The “DUI eye test” refers to the Horizontal Gaze Nystagmus, or “HGN.” It is one of the three “verified” Standardized Field Sobriety Tests (SFSTs) recognized by the National Highway Traffic Safety Administration (NHTSA), along with the walk-and-turn and the one-leg stand.

HGN is a “sobriety test” where an officer holds a stimulus, usually a pen, a fingertip, or a small flashlight, in front of the suspect’s face and asks the driver to follow it with their eyes without moving their head. The officer is watching for an involuntary jerking or twitching of the eyes. Some officers describe it as looking like windshield wipers going back and forth on a dry windshield. That jerking motion is referred to as a nystagmus and it is purportedly an indicator of appreciable impairment by alcohol and possibly other substances.

The test sounds simple. The science behind it is not. Whether HGN is actually a reliable indicator of impairment in any given case is a separate question. Even when scientifically reliable, the accuracy and reliability of field sobriety testing largely depend on the training, experience, and objectivity of the charging officer.  That matters when the test results are summarized in a police report, described on the witness stand, or used to support probable cause for arrest under North Carolina’s impaired driving statute, N.C.G.S. § 20-138.1.