How does alcohol specifically affect HGN?

Alcohol has a significant impact on the neurological systems that control eye movement, specifically through its effects on the vestibular system and the brain's processing of visual stimuli. When a person consumes alcohol, it can lead to the induction of nystagmus, which is characterized by involuntary eye movements. This impairment makes it difficult for individuals to smoothly follow an object moving from side to side, as would be assessed in a Horizontal Gaze Nystagmus (HGN) test.

In the context of the HGN test, the presence of nystagmus observed after alcohol consumption indicates a loss of coordination and control over eye movements. This is because alcohol affects the brain regions responsible for balance and eye coordination, disrupting their function. Nystagmus can manifest as jerking or oscillation of the eyes, which becomes more apparent when the eyes are moved to the extremes of gaze.

Thus, the correct understanding of how alcohol affects HGN revolves around its ability to induce nystagmus and impair the ability to follow a stimulus clearly, which is crucial during field sobriety assessments.