Field Sobriety Testing: Horizontal Gaze Nystagmus (HGN)

Most people are familiar with field sobriety tests such as saying the alphabet backwards or walking in a heel/toe pattern on the side of the road.  However, there are three National High Traffic Safety Administration (NHTSA) approved field sobriety tests.  Those tests are the horizontal gaze nystagmus, one leg stand, and the walk and turn.  

What is HGN?

The horizontal gaze nystagmus (HGN) checks for involuntary jerks of a person’s eye. This involuntary jerking starts when a person gazes to the side and is present in all people, but it is exaggerated by alcohol consumption.  It can also be caused by some illicit and prescription drugs, fatigue, sickness, and other medical conditions.

Although nystagmus refers to any bouncing or jerking of the eye, police officers are concerned with horizontal gaze nystagmus because alcohol depresses the nervous system. This has a noticeable effect on the ability to control sideways eye movements smoothly and accurately. Because HGN has no noticeable effect on one’s vision, those who exhibit the condition are not even aware of it.

Police should conduct the HGN test in a well-lit area or with a flashlight with the suspect facing away from the police car’s flashing lights and passing cars which could lead to inaccurate results. The officer stands in front of the suspect and asks them to remove their glasses and note if contacts are being worn. The officer then verbally instructs the suspect to put their feet together and keep their hands at their sides. 

After verbally explaining the process to the suspect and obtaining an acknowledgement of understanding, the officer will position a stimulus such as the tip of a pen or a finger 12 to 15 inches away just above eye level and ask the suspect to track the stimulus with only their eyes. The object will pass multiple times between each eye from the center to the outside in segments. Each pass must last a specific length of time. If the suspect’s eyes do not track equally or if the pupil size is unequal, the officer may terminate the test and refer the person for medical evaluation and treatment.  This exercise lets the officer know if the suspect is impaired in three ways with each eye:

  • Does the eye follow the object smoothly as it moves from the center of the face toward the outer edge?
  • Does the eye have a distinct jerking motion after being held toward the outer edge?
  • As the officer moves the object towards the edge of the suspect’s shoulder, does the eye jerk before the object is 45 degrees from the center of the suspect’s face?

The test provides up to six “clues” (three for each eye) to the officer that a suspect is impaired. It only takes as few as two “clues” for  a suspect to fail the HGN test.  This test is the most accurate indicator in the three-part test for alcohol impairment according to multiple studies funded by the National Highway Transportation Safety Administration (NHTSA) in the past 45 years.  It can establish probable cause for an arrest.  Police officers may also ask to test the suspect’s blood alcohol content (BAC) using a portable breath alcohol monitor (Breathalyzer/PBT).  Blood samples are generally only taken after an accident when field sobriety testing cannot be performed.  A blood sample may be drawn with consent of the person or after the police obtain a search warrant signed by a judge.

If you have been charged with a DWI or DUI based on HGN test results, your defense attorney can check to see if the test was conducted according to the Standardized Field Sobriety Test (SFST) guidelines.

A 2001 study in Science and Justice showed that over 90% of HGN tests were done incorrectly. The protocol set out by the NHTSA for all three parts of the field sobriety test must be strictly followed or the results are unreliable and invalid. Common mistakes in administering the test include:

  • Moving the stimulus too slowly or too quickly on individual passes;
  • Holding the stimulus close than 12 inches or further away than 15 inches;
  • Not holding the stimulus for at least four seconds at maximum deviation; and
  • Looping the stimulus through the passes

HGN results are only “for the purpose of proving that a defendant may have consumed alcohol”, not that the person has a specific blood-alcohol level.

Your first call after getting a DWI or DUI is Worman Law for a free consultation 24/7 at (314)695-9529. We will go through your medical history and notes from the HGN test to see if the testing was performed correctly. 


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