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Horizontal Gaze Nystagmus Test, or HGN Test, Used in a DUI

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When facing a DUI stop in Arizona, one of the key assessments law enforcement may utilize is the Horizontal Gaze Nystagmus (HGN) Test. A cornerstone in DUI investigations, the HGN test’s validity and administration are as critical as they are complex. This evaluation is more than a roadside inconvenience; it’s a scientifically rooted procedure that carries significant weight in the determination of impairment.

The HGN test is specifically designed to detect impairment by observing the involuntary jerking of the eyes—a phenomenon known as nystagmus.

Here in Arizona, police officers trained in its administration use this test to seek out subtle signs of intoxication that may not be otherwise apparent.

In the following sections, we will examine the mechanics of the HGN test, scrutinize its reliability, and consider the factors that can influence its outcomes. An exploration of Arizona Revised Statute will also be provided to contextualize the legal framework surrounding this test.

 According to research by the Southern California Research Institute, the HGN test has a reported accuracy of 77% in identifying individuals with BAC levels at or above 0.10. However, this leaves a noteworthy margin for false positives and interpretation errors that must be carefully considered in legal proceedings.

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What is the Horizontal Gaze Nystagmus Test and How Officers Use it for a DUI?

What is the HGN Test, or Horizontal Gaze Nystagmus Test?

If you are pulled over for DUI in Arizona, and an officer suspects that you may have been drinking, chances are the officer will administer the Horizontal Gaze Nystagmus test.

Mainly referred to by its acronym, the HGN test is part of a battery of three standardized field sobriety tests used by police officers in Arizona and throughout the United States for DUI investigations.

The HGN test was developed by the National Highway Traffic Safety Administration (NHTSA) as a tool for officers to detect impaired drivers. This test consists of an officer moving a stimulus, most often a pen or their finger, back and forth across your face as they watch how your eyes track the stimulus.

Police officers are looking for nystagmus, which is an involuntary jerking of the eyes.

Officers are taught during training that alcohol and certain drugs will cause pronounced nystagmus that is visible to the human eye. We will often hear officers tout that the HGN test is the most important and reliable test of the three-test battery of field sobriety tests.

Officers will often say that a particularly athletic person may be able to pass the balancing, or divided attention, field sobriety tests, but since nystagmus is involuntary, a person will not be able to compensate for it by being athletic.

In fact, we recently reviewed body camera footage of an officer telling our client that “the eyes don’t lie,” something that the officers are told in training. Unfortunately, it seems that particular officer didn’t learn much more than that phrase during his training, as he called the test the horizontal “guise” nystagmus test and his administration of the test was not done in a standardized manner, making his findings questionable at best.

Although the NHTSA standardized field sobriety test, or SFST, training manual states that the techniques of the tests can readily be grasped by “anyone of average competence,” officers are often quick to overstate the rigor of their training on the stand.

Many of our clients will tell us that they believe they “passed” the field sobriety tests, only to learn that the officer wrote in his report that the person showed multiple clues of impairment.

Part of the misunderstanding is that the officers do not tell you how the tests are graded. If the officer tells you to follow a pen with your eyes, and you are able to do that, you may believe that you passed the test. What the officer is really looking for are three clues for each eye, for a total of six clues on the HGN test.

The clues officers look for in an HGN Test are:

  • Lack of Smooth Pursuit
  • Distinct and Sustained Nystagmus at Maximum Deviation, and
  • Onset of Nystagmus Prior to 45 Degrees.

Lack of Smooth Eye Pursuit

During this portion of the test, the officer is looking at a person’s eyes to see if they follow the stimulus smoothly or if there is a jerking or bouncing motion as the pupils move across the horizontal plane.

Officers will describe a lack of smooth pursuit as similar to windshield wipers skipping across a dry windshield.

To look for lack of smooth pursuit, the officer must start with the left eye and hold the stimulus approximately 12-15 inches from the subject’s nose. The stimulus is also supposed to be held slightly higher than the subject’s eye level.

The officer starts with the stimulus in the center of the subject’s face and then moves the stimulus out beyond the subject’s shoulder. This should take approximately two seconds.

Next, the officer moves the stimulus across to the other shoulder, keeping a distance of 12-15 inches in front of the subject’s face. This “pass” should take approximately 4 seconds.

The officer must do two passes for each eye to look for lack of smooth pursuit. If lack of smooth pursuit is present in each eye, the officer marks 2 clues.

Distinct and Sustained Nystagmus at Maximum Deviation 

For this clue, the officer will move the stimulus from the center of the subject’s face and out to the side until the eye is looking as far to the side as possible. The officer should not be able to see any white between the iris and the outside corner of the subject’s eye.

The officer must hold the eye at that point for a minimum of four seconds, but not longer than 10 seconds.

The officer is looking for “distinct” and “sustained” nystagmus, which is somewhat subjective and not clearly defined.

If the officer determines that the nystagmus is distinct and sustained, then the officer will mark down 2 clues, one for each eye.

Onset of Nystagmus Prior to Forty-Five Degrees

During this portion of the test, the officer is looking to see if nystagmus is present prior to bringing the eyes to a 45-degree angle from looking straight forward. The officer will estimate this 45-degree angle by moving approximately 15 inches to the side starting from the center of the face.

The officer should still see some white between the iris and the corner of the subject’s eye and the stimulus should be slightly beyond the subject’s shoulder at 45-degrees.

It should take the officer approximately four seconds to move the stimulus to slightly beyond the shoulder. If the officer sees nystagmus prior to 45-degrees, he will mark a clue for each eye.

How Many Clues Does It Take to Fail the HGN Test?

There are a total of 6 clues on the HGN test, 3 clues for each eye. Officers will tell you that this is not a “pass or fail” test. However, at a certain number of clues they will consider you impaired—and arrest you—which sure sounds like “failing.”

An officer will consider a person impaired, and above the legal limit of .08, if they exhibit 4 or more clues on the test.

The clues on the HGN test are cumulative, meaning that they should be exhibited in order. Distinct and Sustained Nystagmus at Maximum Deviation should only be present if the officer also observed a Lack of Smooth Pursuit.

Similarly, Onset of Nystagmus Prior to 45-Degrees should only be present if the officer also saw the first 4 clues. On the subject of clues, it is possible for someone to have an odd number of clues (1, 3, or 5) if they only have one eye or if they have a serious medical impairment.

Vertical Nystagmus – Not One of the Six Clues

You may recall that after the officer moved the stimulus from side to side, that he raised the stimulus up above your head in a vertical manner. Although vertical nystagmus is not one of the 6 clues, officers will often check for vertical nystagmus at the end of the HGN test.

Vertical nystagmus, much like horizontal gaze nystagmus, is an involuntary jerking of the eyes. However, it is present when the eyes are looking up, rather than to the left or right.

An officer will claim that the presence of vertical nystagmus is an indicator that the subject is extremely intoxicated, meaning they consumed a large amount of alcohol for their individual tolerance.

What Can Affect the Reliability of the HGN Test?

There are a number of issues that can occur during the administration of the HGN test. First, prior to beginning the test, the officer should ask the subject about any medical issues that could affect the reliability of the test.

The officer should ask the person if they have had any head or eye injuries. If a person was involved in a car accident, there is a possibility that they sustained a head injury, especially if the airbags deployed.

Yet, in many cases, an officer will still attempt to conduct the HGN test. An officer should also inquire as to whether the person has been diagnosed with natural or resting nystagmus prior to attempting to administer the HGN test. If a person’s eyes do not track equally, or their pupils are unequal in size, this could be the result of a serious medical issue and the officer should call for medical assistance rather than conducting the HGN test.

It is important to note that there are 47 types of nystagmus. Officers are taught that alcohol, inhalants, PCP, and other central nervous system (CNS) depressants such as Xanax will enhance nystagmus.

However, there are many other causes of nystagmus that are not attributed to alcohol or drugs. In fact, it is commonly said that everyone has nystagmus, but to such a small degree that it cannot be seen by the human eye.

Some other causes of nystagmus include

  • inner ear problems
  • caffeine
  • nicotine
  • aspirin
  • head trauma
  • eye muscle fatigue
  • antihistamines
  • hypertension
  • sun stroke
  • some prescription medications

It is possible that someone could exhibit nystagmus from something entirely unrelated to alcohol or illicit drugs.

The way that the HGN test is administered can also affect the reliability of the tests. The officers are trained to turn off their emergency lights prior to conducting the test so that they are not visible to the subject. Similarly, they are told to face the subject away from rapidly moving traffic.

Rotating lights, strobe lights, and rapidly moving traffic can cause a person’s eyes to momentarily fixate on the rapid movement as it comes in and out of the person’s field of vision.

This momentary fixation is called optokinetic nystagmus, which an officer could mistake for horizontal gaze nystagmus.

If an officer does not follow the standardized instructions for the HGN test, they may not be able to properly view nystagmus. For instance, if the officer’s passes across the body are shorter than 4 seconds, the officer would not have viewed the eyes for long enough to determine whether there was a lack of smooth pursuit.

If the officer holds a person’s eyes at maximum deviation for longer than 10 seconds, there is a risk of eye muscle fatigue, which can cause nystagmus.

How Accurate is the HGN Test?

From 1977 to 1983, NHSTA commissioned the Southern California Research Institute (SCRI) to conduct studies on the field sobriety tests, including the HGN test.

Per the SCRI studies, the HGN test was supposedly only 77% accurate in determining BACs at or above .10. Since that time, NHSTA has modified its findings to fit the legal limit of .08.

Even so, NHTSA’s own research showed that the test was inaccurate 23% of the time. Further studies have been commissioned by NHTSA, with the most recent study being the 1998 San Diego study.

This study boasts that the HGN test is 88% accurate. This statistic, however, ignores the underlying data that points to a rate of false positives much higher than the purported 12% rate of inaccuracy.

Can You Beat the HGN Test?

While officers are trained to parrot the idea that the HGN test is the most reliable and accurate field sobriety test that they administer, there are a number of pitfalls that can discredit the results of the test.

From alternative causes of nystagmus, to picking apart the “research studies” that have been conducted, the Phoenix DUI attorneys at Colburn Hintze Maletta, PLLC, have successfully cross-examined countless police officers and expert witnesses on the subject of HGN.

We have extensively researched the subject of HGN, from analyzing the police departments’ own NHTSA standardized field sobriety test manuals (for both the instructors and the students), to reading and scrutinizing the studies that have been conducted regarding the HGN test, to consulting with medical experts and toxicologists, the attorneys at CHM Law are well equipped to attack the findings of an HGN test.

Not only have we done our research, but we have put our strategies to the test during numerous DUI jury trials with successful results.

Our Phoenix DUI lawyers have even been sought out by other Arizona criminal defense attorneys for advice and tactics to rebut the HGN test during trial.

While every case is unique and past performance is not a guarantee of future success, Colburn Hintze Maletta, PLLC, has the tools and experience to go up against even the best DUI officers and prosecutors. Call our Phoenix DUI lawyers at (602) 825-2500 for a confidential and free consultation.

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