During the HGN test, what specific eye movement is being observed?

The answer focuses specifically on nystagmus, which is defined as the involuntary jerking movements of the eyes. During the Horizontal Gaze Nystagmus (HGN) test, law enforcement officers observe the eyes as they track a moving object, typically a pen or flashlight, from side to side. The presence and degree of nystagmus can indicate impairment due to alcohol or other substances.

When a person is sober, their eyes normally move smoothly when tracking an object. If they are impaired, the eyes may exhibit nystagmus, which involves a jerking motion that can occur during lateral gaze. This specific type of eye movement is crucial in the assessment of potential intoxication, as it reflects neurological function and helps officers determine a driver’s level of impairment.

In contrast, convergence refers to the inward movement of the eyes toward each other when focusing on a close object, pupil dilation involves changes in the size of the pupils, and fixed gaze refers to an inability to move the eyes from their central focus. These do not pertain to the HGN test's main focus of identifying nystagmus as a sign of possible impairment.