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How eye movement can demonstrate impairment

Law enforcement uses standardized field sobriety tests all across the U.S. to assess driver impairment due to drugs and/or alcohol. This means that wherever you go in the country, if you get pulled over for possible DUI and the officer chooses to administer FSTs, you will undergo the same three tests.

In addition to the one-leg-stand and walk-and-turn tests, you will likely undergo a test known colloquially as the follow-the-finger test. Its scientific name, however, is the horizontal gaze nystagmus test.

What is horizontal gaze nystagmus?

Nystagmus is a medical term for involuntary eye movement, e.g., twitching, bouncing or jerking. There are many different kinds of nystagmus, and they have many different causes. However, authorities who pull you over are looking for a specific pattern called horizontal gaze nystagmus.

HGN is involuntary, meaning that you cannot control it, and it does not affect your vision, so you may not even be aware of it. It is less pronounced when you are sober because your nervous system is usually effective at maintaining smooth and accurate sideways eye motion. However, certain substances, including alcohol, depress your neurologic function, meaning that it is no longer able to compensate for horizontal gaze nystagmus as effectively as it would ordinarily.

What do authorities look for during an HGN test?

During a horizontal gaze nystagmus test, the law enforcement officer will ask you to keep your head still and follow the back-and-forth movement of an object, e.g., a pen, a finger or a small penlight, with your eyes alone. He or she will be looking for the following signs:

  • Distinct nystagmus at maximum deviation
  • The onset of nystagmus prior to 45 degrees
  • Lack of smooth pursuit

Before administering the test, the officer will ensure that you face away from the road where cars may pass with their lights on, as well as the flashing lights of the cruiser, in the interest of obtaining accurate results. He or she may shine a flashlight in your eyes if the test takes place in a poorly lit area.