What is assessed during the Horizontal Gaze Nystagmus test for impairment?

During the Horizontal Gaze Nystagmus (HGN) test, the primary focus is on the ability of an individual to follow a moving object, which typically involves the officer guiding the subject's gaze in a smooth motion with a stimulus, such as a pen or a finger.

The test assesses the presence of nystagmus, an involuntary eye movement that can indicate impairment. Specifically, the HGN test looks for how well the eyes can track the stimulus without significant jerking or deviation, which is often affected by alcohol consumption or certain drugs. The presence of nystagmus, especially as the eyes reach the extremes of their lateral range, suggests impairment affecting the central nervous system.

Other options, while related to overall motor function or vision, do not directly pertain to the specific assessment performed during the HGN test. For instance, coordination of hand movements and speed of eye reaction are evaluated in different contexts and tests, while clarity of vision is not the focus of HGN, as HGN specifically monitors eye movement rather than visual acuity.