What is the expected response of the eyes during a successful HGN test?

In the Horizontal Gaze Nystagmus (HGN) test, the expected response of the eyes during a successful assessment is for the eyes to move smoothly. When a person is sober, their eyes should follow a stimulus, such as a pen or finger, in a controlled manner without any jerking or abrupt movements. Smooth pursuit is an indicator of normal eye function and motor control.

If the eyes do not move smoothly and instead exhibit any jerking or fluttering indications, it typically suggests that the individual may be experiencing impairment, often due to substances such as alcohol or drugs. Recognizing smooth eye movements versus other reactions helps law enforcement and trained personnel determine a driver's sobriety level effectively.