What is the Horizontal Gaze Nystagmus test?
The horizontal gaze nystagmus test is a field sobriety test that is sometimes used by police officers to detect alcohol impairment in drivers. The test looks for sudden and quick movements when the eye is looking sideways. This condition is referred to as nystagmus. Nystagmus becomes very pronounced when alcohol has been consumed. This test can detect blood alcohol content because alcohol consumption suppresses the nervous system and brain’s ability to control the smooth movement of the eye. The jerking motion of the eye is involuntary and occurs when light is held to the eye at 45 degrees or less. The person is unaware that it is happening and it does not affect their vision.
There are pros and cons to administering this type of test. Although positive test results are grounds for a DUI arrest, the test results are often denied as evidence in DUI court proceedings.
The National Highway Traffic Safety Administration (NHTSA) has conducted studies which found the HGN test to be 77% accurate in detecting the content of alcohol in the blood at 10% and higher. NHTSA also found that this test is the most powerful of the three standardized field sobriety tests conducted during traffic stops.
HGN test results are not affected by the wearing of eyeglasses and contact lenses. Police officers often ask the driver to remove their eyeglasses for easier and more accurate detection of eye movement. Drivers are not typically asked to remove their contacts although hard contact lenses may fall out when the pupil moves extremely far to the side of the eye. Officers will make a note of whether the driver is wearing contact lenses.
Nystagmus test results are often not admitted in DUI court proceedings. This condition may exist due to factors other than the presence of alcohol in the blood. If the driver is facing traffic lights or the lights on the police car, the test results will be positive for a condition referred to as optokinetic nystagmus. In addition, nystagmus is also caused by barbiturates, seizure medication and PCP. A head injury, brain damage, glass eye or unequal pupil sizes may also cause a positive test result indicating intoxication incorrectly.
The police officer must be trained on HGN testing to properly administer the test. An object must be moved in front of the driver’s eye at no more or less than eight seconds from the center of the eye to the far side of the eye. The test must be administered according to NHTSA guidelines.
Proper lighting needs to be present to prevent false positive results.
The HGN test cannot produce a specific quantitative level of alcohol content in the blood.
It may be hard for the police officer to accurately determine a 45 degree angle. In addition, the pen or object used to follow the gaze must be 12 to 15 inches from the eye. The police officer must be the same height or taller than the driver when administering the test. All of these conditions can cause the test results to be tainted.
Although the NHTSA found the HGN test to be 77% accurate, this finding also means that one of every four people who fails this test are not impaired. The HGN has scientific support and is deemed reliable when administered by a properly trained police officer.