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What is the Vertical Gaze Nystagmus Field Sobriety Test?

Ben Michael

If you’ve been arrested for a DWI in Texas, you need to know about the nystagmus tests that traffic safety officers use to determine if a driver is under the influence. 

These tests are simple to administer and take little time. The most popular options are the vertical gaze nystagmus test and the horizontal gaze nystagmus test. 

This post will go over these tests in detail. We’ll cover what they are, the procedures, and the interpretations of the results. We’ll also talk about some of the limitations of the tests and how you can defend against them in court.

What is Nystagmus Testing in Texas?

Nystagmus testing in Texas is a standardized field sobriety test that is given to people suspected of driving while intoxicated (DWI) or driving under the influence (DUI). The test helps law enforcement officers determine if a suspect has alcohol or drug-related impairment.

To perform the test, the officer will ask the suspect to follow a moving object with their eyes, usually a pen, small flashlight, or finger. 

The officer would then observe the suspect’s eyes for any signs of nystagmus—rapid, uncontrolled movement of the eyes—often caused by alcohol consumption.

If an officer notices nystagmus, that’s a sign of intoxication. But other things like being tired or having a medical condition can cause nystagmus. So, the officer will also look at other things, like the suspect’s behavior and speech, when deciding if the suspect is drunk.

They might also take the suspect to the police station for further testing. Texas law enforcement officers usually give a breathalyzer test or a blood test to determine the potential offender’s blood alcohol content (BAC).

Texas considers a person with a BAC of 0.08% or higher to be legally intoxicated.

The Types of Nystagmus Testing

Texas law enforcement officers usually conduct two types of nystagmus tests: horizontal gaze nystagmus and vertical gaze nystagmus tests. Let’s break down what each one means, the steps of the test, and how to interpret the results.

So, let’s begin.

Horizontal Gaze Nystagmus Test (HGN)

If law enforcement officers pull you over on suspicion of driving under the influence, they’ll probably ask you to perform the Horizontal Gaze Nystagmus (HGN) test. It’s the National Highway Traffic Safety Administration (NHTSA)-approved field sobriety test, which makes it the most common.

The test involves an officer moving a stimulus, such as a pen or flashlight, side to side, about 12 to 15 inches away from the driver’s face. They’ll then ask the driver to follow the stimulus with their eyes while the officer observes the eye for nystagmus.

As you already know, nystagmus is when the eyes jerk involuntarily or move back and forth. People who are intoxicated will experience more noticeable nystagmus when they look sideways at a certain angle. 

Law enforcement officers look for these signs to determine if a suspect is sober.

Steps Involved in Horizontal Gaze Nystagmus Testing in Texas

Suspects can challenge the reliability and admissibility of an HGN test in a court. So, law enforcement officers follow strict guidelines to test integrity.

Let’s look at these guidelines. 

Initial observation

In Texas, police officers start a HGN test by looking for signs that a suspect is impaired. They’ll check for slurred speech, open alcohol containers, or the smell of alcohol. 

The officer might also consider the suspect’s age, gender, and weight when deciding whether or not to give a HGN test.

Pre-test interview 

If the officer thinks the suspect is impaired based on their initial observations, they’ll talk to the driver to assess their demeanor and if they can follow instructions. The officer is more likely to administer the test if the driver is acting erratically, belligerently, or is incoherent.

Test instructions

If the officer decides to give the HGN test, they will first explain the test to the driver. They will provide clear and concise instructions, such as:

  • “I am going to ask you to follow my finger with your eyes only. Do not move your head.”
  • “Do not blink or close your eyes.”
  • “Keep your eyes on my finger at all times.”
Equal pupil size check

Before giving the test, the cop will check the suspect’s eyes to ensure the pupils are the same size. If they’re not, the driver may have trouble focusing on near objects. Proceeding with the test could give a false positive result.

Test administration

The officer will then begin the test by holding their finger up in front of the driver’s face, about 12 inches away. They’ll ask the driver to follow the officer’s finger as it moves in various directions.

The officer will continue the test for several seconds while observing for signs of nystagmus. If the officer notices nystagmus, they may ask the driver to repeat the test. If the driver continues to exhibit nystagmus, the officer may conclude that the driver is intoxicated.

Nystagmus observation

The officer will observe the driver’s eye for three clues to determine nystagmus.

  •  Lack of smooth pursuit: The eyes twitch or jerk as they track a smoothly moving stimulus. However, unimpaired people can follow it smoothly.
  • Distinct nystagmus at maximum deviation: The eyes will exhibit more pronounced nystagmus at maximum deviation for more than four seconds. But unimpaired people will only sustain the nystagmus for a few seconds.
  • The onset of nystagmus before 45 degrees: The eyes will experience observed nystagmus before the stimulus reaches 45 degrees for impaired persons.
Recording observations

While observing for nystagmus, the officer will count the number of times the suspect’s eyes exhibit nystagmus during each pass of the testing object. They will then record their observations, noting any impairment during the test.

Interpreting the Results

After the HGN test, the officer will score the suspect.

The HGN test is scored on a scale from 0 to 4, with 0 being no nystagmus and 4 being severe nystagmus. Texas considers a score of 4 to be a positive indication of impairment.

But keep in mind that a positive HGN test result doesn’t always mean the driver is intoxicated. Other factors, like fatigue or a medical condition, can cause nystagmus.

Vertical Gaze Nystagmus Test (VGN)

Vertical Gaze Nystagmus (VGN) is the opposite of Horizontal Gaze Nystagmus (HGN). But it’s less commonly used than the HGN in standard field sobriety testing in Texas.

The test involves an officer holding a small stimulus, like a penlight, about 12 to 15 inches in front of your nose. Then, they’ll move the stimulus up and down, and you’ll follow it with your eyes while keeping your head still. The officer will watch your eyes for any signs of nystagmus.

The test is similar to the HGN, but the officer moves the stimulus up and down instead of side to side.

Steps Involved in Vertical Gaze Nystagmus Testing in Texas

The VGN test in Texas is similar to the HGN test. Here’s a quick breakdown of the steps law enforcement officers follow to ensure test integrity:

  • Initial observation: The officer begins by observing the driver for signs of intoxication.
  • Pre-test interview: Next, the officer will interview the suspect for further signs of impairment, like being erratic, belligerent, or unable to follow instructions or converse coherently.
  • Test instructions: If the officer suspects impairment, they’ll explain the test to the driver and give them the test instructions.
  • Pupil check: The officer will check to see if the driver’s pupils are the same size. If they’re unequal, the suspect may find it challenging to focus on near objects.
  • Test administration: Texas law enforcement officers usually administer the test after the pupil check. They’ll hold the stimulus a few inches from the suspect’s nose and move it up and down for seconds.
  • Nystagmus observation: The officer will look for nystagmus in the suspect’s eyes, looking for signs such as lack of smooth pursuit (problem following the object smoothly), distinct nystagmus at maximum deviation (when the driver looks up or down), and the onset of nystagmus before the eyes reach a 45-degree angle.
  • Observation recording: Finally, they’ll count the number of times the eyes showed nystagmus during each stimulus pass and then record their observations. The officers will meet the arrest criteria if they find four or more clues.

Interpreting the Results

Like in the HGN test, officers score a VGN test on a scale of 0 to 4. 

A score of 0 means no nystagmus, while a score of 4 signals severe nystagmus. Texas law considers a VGN score of 4 as a positive impairment.

The VGN test is less accurate than the HGN test, but law enforcement officers still find it helpful. If an officer suspects a driver is intoxicated, they may administer the VGN test to help them decide.

Categories of Nystagmus and Their Causes

Alcohol is just one of the many causes of nystagmus. Here, we’ll explore the different categories of nystagmus and what can cause them.

Natural Nystagmus

Nystagmus can occur naturally in some people. 

Issues with the vestibular system, which is responsible for balance and eye movement, usually cause it. The condition can be present at birth or develop later in life. It can be mild or severe and can affect one or both eyes.

Though natural nystagmus is not a severe condition, it can be disruptive. It makes it difficult to focus on objects, read, or drive. In some cases, it can cause dizziness or nausea.

The good thing is that those with natural nystagmus usually know they have it. So, if you do, you must tell law enforcement officers before you take any nystagmus test.

Also, natural nystagmus is only visible at a certain gaze angle, not before or after. An experienced officer should be able to tell between natural and alcohol-induced ones.

Physiological Nystagmus

Physiological nystagmus is the eyes’ normal, involuntary rhythmic movement in response to certain stimuli or movements. 

It plays a few roles in normal vision and balance, like keeping your eyes from tiring out when you stare at something for a long time. This nystagmus ensures that light entering your eyes falls constantly on the non-fatigued part of the retina.

It’s challenging to see physiological nystagmus with the naked eye, so there is a slim chance of a law enforcement officer confusing this nystagmus with HGN or VGN.

Several things can cause physiological nystagmus, like fatigue (staring at stationary objects for a long time can lead to fatigue-induced physiological nystagmus) and the vestibular system. 

For example, quickly turning your head can cause your eyes to exhibit a nystagmic response, which stabilizes your gaze and helps you maintain visual fixation.

Positional Alcohol

Alcohol can trigger visible nystagmus at extreme gaze angles in both vertical and horizontal directions. Traffic safety officers in Texas rely on this nystagmus to file DUI or DWI charges. 

The reason is simple.

When someone is under the influence of alcohol, their eyes may move in an uncontrolled manner. Alcohol affects the central nervous system, which controls eye movement. 

The nystagmus is typically more pronounced when the person looks to the side. It may come with other symptoms; for instance having double-vision or difficulty focusing may be symptoms of nystagmus.

If an officer thinks a driver might be driving under the influence, they can ask the driver to do the HGN eyes test. If their eyes show nystagmus, the officer might have a good reason to arrest them for DUI or DWI.

Result from Neural Activity

Neural and muscle activities can induce nystagmus.

It usually occurs in response to the eyes trying to catch up with moving objects. For instance, watching moving scenes, like passing landscapes from the window of a moving car, can trigger nystagmic responses. This rhythmic eye movement helps it track the motion. 

Also, neural activities-induced nystagmus can result from watching an object that displays contrasting moving objects, like white and black spokes on a spinning wheel. Some people can experience slight nystagmus when they gaze at extreme deviation.

So, during HGN or VGN tests, officers look for distinct nystagmus at maximum deviation for at least four seconds. It ensures that neural activity does not cause the nystagmus.

Nystagmus caused by neural activities will go away when the object is no longer moving or is out of sight.

Pathological Disorders

People with pathological disorders like brain damage, brain tumors, or inner ear diseases may experience nystagmus.

Brain damage can disrupt the signals that control eye movement, causing nystagmus. Conversely, brain tumors can cause nystagmus by pressing on or damaging the nerves controlling eye movement. At the same time, inner ear diseases can affect the balance mechanism in the inner ear, leading to involuntary eye movements.

However, it’s unlikely that people with this condition would be driving.

Again, they might experience pendular nystagmus, characterized by quasi-sinusoidal eye oscillation that disrupts visual acuity and causes oscillopsia rather than jerk nystagmus.

Is the HGN or VGN Test Mandatory for Drivers in Texas?

No, taking a field sobriety test is not mandatory in Texas.

However, Texas Implied Consent law presumes that individuals operating a motor vehicle have consented to chemical testing, like breath, blood, and urine tests. So, by driving a vehicle or operating a watercraft in Texas, you have automatically consented to substance intoxication tests if an officer has grounds to test you.

Drug or alcohol intoxication tests often follow an arrest for DWI or if an officer believes you’ve violated any applicable alcohol or intoxication laws.

Refusing to comply to a sobriety test can lead to any of these consequences:

  • A driver’s license suspension for at least 180 days whether a prosecution follows the arrest or not.
  • The refusal can be used as evidence in a legal case.
  • An officer may ask for permission from a court to take urine, blood, or breath samples to test a suspect for intoxication.

Although freely consenting to a sobriety test eases those consequences, it doesn’t eliminate them. For example, a suspect may still lose their license for at least 90 days whether they face prosecution resulting from the DWI arrest or not. Moreover, consenting to a sobriety test may help officers produce evidence to build their case against you.

Conversely, field sobriety tests like HGN and VGN are voluntary in Texas. A driver can refuse to take a field sobriety test without facing the penalties for implied consent. However, an officer may use other observations, like the driving pattern, refusal to take the test, smell of alcohol, or other evidence to establish probable cause for an arrest.

Drivers have the right to legal representation. You can consult with an experienced DWI attorney in Texas to understand your rights and the potential consequences of a DWI charge and to get representation in court.

Legal Consequences for Rejecting the HGN or VGN Test in Texas

Texas has no legal consequences for refusing the HGN or VGN tests. 

However, if you refuse to submit to a field sobriety test, the officer can still choose to have you take a chemical test to determine if you are driving under the influence or while impaired (DWI). 

Like in the field sobriety tests, you can legally refuse to submit to chemical testing. But if you do, you could lose your driver’s license. A first refusal could cause you to lose your license for 180 days and up to two years if you have a prior DWI conviction.

Whether or not you choose to take a field sobriety test is entirely up to you. However, refusing can lead to serious criminal charges and penalties. 

We recommend you exercise your right to remain silent and have an attorney present before answering any questions if arrested for DWI offense.

How to Mount a Defense Against the HGN or VGN Test During Trial

Let’s explore how to help you get the best possible outcome. You can legally challenge the nystagmus test in Texas like any other field sobriety test. Defending yourself against the test involves questioning its reliability, the procedures used, and the qualifications of the administering officer. 

However, you’ll need the help of an experienced Texas DWI attorney to mount a successful challenge. Here’s how a skilled lawyer can help you get the best possible outcome:

  • Cross-examining the officers.
  • Getting expert witness testimonies.
  • Proving medical condition.
  • Looking for inconsistency in the officer’s testimony.
  • Reviewing the field test sobriety test video.
  • Challenging the officer’s qualification.

Cross-examining the officer

The officer’s testimony is often the most crucial evidence in a DWI trial. 

Your attorney will grill them to scrutinize their testimonies, procedures, and evidence. Cross-examination is a legal procedure that helps to expose weaknesses in the prosecution’s case and to discredit the witnesses.

An experienced DWI attorney might grill the officer about:

  • Their training and experience in conducting field sobriety tests.
  • Their observations of the defendant’s behavior during the field sobriety tests.
  • Their procedures for administering the field sobriety tests.
  • Their conclusions about the defendant’s sobriety.

Also, the attorney will try to get the officer to admit mistakes or to make one in the past to discredit them. They may also try to get the officer to admit bias against the defendant, especially if the officer has had prior contact with the defendant.

Getting expert witness testimonies

Expert witnesses can provide credible alternative explanations for observed nystagmus, which can help you challenge the reliability and accuracy of the nystagmus tests. 

Your attorney may engage forensic toxicologists or medical professionals specializing in nystagmus to educate the jury on the limitations of nystagmus tests.

For example, a forensic toxicologist may be able to testify that a variety of factors other than intoxication, such as fatigue, stress, or certain medications, can cause nystagmus. A medical professional specializing in nystagmus can testify that nystagmus can be a normal variation in eye movement and that it is not always indicative of intoxication.

With these testimonies, your attorney may be able to cast doubt on the credibility of the nystagmus tests and, ultimately, help to win your case.

Proving medical condition

Certain medical conditions can cause nystagmus that’s unrelated to drinking alcohol. If you have this condition or are on medication when you take the test, your attorney can use that information to help you challenge the test and dismiss the charges.

They do this by demonstrating how the medical condition could have affected the driver’s performance during the test or contributed to the observed symptoms of impairment.

The steps your attorney may take to prove your medical condition include the following:

  • Obtaining and tendering relevant medical reports, including records of diagnoses, treatments, and medications
  • Engaging a qualified medical doctor or specialist to testify about your condition
  • Providing evidence of prescription medications and the potential side effects
  • Tendering a detailed history of your treatment, including any surgeries, therapies, and ongoing treatment, to build a comprehensive picture of your health

Looking for inconsistency in the officer’s testimony

Proving that an officer’s testimony is inconsistent can be a powerful tool for building a solid defense against a DWI charge. It can undermine the credibility of the prosecution’s case and make it more likely for the jury to dismiss or reduce the charge. 

And one common way is to show that the officer’s story has changed over time. For example, if the officer’s initial incident report differs from their testimony at trial. Another way is to show that other evidence in the case contradicted the officer’s testimony. 

Your attorney can also prove inconsistency by showing that the officer’s testimony is not believable. For instance, they can show that the officer has a history of lying, making misleading statements, or being biased against you.

Reviewing the field sobriety test video

The prosecution will probably tender the field sobriety test video as evidence. 

Getting a copy of the video and reviewing it carefully for procedural errors or constitutional violations is essential. For example, the officer may have failed to properly advise the defendant of his or her rights, or the test may have been conducted unfairly. 

Also, with the video, your attorney may be able to dispute the officer’s interpretation of the nystagmus test. The attorney may be able to prove that the observed impairment was due to other factors like fatigue or medical condition and not caused by intoxication. 

Your attorney may also challenge the credibility of the test if the officers don’t produce a video recording of the test to support their claims.

Challenging the officer’s qualification

You can defend against DWI charges by challenging the officer’s qualifications to administer the field sobriety test. In Texas, officers must have current certification in field sobriety testing to administer the test. 

Your attorney can ask to see their certification. If the officer is not certified to administer a field sobriety test or has expired certification, the test results may not be allowed in court.

They can also challenge the officer’s conduct during the field sobriety test. For example, if the officer was rude or unprofessional or did not follow the proper procedures for administering the test, the test results may be called into question.

Nystagmus Testing: Why It Shouldn’t Be Overlooked

Several studies have validated field sobriety tests as effective for determining whether a suspect is driving under the influence of alcohol or while impaired.

The National Highways Traffic Safety Administration (NHTSA), through a contract with the Southern California Research Institute (SCRI), conducted extensive scientific research between 1975 and 1983 to determine the effectiveness of these tests.

The laboratory tests show that HGN is 77% accurate when administered alone in a standardized manner and 80% accurate when combined with Walk-and-Turn testing.

NHTSA conducted three standardized field sobriety tests between 1995 and 1998 in Colorado, Florida, and San Diego to validate the research findings. All the tests reported that the officers correctly made over 90% of arrest decisions, validating the SFSTs.

Additional Field Sobriety Tests and Essential Information to Be Aware Of

Horizontal Gaze Nystagmus (HGN) and Vertical Gaze Nystagmus (VGN) tests are not the only field sobriety tests in the United States. Let’s explore the other popular ones in Texas.

Walk-and-Turn Testing

This field sobriety test involves a suspect taking nine heel-to-toe steps along an imaginary straight line, then turning around and taking nine steps back. It requires the driver to keep their arms at their sides and count their steps out loud as they perform the test.

The test consists of two phases: the instruction phase, where the officer explains how the test works, and the performance phase, where the suspect takes the test based on the instruction.

During the performance phase, the officer looks out for clues like:

  • Starting the test during instruction.
  • Using the arms for balancing.
  • Stepping off the line.
  • Taking an incorrect number of turns
  • Stopping while walking
  • Staggering while walking or turning incorrectly. 

One-Leg Stand Testing

The test requires suspects to raise one foot about six inches above the ground and count out loud for approximately 30 seconds. And their arms must be at their side throughout the test.

Officers look out for clues like:

  • Using the arm to maintain balance.
  • Inability to complete the test.
  • Swaying or hopping to maintain.

The test only works for people who don’t need help walking or balancing properly. They can fail the test even if they’re not impaired or under the influence.

The Best Strategies for Challenging the HGN or VGN Field Sobriety Test in Court in Texas

Getting charged with a DWI in Texas doesn’t mean your life is over. An experienced DWI attorney can help you understand your rights and options and fight for the best possible outcome. Working with them can help you successfully challenge the test in court.

And these strategies can help:

  • Thorough case review
  • Expert witnesses
  • Educating the jury
  • Legal precedents

Now, let’s explore them in detail.

Thorough case review 

A good defense starts with understanding the charges against you.

So, review the case thoroughly, including the police report, witness statements, and arrest records. Look for procedural errors or constitutional violations. These can help you strengthen your case, giving you a better chance of walking out of the courtroom as a free person.

Some of the specific things to look for when reviewing the case include:

  • Procedural errors. Did the police follow the proper procedures when they arrested you? Were your Miranda rights read to you? If the police made a procedural error, it could be grounds for dismissal of the charges against you.
  • Constitutional violations. Did the police violate your constitutional rights during the investigation or arrest? For example, did they search you without a warrant? Did they interrogate you without an attorney present? 

Expert witnesses

Get an expert witness on your side to help you out. They can help you develop solid counterarguments against the prosecution’s claims, especially about the results of your VGN or HGN tests.

For example, your doctor could write a medical report that shows you have a medical condition that could cause the results you got.

VGN and HGN tests can measure your ability to coordinate your eye movements. 

If you fail these tests, it can be used as evidence that you were driving under the influence. However, several factors can affect your ability to perform these tests, including fatigue, medication, and medical conditions.

An expert witness can help you explain these factors to the jury and show that your VGN or HGN results were not necessarily caused by alcohol consumption. 

For example, if you have a history of migraines, your doctor could testify that a migraine attack could have caused your VGN results.

Educating the jury

In addition to using expert witnesses, you can educate the jury on the limitations of field sobriety tests to strengthen your defense. For example, your attorney can point out how not following standardized procedures can affect the reliability of the test. 

For instance, if the officer did not correctly instruct you on how to perform the test or if they did not observe you closely while you were taking the test, then the results may not be accurate.

Your lawyers can also educate the jury on the other causes of nystagmus, like fatigue, stress, and certain medical conditions. 

Suppose the jury is aware of the other nystagmus causes. In that case, they may be less likely to believe that you were intoxicated simply because you exhibited nystagmus during the field sobriety test.

Legal precedents

Review recent court cases that have dealt with VGN or HGN tests to see how you should approach your case. Build your defense on precedents that limit the use of these tests or require specific procedures for their administration.

Several court cases have dealt with the admissibility of VGN and HGN tests in the past few years. 

One key precedent is the case of Birchfield v. North Dakota, in which the Supreme Court ruled that breath tests are more reliable than field sobriety tests, such as VGN and HGN, and, therefore, should be the preferred method of testing for DWI.

Speaking with an experienced DWI attorney is essential if you’re facing DWI charges. 

That’s where Michael and Associates comes in. 

We can help you build a strong defense against your charges and get them dismissed. Our attorneys have won over 1,000 criminal defense cases with a 98% success rate. We have the experience to help you navigate the legal system and get the best possible outcome. Reach out to us for a free case review.

Ben Michael

Ben has vast experience in defending criminal cases ranging from DWIs to assault, drug possession, and many more. He has countless criminal charges dismissed and pled down. Among many other awards, one of the Top 10 Criminal Defense Attorneys in Texas and winner of Top 40 under 40.

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