus, which is an involuntary movement of the eyes that can indicate impairment.

When a person is sober, their eyes should move smoothly as they follow the object. However, if they are impaired, the eyes may exhibit jerking or bouncing movements, especially as they are moved to the side. This response is particularly pronounced under the influence of alcohol and is a key indicator of the level of impairment, making the HGN test a valuable tool for determining whether someone is fit to drive.