Which of the following is a sign of impairment when conducting HGN testing?

The identification of distinct and sustained nystagmus at maximum deviation as a sign of impairment during HGN testing is grounded in the physiological responses of a person under the influence of alcohol or other impairing substances.

During a typical HGN test, when a person is sober, their eyes should be able to smoothly track a moving stimulus without exhibiting nystagmus, which is an involuntary oscillation of the eyes. However, when there is impairment, particularly at the extreme position of gaze (maximum deviation), the eyes will demonstrate distinct and sustained nystagmus. This observed nystagmus indicates that the brain's ability to control eye movement is compromised, which is often linked to the effects of alcohol or certain drugs on the central nervous system.

The other choices, such as smooth pursuit of the stimulus, ability to follow the stimulus with both eyes, and focused gaze, suggest that the individual is functioning normally without significant impairment. Thus, they do not indicate signs of impairment and can be seen in individuals who are not affected by substances.