A gaze test, most commonly referring to the Horizontal Gaze Nystagmus (HGN) test, is a field sobriety test used to detect involuntary eye movements that can indicate intoxication.
Understanding the Horizontal Gaze Nystagmus (HGN) Test
The Horizontal Gaze Nystagmus (HGN) test is a specific type of gaze test widely used in law enforcement. It primarily measures nystagmus, which is the medical term for an involuntary jerking of the eyeballs.
What it Measures
The core purpose of the HGN test is to observe and measure this involuntary eye movement. While nystagmus can occur for many reasons, it becomes significantly more pronounced when an individual is intoxicated, particularly by alcohol or certain drugs.
How it Works
During an HGN test, an officer typically asks a subject to follow a moving object (like a pen or finger) with their eyes, keeping their head still. The officer then observes the eyes for specific clues of nystagmus, such as:
- Lack of smooth pursuit: The eyes cannot follow the stimulus smoothly.
- Distinct and sustained nystagmus at maximum deviation: Jerking is evident when the eye is moved as far to the side as possible and held there.
- Onset of nystagmus prior to 45 degrees: Jerking begins before the eye has moved 45 degrees from the center.
Importance in Field Sobriety Testing
As a type of field sobriety test, the HGN test is a key tool for law enforcement officers to assess a driver's impairment without requiring a breathalyzer or blood test on the spot. Its effectiveness stems from the physiological impact of intoxicants on the central nervous system, which directly affects eye muscle control.
- Indicator of Impairment: The presence and characteristics of nystagmus observed during the test serve as indicators of potential impairment due to alcohol or drug consumption.
- Non-Invasive: It is a non-invasive test that can be administered quickly and safely at the roadside.
While the HGN test is a valuable tool, it is typically used in conjunction with other field sobriety tests and not as the sole determinant of intoxication.
