Horizontal Gaze Nystagmus (HGN) Test
Simple physical and cognitive tests for drivers stopped on suspicion of driving under the influence have been used for decades. While the one-leg stand and the walk-and-turn tests are familiar to most people through television, the ‘horizontal gaze nystagmus test’ (HGN) is relatively new to field sobriety tests.
This natural phenomenon is an involuntary twitching of the eye that occurs when a person looks in a particular direction at an angle of about 45 degrees while the head is held still in the forward position. Nystagmus occurs constantly, but is undetectable in normal circumstances. It increases notably when a person experiences vertigo or is under the influence of drugs or alcohol.
A penlight or a finger is moved in front of the driver’s eyes – about one foot distant – and the driver is asked to follow the light or finger with the 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 or the driver may have ingested any of a number of drugs.
NHTSA research indicates that this test allows proper classification of approximately 77 percent of suspects.
While the National Highway Traffic and Safety Administration (NHTSA) supports HGN testing to determine intoxication or drug influence, public interest groups cite a range of circumstances that can cause impaired nystagmus reactions:
- motion sickness or illnesses that affect the sinuses
- eye disease or eye muscle fatigue, such as created by long hours of driving
- contact lenses
- ear infections
- sunstroke
- vertigo
- over-the-counter medications such as nicotine, aspirin, and cold remedies or caffeine
- congenital defects or side effects of neurological disease.