What is the primary purpose of the HGN test?

The primary purpose of the Horizontal Gaze Nystagmus (HGN) test is to assess eye movement for signs of impairment. This test is specifically designed to identify involuntary eye movements that can indicate intoxication or other types of impairment, particularly associated with alcohol consumption. When a subject is instructed to follow a moving object with their eyes, a trained observer looks for specific indicators such as nystagmus—the jerking or oscillation of the eyes—which can reveal the effects of substances on the nervous system.

While the other options may involve aspects of physical or cognitive abilities, they do not capture the essence of what the HGN test is intended to measure. The focus of the HGN test is solely on the condition and movement of the eyes in relation to discernible signs of impairment, making the assessment of eye movement the correct and relevant purpose of the test.