One of the three main standardized field sobriety tests is the Horizontal Gaze Nystagmus Test. Nystagmus refers to the involuntary jerking of the eyes. Alcohol and other drugs can impair your involuntary motions and can cause horizontal gaze nystagmus. But how do police officers know whether or not you’re impaired?
Procedures of Horizontal Gaze Nystagmus Testing: The Three Clues
The test officers use at the roadside is to look for “Horizontal Gaze Nystagmus” – an involuntary jerking of the eyes occurring as the eyes gaze toward the side. Some jerking will be seen if the eyes are moved far enough to the side.
The officer will need a contrasting stimulus for the suspect to follow with their eyes. This may be a tip of the officer’s index finger, penlight or pen. The stimulus used should be held slightly above eye level. It should be held approximately 12-15 inches in front of the nose.
- The Lack of Smooth Pursuit (Clue Number One) – The eyes can be observed to jerk or “bounce” as they follow a smoothly moving stimulus, such as a pencil or penlight. The eyes of an unimpaired person will follow the stimulus smoothly, i.e., a marble rolling across a smooth pane of glass, or windshield wipers moving across a wet windshield.
- Distinct and Sustained Nystagmus at Maximum Deviation (Clue Number Two) – Distinct and sustained nystagmus will be evident when the eye is held at maximum deviation for a minimum of four seconds. People exhibit slight jerking of the eye, even when unimpaired, but this will not be evident or sustained for more than a few seconds. When impaired by alcohol, the jerking will be larger, more pronounced, sustained for more than four seconds, and easily observable.
- Onset of Nystagmus Prior to 45 Degrees (Clue Number Three) – The point at which the eye is first seen jerking. If the jerking begins prior to 45 degrees it is evident that the person has a blood alcohol content (BAC) above .08, as shown by recent research. The higher the degree of impairment, the sooner the nystagmus will be observable.
Original research shows that if a subject exhibits four or more of the clues, or cannot complete the test, the subject’s blood alcohol content (BAC) is likely to be above .10. This criterion has been shown to be accurate 77 percent of the time.
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