Why is an officer shining a penlight in someone's face and asking them to follow it with their eyes? That is part of a roadside sobriety test called horizontal gaze nystagmus, or HGN. It looks for involuntary jerking of the eyes as they move side to side. Police use it as one of the standardized field sobriety tests to look for signs that alcohol or certain drugs may be affecting the brain and nervous system.

In practice, an officer watches for specific clues, such as whether the eyes jerk before reaching a certain angle or cannot track smoothly. HGN can matter a lot in a DWI case because prosecutors may use it to support probable cause, an arrest, or later testimony about suspected impairment. But it is not foolproof. Fatigue, flashing lights, certain medications, neurological conditions, and head or eye injuries can affect the test, and poor roadside conditions can make observations less reliable.

In North Carolina, HGN evidence often appears in cases charged under G.S. 20-138.1 for impaired driving. After a crash, it can also affect a related injury claim. If the driver who caused the wreck showed HGN clues, that may strengthen arguments about fault, negligence, or even punitive damages in a civil case. At the same time, a defense lawyer may challenge how the test was given, whether the officer was trained, and whether another medical issue explains the eye movement.