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What is a Horizontal Gaze Nystagmus (HGN) Test?


The Horizontal Gaze Nystagmus (HGN) Test is one of the three standardized field sobriety tests approved by the National Highway Traffic Safety Administration (NHTSA).  Although the name of the test may be unfamiliar, most people would recognize the HGN test when they see a police officer asking someone to follow the tip of a finger (or some other object) with their eyes as the police officer moves it back and forth in front of the test subject.

Field sobriety tests are not “pass/fail tests”, but instead are designed to give “clues” or “indicators” to law enforcement officers that suggest whether a driver is impaired due to alcohol or drugs.  Each of the standardized tests is divided up to look for the presence of pre-defined indicators of impairment.  When the police administer one of these tests, this is what they are looking for.  A person being tested may exhibit none, some, or all of the clues on any given test.  It is ultimately up to the court to determine the significance of a person’s performance, but the police use these tests to establish probable cause to arrest somebody for DWI.  Officers are required to attend training to learn how to conduct these field sobriety tests in accordance with the guidelines established by the NHTSA.

What is Nystagmus?

Nystagmus is an involuntary bouncing or jerking of the eyeball.  Scientific evidence has established that alcohol and certain drugs impair the brain’s ability to control the eye muscles, causing the bouncing and jerking associated with nystagmus.  The theory is that the more the driver has to drink, the more pronounced the nystagmus becomes as evidenced by an increase in the bouncing or jerking motion of the eye.

The Horizontal Gaze Nystagmus (HGN) Test is designed to detect a driver’s impairment due to alcohol or certain drugs by measuring the level of bouncing or jerking in the eye.  This test, as well as the other field sobriety tests, is used to establish probable cause for an arrest. To administer the HGN test, the officer holds a pen or other object twelve to fifteen inches from the driver’s face slightly above eye level.  The officer instructs the driver to follow the object with his or her eyes while keeping his or her head still.  The officer looks for signs of nystagmus during the test to suggest that the driver is impaired.

There are a total of 6 “clues” to be looked for in an HGN Test (3 for each eye).  NHTSA teaches officers to look for the inability of the eye to smoothly follow an object (“lack of smooth pursuit”), distinct and sustained nystagmus at the edge of the eye’s range of movement (“sustained nystagmus at maximum deviation”), and an onset of nystagmus before the eye moves 45 degrees from center (“onset prior to 45 degrees”).

Defenses to the HGN Test

We have several ways to attack field sobriety tests to prevent the information from being used in court or to attack the validity of the information if it is presented to a judge or jury.  One way is to attack how the test was administered.  When officers are trained to perform the HGN test, they are provided a manual outlining in specific detail how to perform the test.  Because we are familiar with the training material, we ask specific, directed questions to the officer about how he administered the test to the driver.  If the officer failed to provide the proper instructions to the driver or failed to conduct the test exactly as prescribed by the NHTSA guidelines, we use this to attack the “clues” the officer noted as invalid due to the test not being performed correctly.  Standardized testing is only valid if the tests are given in a standardized way.

Another avenue for attacking the HGN Test relates to the source of the nystagmus itself.  Nystagmus is caused by several other conditions in addition to alcohol or drug impairment. For example, if the driver has bright lights moving rapidly in and out of his vision, he might exhibit nystagmus.  The lights of oncoming traffic can cause this to happen; therefore, if the test is performed on the side of the road with the driver facing oncoming traffic, it may result in a false positive. A number of health conditions can cause nystagmus. Unless the officer is able to differentiate alcohol nystagmus from other types of nystagmus, the “clues” the officer noted may be a false positive because they were caused by another condition. An attorney with experience handling driving while impaired cases knows the best way to attack the results of an HGN test.

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