The horizontal gaze nystagmus test is one of three tests that make up the standardized field sobriety tests that the National Highway Traffic Safety Administration introduced in the early 1980s. It is used to help police officers in Nebraska and around the country determine whether motorists are operating their vehicles under the influence of alcohol. Nystagmus is an involuntary movement of the eyes, which becomes more pronounced after alcohol has been consumed.
Conducting the horizontal gaze nystagmus test
When a police officer conducts a horizontal gaze nystagmus test, they ask the driver to follow an object like a pencil or pen without moving their head as it moves it from side-to-side. The police officer will look for involuntary eye movements at the beginning of the test, when the eyes reach an angle of about 45 degrees and when the eyes reach their maximum deviation. Failing the horizontal gaze nystagmus test gives police officers the probable cause they need to make a drunk driving arrest.
The reliability of the horizontal gaze nystagmus test
Even the NHTSA concedes that the horizontal gaze nystagmus test is only accurate about 77% of the time when motorists have blood alcohol concentrations of 0.10% or higher, but many people will find this test difficult or impossible to pass even while completely sober. That is because there are dozens of medical conditions that can cause nystagmus. These conditions include retina or optic nerve damage, head trauma, stroke, Meniere’s disease and other inner ear conditions, central nervous system disorders and multiple sclerosis.
Not used in court
Failing a standardized field sobriety test may provide police officers with enough evidence to make a drunk driving arrest, but it will not provide prosecutors with proof of intoxication. Like all parts of the NHTSA test, the horizontal gaze nystagmus test can detect a long list of medical conditions as well as alcohol impairment. This means that more reliable evidence, such as the results of chemical toxicology tests, is needed to prosecute DUI cases.