Field sobriety tests explained: Horizontal Gaze Nystagmus test

Even if you've never been pulled over, you have probably watched enough television to have some idea of how a DUI traffic stop is conducted. Whether it's blowing into a breathalyzer or taking field sobriety tests, we all think we have some idea about what to expect.

In today's post, we'll be talking about field sobriety tests, and one test in particular. It's known as the Horizontal Gaze Nystagmus test, and it is one of three field sobriety tests developed and recommended by the National Highway Traffic Safety Administration.

Horizontal Gaze Nystagmus (HGN) is a phenomenon that occurs naturally as a person's eyes move from side to side. Specifically, it refers to an involuntary jerking of the eye. Under normal conditions (when a person is sober), "nystagmus occurs when the eyes are rotated at high peripheral angles," according to the NHTSA.

You have probably seen an HGN test without knowing what it was called. Police officers ask the suspect to follow a moving object with their eyes, usually a finger, a flashlight or a pen. The officer is looking to see if the suspect's eyes can track movement accurately, and if the nystagmus looks typical.

If the suspect is inebriated, the nystagmus may occur at lesser angles of eye movement and may be more exaggerated than what's considered normal. The suspect may also have difficulty tracking the moving object.

While an HGN test can indicate intoxication, it by no means proves intoxication. An officer's assessment of what a normal nystagmus looks like can be subjective. Moreover, certain medical conditions and prescription drugs can cause an abnormal nystagmus.

As we have previously written, the details of a DUI stop are very important. Fighting the charges often involves scrutinizing test results as well as how testing was conducted. Therefore, you should work with an experienced criminal defense attorney and be as thorough as possible about what you remember from the traffic stop.

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