How is nystagmus indicated during the HGN test?

Nystagmus during the Horizontal Gaze Nystagmus (HGN) test is indicated specifically by the involuntary jerking of the eyes. This jerking can be observed as a person’s eyes follow a moving object, such as the stimulus used in the HGN test. The presence of nystagmus is significant because it can suggest impairment related to alcohol or certain drugs. The test evaluates the smoothness of eye movement, and the jerking is a clear sign of dysfunction in the vestibular system or neural pathways, which may indicate intoxication.

The other choices do not accurately describe the presence of nystagmus in this context. For instance, smooth movement of the eyes or a prompt reaction to light would suggest normal functioning, while loss of balance is not an indicator of nystagmus per se but rather a separate aspect that could also suggest impairment.