Written by attorney Bill Powers

"Strobe Effect" and Optokinetic Nystagmus - Standardized Field Sobriety Testing HGN


[n?stag′m?s] Etymology: Gk, nystagmos, nodding involuntary, rhythmic movements of the eyes. The oscillations may be horizontal, vertical, rotary, or mixed. Jerking nystagmus, characterized by faster movements in one direction than in the opposite direction, is more common than pendular nystagmus, in which the oscillations are approximately equal in rate in both directions. Jerking nystagmus occurs normally when an individual observes a moving object, but on other occasions it may be a sign of barbiturate intoxication or of labyrinthine vestibular, vascular, or neurologic disease. Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier OPTOKINETIC NYSTAGMUS: Nystagmus induced by looking at objects moving across the visual field. Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition.


Standardized Field Sobriety Testing

The Standardized Field Sobriety Test (SFST) is a battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest. These tests were developed as a result of research sponsored by the National Highway Traffic Safety Administration (NHTSA) and conducted by the Southern California Research Institute. A formal program of training was developed and is available through NHTSA to help police officers become more skillful at detecting DWI suspects, describing the behavior of these suspects, and presenting effective testimony in court. Formal administration and accreditation of the program is provided through IACP. The three tests of the SFST are:

  • the horizontal gaze nystagmus (HGN)

  • the walk-and-turn

  • the one-leg stand.

These tests are administered systematically and are evaluated according to measured responses of the suspect.

HGN Testing Horizontal gaze nystagmus is an involuntary jerking of the eyeball which occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.10 or greater. NHTSA research indicates that this test allows proper classification of approximately 77 percent of suspects. HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.

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