What is a key characteristic of the horizontal gaze nystagmus test?

The horizontal gaze nystagmus test is specifically designed to identify involuntary eye movement, which is a critical indicator of potential impairment due to alcohol or certain drugs. When a trained officer observes a person's eyes following a moving object, they look for nystagmus, a condition characterized by jerking eye movements that can occur when the vestibular system is affected, often due to intoxication. The presence of these involuntary movements provides evidence that the individual may be under the influence, as impaired motor control often manifests in the eye's ability to smoothly track an object.

The other choices relate to different aspects of sobriety testing. Following a moving object is indeed part of the test but does not highlight the key characteristic itself, which is the involuntary nature of the eye movement. Walking in a straight line and standing on one leg are components of other sobriety tests that measure coordination and balance, but they do not pertain to the specific characteristics of the horizontal gaze nystagmus test.