What is indicated if a test subject cannot follow moving objects with their eyes during the Horizontal Gaze Nystagmus test?

When a test subject cannot follow moving objects with their eyes during the Horizontal Gaze Nystagmus (HGN) test, it suggests a possible sign of intoxication. The HGN test is specifically designed to detect the presence of alcohol and certain drugs in an individual's system by analyzing the involuntary eye movement known as nystagmus. When a person is intoxicated, their ability to smoothly track an object decreases, and they may exhibit involuntary jerking of the eyes. This reaction occurs due to the impairment of the central nervous system caused by alcohol or other substances, making it a key indicator used by law enforcement to assess sobriety.

While other factors such as fatigue, visual impairment, or distraction might theoretically affect a person's ability to follow moving objects, in the context of the HGN test, the inability to properly perform this task is most closely correlated with signs of intoxication specifically.