There are three field sobriety tests that have been “standardized” by the National Highway Traffic Safety Administration:
Nystagmus is the rhythmic back and forth oscillation of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye.
All people have slight oscillation of the eyes normally. There are two major types of eye movements: pendular and jerk. Horizontal Gaze Nystagmus (HGN) is a type of jerk nystagmus with the saccadic movement toward the direction of the gaze.
HGN is an involuntary motion, meaning the person exhibiting the nystagmus cannot control it. Furthermore, the subject exhibiting nystagmus is unaware that it is happening because the bouncing of the eye does not affect the subject’s vision.
The suspect follows a stimulus with the eyes. The officer observes smooth pursuit, nystagmus prior to a 45 degree angle, and nystagmus at maximum deviation.
The officer is instructed to give the test as follows:
The Horizontal Gaze Nystagmus test has three clues of impairment:
The test is administered with two passes, so a total of six clues are possible. A minimum of four clues are needed to determine if the suspect’s BAC level is above 0.10 percent.
The officer is warned that there are conditions that may interfere with the test. Such conditions include wind or dust irritating the subject’s eyes, or visual or other distractions impeding the tests. Examples include rotating or strobe lights or traffic passing in close proximity.
In addition, the Officer is warned that nystagmus may be due to causes other than alcohol, and that fatigue nystagmus may be present if the subject’s eyes are kept at maximum deviation for more than 30 seconds.
Also, various medications, as well as Caffeine, nicotine, or aspirin may cause nystagmus.