Which clue indicates the inability of a subject to follow a moving stimulus smoothly in the HGN test?

The indication of the inability of a subject to follow a moving stimulus smoothly in the Horizontal Gaze Nystagmus (HGN) test is observed through the presence of failure to track smoothly. During the HGN test, a trained officer moves a stimulus, typically a pen or similar object, horizontally in front of the subject’s eyes. If the subject cannot smoothly follow this movement and exhibits jerking or a noticeable discontinuation in the movement of their eyes, it indicates potential impairment.

This failure to track smoothly is significant because it reveals that the subject's neurological function may be compromised, which is a key indicator of possible intoxication or impairment. The officer looks for distinct signs of this jerking motion, as it affects the reliability of the test results.

In contrast, the other options pertain to different aspects of eye movement that might indicate impairment, but they do not specifically describe the inability to follow a smooth motion. For instance, nystagmus maximum deviation and onset of nystagmus prior to 45 degrees provide different but relevant indicators of impairment severity and timing, while excessive blinking does not directly relate to tracking ability.