DWI Street Science: Horizontal Gaze Nystagmus

By Allen, Texas Criminal Defense Lawyer Kyle T. Therrian
Office Number: (972) 562-7549
24 Hr Jail Release: (214) 403-6522


Part 1: Horizontal Gaze Nystagmus
Part 2: Walk and Turn
Part 3: One Leg Stand


Of the three standardized field sobriety tests, the Horizontal Gaze Nystagmus Test (HGN Test) is regarded by law enforcement as the most “scientific” or “technical.” Horizontal gaze nystagmus is the involuntary jerking of the eyes which occurs when the eyes gaze toward the side. Nystagmus is naturally occurring phenomenon that is normally unnoticeable. In a DWI case, the prosecutor will explain that once a person consumes enough alcohol, this jerking will eventually become noticeable (and was noticeable to the officer conducting the tests). This is true, but misleading for two reasons: (1) alcohol is just one of a several dozen of causes of nystagmus, and (2) even when alcohol can be isolated as the cause–alcohol can enhance nystagmus at blood alcohol concentration (BAC) levels as low as of .04.

Obviously this will not be the song that the State will be singing, and even when confronted with these facts, most officers will authoritatively disagree (ironically, without reference to any authority). At the end of the day, the result of any HGN test is purely circumstantial–the jury may or may not consider it as tending to establish the existence of alcohol.


When an officer conducts the HGN test, he is looking for a total of 6 clues (an identical set of 3 clues in each eye). These clues include:

  1. Lack of smooth pursuit: jerking of the eye as it follows a smoothly moving stimulus
  2. Distinct and sustained nystagmus at maximum deviation: jerking of the eye when gazing completely to one side for four seconds
  3. Onset of nystagmus prior to 45 degrees: jerking of the eye prior to looking to either side at a 45 degree angle.

Because Texas Courts automatically recognize the science behind the HGN test as a reliable method of determining whether alcohol is in the system, the results of a properly administered test will usually be admitted as circumstantial evidence against the accused. The emphasis is on proper administration because it is the science behind the test and not the officer administering the test which the Court deems as automatically reliable. Because the officer is merely a practitioner of the method designed by doctors and scientists he must first assure the court that he conducted the tests properly, then he must convince the jury.

As you can imagine, there are a number of ways that an officer can administer the HGN test improperly. There are also a number of things outside of the officer’s control which may affect the scientific impact of the test. The following are some of the most common issues that can become an obstacle in presenting HGN evidence at trial:

  • Officers may fail to use or improperly administer screening techniques to rule out medical conditions as the cause of nystagmus.
  • A person could have a medical disorder or injury that causes nystagmus that cannot be ruled out by the officer’s standard screening techniques.
  • The proper administration of the test requires the officer to hold his stimulus or pen at a certain distance from the face and move it from side to side at a reasonable speed.
  • When checking for nystagmus at maximum deviation, the officer must not hold the gaze for too long (approximately 4 seconds).
  • The officer should position an individual so that their field of vision will not include fast moving objects or flashing lights–sometimes this is not even possible.

Even when an officer administers the HGN in an airtight fashion, it can sometimes be hard for the State to overcome a line of questioning which illustrates that the officer is merely playing the role of scientist. While officers will support their testimony with training and experience, at the end of the day, even the most experienced officer’s expertise will usually be limited to what he or she learned from the instruction manual.

*Kyle Therrian is an attorney licensed to practice in the State of Texas. Nothing in this article is intended to be legal advice. For legal advice on any case you should contact an attorney directly.