Challenges to Field Sobriety Tests: The Horizontal Gaze Nystagmus Test in Washington, D.C.

by Jamison Koehler on December 1, 2009

The manual used to train Metropolitan Police Department officers in Washington, D.C. describes three different phases in detecting a DWI, DUI, or OWI.  The first phase involves the officer’s decision whether or not to stop the vehicle.  The second phase involves the decision whether or not to ask the driver to exit the vehicle.  Finally, once the vehicle has been stopped and the driver has been taken out of the car, the police officer needs to decide whether or not there is “probable cause” to arrest the driver for suspicion of DWI, DUI or OWI.


The officer is instructed to determine, during each phase of the “detection,” whether there is sufficient evidence to proceed to the next phase in the detection process.  Otherwise, the driver must be sent on his or her way.  In making the ultimate decision whether or not to arrest the driver, the officer is to consider all of his/her observations up to that point.

A criminal defense lawyer representing a person charged with a drinking-and-driving offense in D.C. should challenge each step in this “detection process.”  A week or so ago, I discussed potential challenges to the legality of the stop itself.  (Read post here.)  A day later, I focused on possible challenges to the police officer’s initial observations of the driver and the officer’s subsequent decision to further detain the driver for a field sobriety test or other form of investigation. (Read post here.)

Today’s entry is the first of two posts focusing on the next step in a police investigation into a possible DUI, DWI or OWI; namely, the administration of field sobriety tests.  Specifically, after a brief description of the field sobriety tests that are used in Washington, D.C., today’s post will focus on potential challenges to the Horizontal Gaze Nystagmus (HGN) test.  A future post will discuss the Walk-and-Turn and the One-Leg-Stand.

Standardized Field Sobriety Test Used in Washington, D.C.

Police have historically used a range of different field tests to measure the sobriety of a driver who has been pulled over for suspicion of drinking-and-driving.  The ones that probably come most quickly to mind – from what we have seen in movies or on T.V. — are the finger-to-nose, backward counting, and reverse-citation-of–the-alphabet tests.

After the accuracy of these tests was successfully challenged in many jurisdictions during the 1960s and 1970s, the National Highway Traffic Safety Administration (NHTSA) developed a more uniform approach in the early 1980s for conducting field sobriety tests:  the so-called Standardized Field Sobriety Test (SFST).  Developed by NHTSA in cooperation with the Southern California Research Institute, the SFST is a “battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest.”  The flagship of the SFST, and the focus of today’s post, is the Horizontal Gaze Nystagmus Test.

Horizontal Gaze Nystagmus (HGN) Test

The police training manual defines “nystagmus” as the involuntary jerking of the eyeball.  As such, nystagmus is roughly akin to what some people have described as a windshield wiper becoming stuck on a dry windshield.  “Horizontal gaze” nystagmus describes this involuntary jerking when the eyes rotate from the middle to one side.

Details of beauty

Even a perfectly sober person’s eyes will occasionally jerk momentarily when the eye is moved from the middle to the side.  “Physiological nystagmus” is a naturally occurring phenomenon that keeps the sensory cells of the eye from tiring.  However, a sober person’s eyes will normally stabilize after a moment or two.  Studies have shown that the jerking will be “exaggerated” and will occur at “lesser angles” (that is, will occur sooner during the eye rotation) when a person is under the influence of alcohol.  The eyes of an alcohol-impaired person will also have difficulty “smoothly tracking” a moving object.  The police manual uses the analogy of a rolling marble to describe the “smooth pursuit” required.

The first step in the administration of the test is to check both of the suspect’s eyes for (1) equal tracking (that is, can they follow an object together?) and (2) equal pupil size.  The officer will position a pen or other small object 12-15 inches from the suspect’s nose at a slightly raised level and then “move the stimulus smoothly” first to the right, then to the left, then back to the middle.  If the eyes do not track together, or if the pupils have noticeably different sizes, the suspect may be suffering from a medical disorder or injury that could compromise the results of the test.  The officer will also ask the suspect to remove eyeglasses or contact lenses.

Assuming there is no problem with equal tracking or pupil size, the officer will repeat this process three times, each time checking for a different “clue” of intoxication.  Movement of the stimulus should take approximately two seconds to rotate the eye from the middle to the far side of the eye.  The officer then moves the stimulus for two seconds back to the middle.

During the first rotation, the officer determines whether each eye is able to “pursue smoothly.”  During the second rotation, focused on “distinct nystagmus at maximum deviation,” the officer again moves the object so that each eye has gone as far to the side as possible.  (“Maximum deviation” means that there is no white showing between the iris – the colored portion of the eye – and the corner of the eye.)  This time, however, the officer holds the eye at that position for a minimum of four seconds.   The continued jerking of the eye after two or three seconds could be a “clue” of intoxication.

Finally, checking for the third clue of intoxication, the officer moves the stimulus first to the right at a speed that would require approximately four seconds for the stimulus to reach the edge of the suspect’s shoulder.  The officer seeks to determine whether the onset of the jerking occurs prior to 45 degrees from the starting position of the stimulus.  (If the stimulus were 12 inches from the suspect’s nose, 45 degrees would equal 12 inches out from the nose.)

If the examiner finds that at least four of the six clues have been satisfied, NHTSA research suggests that there is an 88% likelihood that the suspect has a blood alcohol content of 0.08 or greater. HGN might also indicate consumption of seizure medications, phenicyclidine, a variety of inhalants, barbituates, and other depressants.

Challenges to the Horizontal Gaze Nystagmus Test

I read through all the NHTSA materials describing the HGN test.  I also read the 300-page manual used to train D.C. police officers in the administration of the SFST, and I struggled to put this into language a person could easily understand.  I’m not sure I succeeded. The HGN may be, as NHTSA claims, the most accurate of the three tests used in the SFST battery.  But it also the most complicated to understand and the most difficult to administer.  Therein lies part of the problem.

I also took a recent training session for D.C. criminal defense lawyers on administering the SFST.  I sat in a conference room with 100 other lawyers as we watched multiple videos demonstrating the correct techniques for administering the HGN.  The video demonstrations were conducted under ideal laboratory conditions by healthy, rested people who were experts in HGN test administration.  The videos included people that were proven to be intoxicated.  When the instructor asked us how many of us could detect the HGN clues that were present in the subjects captured on video, only a handful of us raised our hands.


Therein lies another part of the problem.  None of us present at the training session were optometrists or opthamologists.  We were not scientists or trained laboratory technicians.  Neither are the D.C. police officers who are required to administer the test.

There are, as I see it, two major problems with using the HGN test as a way to establish probable cause for the arrest of a suspect.

The first problem is with the administration of the test.   The training manual itself states in LARGE CAPITAL LETTERS that if any one of the elements of the HGN test is changed, the validity of the entire test is compromised.  And there are a million ways in which a police officer, one who has received a brief training course and perhaps a two-hour refresher every once in a while and one who is working out on the street late at night in sometimes unlit and hazardous conditions, could deviate from the prescribed procedures.

The officer might, for example, begin with the right eye instead of the left eye.  This, the manual tells us, could compromise the test results.  He/she might hold the stimulus 20 inches or 10 inches from the nose instead of the required 12-15 inches.  This could undermine the test.  Did the officer have an instrument to measure the distance from the stimulus to the nose?  And how is the officer able to gauge the “onset of nystagmus at less than 45 degrees?”  Did the officer happen to have a protractor handy when he or she conducted the test?  Yet, again, each of this conditions is essential to the reliability of the test in predicting intoxication.

As one critic of the HGN, Steven Rubenzer, has pointed out, even when officers are freshly trained and use an apparatus to determine the specific angle of the nystagmus onset, a 6-degree range of error is expected:  “If a 6-point spread is acceptable, one officer may estimate 45 degrees at 42 degrees, another at 48.  If the officers are consistent in their scoring, the first officer will score this clue much less often than the second will.”


How is the officer able to separate other observations about the suspect prior to the administration of the test to prevent these observations from contaminating his/her conclusions about the HGN test?  Conducting the test out on the street, where there are many extraneous lighting and other conditions that could compromise the test’s validity, does not have the same reliability as a “double blind” experiment conducted by experienced researchers.

The second problem has to do with the validity of the HGN in predicting intoxication levels, even assuming that the test was properly administered.  In other words, the test may not be an accurate measure of the very qualities it purports to measure.

Not a single one of the SFST field studies has been published in a peer-reviewed scientific journal.  While I am not a scientist, I know from my years of working at the U.S. Environmental Protection Agency the importance of peer review.  As Steven Rubenzer points out, “[p]eer review exposes the work to the criticism of other researchers and authors who may not share the same beliefs and purposes, and who have the training and experience in valid experimental design.  The scrutiny that this process brings is crucial to detecting error and bias.”

In addition to this general indictment of the SFST, there have been multiple other challenges to the specific methodology used to develop and validate the HGN test.  For example, as at least one Board-certified ophthalmologist pointed out, again as reported by Steven Rubenzer, NHTSA’s recommendation of two seconds across each eye during the “smooth pursuit” component invites rapid intermittent eye movement (the jerking described by the manual) because it requires the eye to move too quickly.  Other ophthalmologists have strongly recommended elimination of the distinct nystagmus at maximum deviation part of the test.  They argue that fatigue can induce nystagmus at maximum deviation in 50 percent of people and that nystagmus persists after blood alcohol contents have fallen to zero.

In short, use of the HGN test provides fertile ground for a healthy challenge from defense counsel representing a person charged in D.C. with DWI, DUI, or OWI.  A future post will discuss similar problems with the walk-and-turn and one-leg stand tests.

2 Comments on “Challenges to Field Sobriety Tests: The Horizontal Gaze Nystagmus Test in Washington, D.C.

  1. Conventional field sobriety tests are designed to determine whether a driver is under the influence of alcohol by measuring the driver’s balance, coordination, and ability to follow instructions. These tests are based on the theory that someone who is intoxicated will not demonstrate the balance, coordination, and ability to follow directions which is required to pass the tests. This theory is seriously flawed, because there are a litany of other potential causes for “failing” these roadside gymnastics.

    In addition to the conventional field sobriety tests, many police officers use the horizontal gaze nystagmus test. This test measures various aspects of involuntary eye movements, by having the DUI suspect follow a stimulus (usually a pen) with his eyes, without moving his head. While it is common knowledge that alcohol consumption can diminish balance, coordination, and the ability to follow directions, the medical phenomenon of nystagmus is outside of the common knowledge of a layperson. Therefore, expert testimony should be required as a condition to the admissibility of HGN related evidence. Massachusetts courts have long required this. See Commonwealth v. Sands, 424 Mass. 184 (1997).

    Brian E. Simoneau
    Mass. OUI Attorney

  2. I think DUI attorneys and cops alike have a hard time determining the results of field sobriety tests. If the person is plastered, the results are obvious. But what about if they’ve just had a couple drinks and the results are inconclusive? What do you do then? Nobody can count backwards from 100, so I”m sure all of us would look drunk trying to do that.

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