Field Sobriety Tests: Nystagmus Test
One of the most common Standardized Field Sobriety Tests used by police to determine if probable cause exists for a drunk driving arrest is the Horizontal Gaze Nystagmus (HGN) test. This test works by measuring a driver’s eye reflexes when asked to look at a moving stationary object at a close distance, allowing officers to look for certain key signs of intoxication.
How Does the Test Work?
Nystagmus is the medical term used to describe an involuntary twitch of the eyeballs which occurs when a person looks sideways at an angle greater than 45 degrees. When a person is under the influence of alcohol or certain drugs, this jerking become more pronounced and can occur at angles less than 45 degrees.
To administer the test, an officer must first check a driver’s eyes for any resting nystagmus, equal pupil size, and the ability to equally track and object with both eyes. If a driver cannot do any of these things, there is a possibility that they have an existing medical condition which may render the test result unreliable. To properly administer the test, an officer must hold a small object rough 12-15 inches away from a driver’s nose, usually a flashlight or pen, and slowly move it from side to side while the driver follows it with their eyes.
Officers will look for several signs of intoxication, including:
- A lack of smooth pursuit
- Eye jerking before reaching a 45 degree angle
- Eyes jerking within four seconds while gazing all the way to one side
Are HGN Tests Accurate?
The National Highway Traffic Safety Administration (NHTSA) has determined that HGN tests are reliable as long as they are administered by a police officer who has had proper HGN training. If a subject’s eyes are not properly visible, if they are facing blinking lights or oncoming headlights, or if the subject has their eyeglasses on at the time of the test, the results could be skewed. It is also important to note that HGN tests are not pass/fail, but merely a way for police to gain a possible indicator of a driver’s intoxication. For this reason, many states do not allow the test to be used as evidence in a court of law.
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