Legal Definitions - horizontal-gaze nystagmus test

The Horizontal-Gaze Nystagmus (HGN) test is a standardized field sobriety test used by law enforcement officers to help determine if a person may be impaired by alcohol or certain drugs. During this test, an officer observes a person's eyes as they track a moving object, such as a pen or a finger, from side to side. The officer is looking for involuntary jerking or twitching of the eyes, which is a common indicator of central nervous system impairment.

• Example 1: Routine Traffic Stop
  Officer Miller pulls over Mr. Johnson for weaving slightly within his lane late at night. Upon approaching the vehicle, Officer Miller notices a faint smell of alcohol and Mr. Johnson's slightly slurred speech. To further assess Mr. Johnson's sobriety, Officer Miller asks him to step out of the car and performs an HGN test. Officer Miller holds a pen approximately 12-15 inches from Mr. Johnson's face and moves it horizontally, instructing Mr. Johnson to follow it with his eyes without moving his head. Officer Miller observes Mr. Johnson's eyes for distinct jerking movements at maximum deviation and before the eye reaches 45 degrees. If these involuntary jerks are present, it would provide an indication that Mr. Johnson's ability to operate a vehicle safely might be impaired.

• Example 2: Minor Accident Investigation
  Ms. Chen is involved in a minor fender-bender in a parking lot. When police arrive to investigate, the other driver, Mr. Davis, appears disoriented and has difficulty recalling the sequence of events. Suspecting potential impairment, Officer Rodriguez asks Mr. Davis to perform several field sobriety tests, including the HGN test. Officer Rodriguez instructs Mr. Davis to follow the tip of her flashlight with his eyes. She carefully watches for any involuntary nystagmus (jerking) as his eyes track the light to the left and right. The presence of significant nystagmus would suggest that Mr. Davis's coordination and cognitive functions might be compromised, contributing to the accident and indicating possible intoxication.

• Example 3: DUI Checkpoint
  At a late-night sobriety checkpoint, Officer Green stops a vehicle driven by Ms. Lee. While speaking with Ms. Lee, Officer Green observes that her pupils seem unusually dilated and her responses are somewhat delayed, although she denies consuming alcohol. Given these observations, Officer Green asks Ms. Lee to perform an HGN test. He uses his finger as the stimulus, moving it slowly across her field of vision. Officer Green is specifically looking for the smooth pursuit of her eyes and any signs of nystagmus, which could indicate impairment not only from alcohol but also from certain drugs that affect the central nervous system. If Ms. Lee's eyes exhibit involuntary jerking, it would be a piece of evidence suggesting impairment, prompting further investigation.