What is the primary physical sign looked for during the Horizontal Gaze Nystagmus test?

The primary physical sign looked for during the Horizontal Gaze Nystagmus (HGN) test is nystagmus itself. Nystagmus refers to the involuntary jerking or oscillation of the eyes, which is a key indicator of impairment, particularly in relation to alcohol consumption. When someone is under the influence, their eyes may not track smoothly as they move from side to side; instead, they may exhibit these characteristic jerking movements.

In the context of the HGN test, an officer will observe the eyes while a subject is instructed to follow a stimulus (like a pen or light) with their eyes. The presence of nystagmus at certain angles of gaze is evaluated as a strong indicator of potential impairment. This specific behavior is significant because it provides observable, physical evidence of how alcohol or other substances affect motor control and coordination, which is essential in DUI enforcement.

Other options like delayed reaction time, dizziness, or difficulty balancing, while potentially relevant in overall assessments of impairment, are not the primary signs evaluated during the HGN test itself. The focus is specifically on the degree and presence of nystagmus to make an informed judgment about the individual's level of sobriety.