What does the Horizontal Gaze Nystagmus test primarily evaluate?

The Horizontal Gaze Nystagmus (HGN) test is designed to assess the movement of the eyes in response to a stimulus, typically a pen or finger, that is moved horizontally from side to side. This test is specifically used in Standardized Field Sobriety Testing to determine the presence of nystagmus, which is an involuntary jerking of the eyes that can occur when a person is under the influence of alcohol or other drugs.

When conducted properly, the HGN test looks for three key indicators: the onset of nystagmus before the eye reaches a 45-degree angle, the distinct and sustained nystagmus at that angle, and the ability of the eye to track smoothly. The primary focus on eye movement makes this test a reliable indicator of impairment due to intoxication. Other options like balance or coordination are assessed through different tests, while speech clarity is not a focus of the HGN test at all. Thus, evaluating eye movement is essential for understanding potential impairment in a DUI context.