The horizontal gaze nystagmus test, or “HGN” test is commonly known as the pen test. It is a field sobriety test standardized by the National Highway Transportation Safety Administration (NHTSA). The test is used to determine if a driver is under the influence.
The pen test is a simple enough test for the subject to perform, but it takes special training for the officer to administer it. The officer will ask the subject to follow an object, usually a pen, with his or her eyes as the officer moves it back and forth.
If the subject struggles to track the object properly, it may be a sign of intoxication. Yet, this is not the only purpose of the test. Alcohol and drugs cause an interesting side effect in many individuals, called “nystagmus,” which is the involuntary jerking of the eye muscles.
This phenomenon manifests itself as twitchy eye movements, when the subject is trying to track the object. The officer needs to undergo training to ensure that this test is done correctly. Scientifically, it is one of the more accurate, if not the most accurate, field sobriety test currently available.
While other field sobriety tests rely more on subjective observations, this test shows a consistency and reliability that helps it stand as relatively solid evidence, in court. There are certain conditions like neurological problems, multiple sclerosis, or vision problems that can skew the test. Other than that, however, the HGN or pen test can be generally relied upon to produce accurate results and allow an officer to make a relatively sound judgment on whether to make the arrest or not.