What technique is used to calculate the angle of the eyes in the HGN test?

In the Horizontal Gaze Nystagmus (HGN) test, the 45-degree angle method is utilized to assess the presence of nystagmus, which may indicate impairment due to alcohol or other substances. During this part of the Standardized Field Sobriety Test, an officer observes the subject’s eyes while moving a stimulus, typically a pen or finger, horizontally.

The rationale behind the 45-degree angle is that it is the point where the eyes are expected to show a clear reaction to nystagmus. Officers look for two specific signs at this angle: the onset of nystagmus before reaching 45 degrees and distinct nystagmus within that range. This angle allows officers to accurately gauge eye movement and determine any impairment effectively. The 45-degree threshold is specifically chosen based on research that shows noticeable signs of nystagmus at this position, making it crucial for reliable results in field sobriety testing.