What aspect of the HGN test is primarily assessed through the observation of eye movement?

The correct answer focuses on the presence of nystagmus, which is a critical aspect of the Horizontal Gaze Nystagmus (HGN) test used in field sobriety assessments. This test evaluates the eye movements of an individual as they follow a stimulus, typically a pen or flashlight, moved horizontally across their field of vision.

Nystagmus refers to an involuntary eye movement characterized by a rapid oscillation of the eyes. When a person is under the influence of alcohol or certain drugs, the presence of nystagmus becomes more pronounced and can occur at lower angles of gaze than what would typically be observed in sober individuals.

During the HGN test, law enforcement officers specifically look for two types of nystagmus: the onset nystagmus, which involves the eyes beginning to oscillate before reaching a 45-degree angle, and the maximum deviation, which is noted when the eyes move past this angle. The degree and nature of this eye movement provide clues about the person's intoxication level.

The other options—reaction time, balance and coordination, and the ability to follow instructions—are relevant to assessing a person's overall impairment but do not directly pertain to the specific observations made during the HGN test.