By: DUI Lawyer Cory Yager, Ex-cop with over 1000 DUI arrests, FTO (field training officer) and HGN Certification (participant and instructor)
You get pulled over. An officer asks you to follow a pen with your eyes. Then you are told you are too drunk to drive – even though you have not had a single drink. This shows how unreliable the HGN test can be.
The Horizontal Gaze Nystagmus (HGN) test is a standard field sobriety test used by law enforcement to detect signs of intoxication. But how accurate is it? Can it be challenged in court?
Later in this article, you will see a video showing the HGN test being performed by an officer. You can understand exactly how it works and what to watch for during a DUI stop.
What Is Horizontal Gaze Nystagmus (HGN)?
Horizontal Gaze Nystagmus is an involuntary, rhythmic jerking of the eyes. It occurs when the brainstem and vestibular system struggle to focus the eyes. HGN is a neurological reflex, not just a loss of motor control. While alcohol can increase nystagmus, many other factors can trigger it.
Nystagmus can occur in different directions: horizontally (side-to-side), vertically (up-and-down), or in a pendular motion. The HGN test measures horizontal nystagmus only.
HGN becomes more pronounced when someone is stressed, tired, or intoxicated. Alcohol slows down neural processing in the brainstem and cerebellum. This impairs the vestibulo-ocular reflex and smooth pursuit eye movements. Alcohol affects eye movements in three ways:
How Alcohol Impacts Eye Movements and Nystagmus
Alcohol slows the brain's ability to process eye movement signals. This results in jerky, delayed tracking – the ability of your eyes to follow an object as it moves, like a pen during a sobriety test.
Neurotransmitter function is also compromised, leading to impaired coordination.
Cerebellum interference occurs because your cerebellum helps fine-tune muscle movements, including those in the eyes.
When someone is intoxicated, HGN becomes more noticeable when following a stimulus like a pen or flashlight.
Non-Alcohol Causes of Nystagmus: Medical and Environmental Factors
Your eyes can still show nystagmus even if you have not had any alcohol. Here are common non-alcohol causes that could trigger a false positive during a DUI stop:
Neurological Conditions: Multiple Sclerosis, stroke, or brainstem injuries can cause permanent nystagmus.
Inner ear disorders: Vestibular neuritis can trigger eye jerking similar to HGN.
Medications: Anti-seizure medications, antidepressants, and antihistamines can trigger HGN.
Fatigue: Tiredness can cause temporary nystagmus-like eye movements.
Bright lights: Flashing lights or rapidly changing brightness can induce nystagmus. Officers may mistake this for alcohol-related impairment.
Visual impairments: Some vision problems can also cause nystagmus-like movement.
Failing an HGN test does not automatically mean someone is intoxicated. Many non-alcohol-related conditions can cause false positives.
How HGN Differs From Other Field Sobriety Tests
The HGN test only looks at how your eyes move. Unlike other tests, it checks a reflex your body cannot control. You cannot deliberately hide the effects of alcohol on your eyes the way you might compensate for poor balance during a walk-and-turn test.
The HGN test differs from other Standardized Field Sobriety Tests because it is a psycho-physical test. The Walk-and-Turn and One-Leg Stand tests are divided attention tasks.
Throughout training, officers learn that the HGN test is the most "scientific" of the three standardized tests. Yet its reliability has been widely debated. Officer errors, medical conditions that mimic nystagmus, and the lack of correlation between HGN and actual driving impairment all raise concerns.
The Origins of the HGN Test: How It Became a DUI Tool
In the 1970s, Dr. Marceline Burns proposed her dissertation on uniform, reliable tests. Her goal was to help police officers and support DUI arrests. The National Highway Traffic Safety Administration (NHTSA) liked her idea and funded her research. But her studies were never independently reviewed.
NHTSA was established in 1970 to improve road safety and reduce drunk driving deaths. The agency recognized the need for field sobriety tests with standardized procedures. NHTSA funded Dr. Burns to determine whether eye movement patterns could indicate alcohol impairment. They could. She then received every federal funding contract for multiple studies to support the scientific basis for such tests.
Burns' research led to the creation of the Standardized Field Sobriety Tests. This also included the Walk-and-Turn Test and the One-Leg-Stand Test.
Critics raised vast concerns about the HGN test. Its administration is subjective and variable. Despite these criticisms, NHTSA endorsed the HGN as valid for identifying impaired drivers. To read more about myths surrounding the history of the Standardized Field Sobriety Tests, click here.
How Prosecutors Use the HGN Test in DUI Trials
Prosecutors welcomed "something new and more scientific." In 1999, The American Prosecutors Research Institute published a detailed guidebook. It was called The Horizontal Gaze Nystagmus: The Science and the Law. This book provided clues for prosecuting intoxicated driving cases. It included several lists of questions to ask different types of defense experts.
The Prosecutor's guide has a major flaw. The book relied heavily on NHTSA-backed studies. No independent research was conducted. Prosecutors assumed that no criminal defense attorneys would successfully challenge field test evidence. Also, the report cites favorable, pro-prosecution case law as a guide sheet for other prosecutors.
Prosecutors frequently use the HGN to establish probable cause for DUI arrests. Yet its accuracy and reliability have been widely debated in courtrooms across the United States.
The officer may claim:
You established six out of six clues of impairment.
Nystagmus was clearly visible before 45 degrees, indicating intoxication.
The test was administered according to standardized police procedures.
Court Challenges to HGN: Why Its Reliability Is Questioned
Many courts have ruled against using the HGN test as DUI evidence. Since that guidebook was released, dozens of court opinions have disavowed unsubstantiated police claims. Many have overturned convictions when judges failed to act as gatekeepers of proper forensic evidence at trial. Almost all appellate losses for the state resulted from faulty administration of the HGN evaluation.
The HGN DUI test is one hundred percent voluntary. Detained drivers should never agree to or attempt it. The key reason not to take the police gaze test is that improper administration can induce nystagmus.
Officers receive no formal medical training. In most states, the person administering the test – which is a medical test – only needs a high school diploma or GED. Entry-level police hiring standards confirm that a recruit must have his or her high school GED or diploma to qualify, with no medical or optometric coursework required. You can be arrested based on a botched effort to assess your eye movements.
Even Dr. Burns concedes, in a recorded excerpt from a California DUI seminar, that the NHTSA three-test battery cannot predict alcohol impairment. Based on medical college information cited below, the most common source of error is holding the stimulus too high, which causes nystagmus.
"The eyeballs are controlled by three pairs of extrinsic muscles. The superior and inferior recti muscles control vertical movements. The lateral and medial recti muscles move the eyeball side-to-side. The superior oblique muscle moves the eyeball down and to the side. The inferior oblique muscle moves the eyeball up and to the side."
The 6 Clues for HGN: What Police Look For
Understanding the clues for HGN is essential for anyone facing a Georgia DUI charge. Police officers score six total clues for HGN – three per eye – during the roadside evaluation. Under the NHTSA manual, an officer who observes four or more clues out of six may conclude the subject's BAC is at or above 0.08. However, that conclusion is valid only when the test is administered in strict compliance with the manual's procedures.
Here are the six scored clues, broken down by eye and by what the officer watches for in each pass:
Lack of smooth pursuit – left eye. The officer moves the stimulus from the center of the subject's face toward the left ear over roughly two seconds, then back. The officer watches whether the left eye tracks the stimulus in a smooth, fluid motion. If the eye bounces or jerks as it follows the stimulus – like a ball rolling over gravel instead of glass – the officer scores this clue. Even slight, irregular jerking counts. Officers who move the stimulus too fast can artificially create the appearance of jerky tracking, so speed control matters.
Lack of smooth pursuit – right eye. The same pass is repeated toward the right ear. The officer watches the right eye for the same bouncing or jerking movement during tracking. Two complete passes per eye are required under NHTSA standards. An officer who only makes one pass per eye has not followed the protocol. This may undermine the reliability of this clue in court.
Distinct and sustained nystagmus at maximum deviation – left eye. The officer moves the stimulus to the far left until no white is visible on the outside of the left eye. The officer holds the stimulus at that maximum position for a minimum of four seconds. If the eye shows distinct, sustained jerking throughout that hold, the clue is scored. Fatigue nystagmus – a natural response – can appear when the eye is held too long. Holding past four seconds may produce a false positive.
Distinct and sustained nystagmus at maximum deviation – right eye. The officer repeats the hold at the far right of the right eye for a minimum of four seconds. The same distinct, sustained jerking must be present. If the officer holds the position for fewer than four seconds, the officer did not complete the pass under the manual, and that gives the defense a basis to challenge the result. Two passes per eye are required for this clue as well.
Onset of nystagmus prior to 45 degrees – left eye. The officer slowly moves the stimulus from the center toward the left, taking roughly four seconds to reach the 45-degree angle. If the eye begins jerking before reaching that 45-degree point, the clue is scored. This clue most directly correlates with higher BAC levels. It is also the clue most frequently administered incorrectly. Officers often move the stimulus too quickly or misjudge the 45-degree angle. This leads to scoring errors.
Onset of nystagmus prior to 45 degrees – right eye. The same slow pass is repeated toward the right. The officer watches for the onset of jerking before the 45-degree mark. Two passes per eye are required. Because 45 degrees is an estimate based on the subject's facial structure, the angle measurement is inherently subjective. This creates a strong area for cross-examination.
Just because an officer claims to observe nystagmus does not automatically prove intoxication. Medical conditions, medications, fatigue, and officer error can all produce the same eye movements that alcohol would cause.
HGN Test Steps: The Full NHTSA Procedure
The HGN test steps follow a strict sequence outlined in the NHTSA manual. Each step requires precise timing, distance, and technique. When officers skip steps or rush the process, the test results become unreliable and challengeable in court.
The full NHTSA-standardized HGN test steps proceed in this order:
Pre-test medical questions and verbal instructions. Before touching a stimulus, the officer should ask the subject about head injuries, neurological conditions, medications, and whether the subject wears contact lenses. The officer then instructs the subject to stand with feet together, hands at sides. The subject must follow the stimulus with eyes only while keeping the head still.
Positioning the stimulus. The officer holds the stimulus (pen, penlight, or fingertip) 12 to 15 inches from the subject's nose and slightly above eye level. Holding the stimulus too high engages the wrong eye muscles. This has nothing to do with alcohol.
Checking for equal pupil size and resting nystagmus. The officer observes whether both pupils are the same size. The officer also checks whether either eye shows nystagmus while looking straight ahead with no stimulus movement.
Checking for equal tracking. The officer moves the stimulus across the subject's full visual field to confirm both eyes track together. Unequal tracking may indicate a medical condition rather than impairment.
Checking each eye for lack of smooth pursuit. The officer makes two complete passes per eye, moving the stimulus smoothly from center to side over approximately two seconds, then back. The officer watches for jerky or bouncing eye movement.
Checking each eye for distinct and sustained nystagmus at maximum deviation. The officer moves the stimulus to the far edge of each eye and holds it there for a minimum of four seconds. Two passes per eye are required.
Checking each eye for onset of nystagmus prior to 45 degrees. The officer moves the stimulus slowly toward the side, taking roughly four seconds to reach the 45-degree angle. Two passes per eye are required. This is the pass that officers most frequently perform incorrectly.
Optional vertical gaze nystagmus (VGN) check. This unscored step was added by the IACP. The officer moves the stimulus up and down twice to look for vertical nystagmus.
Remember, the HGN test is voluntary in Georgia. You can refuse to participate. The standardized procedures described in the NHTSA field sobriety test manual exist because deviations from them compromise the test's validity.
Pre-Test Instructions and Medical Screening
Before scoring any clues, the officer should conduct a medical screening. This includes asking whether the subject wears hard contact lenses, which can affect eye tracking. The officer should also ask about head injuries, brain damage, neurological conditions, and any medications the subject takes.
The subject should remove eyeglasses. The officer should instruct the subject to stand with feet together and hands at their sides. The officer should then say something like: "I am going to check your eyes. Keep your head still and follow this stimulus with your eyes only."
Skipping these pre-test steps is one of the most common defense arguments in Georgia DUI cases. When an officer fails to screen for medical conditions that cause nystagmus, the entire test result may be called into question.
Checking for Resting Nystagmus
The resting nystagmus check is a key starting point. The officer's purpose is to check for "resting" nystagmus, meaning the eyes move even when no stimulus is present. The NHTSA manual does not advise officers to stop testing if resting nystagmus is detected. This could lead to false positives.
Ensuring Equal Tracking
The officer must move the stimulus twice for each eye. The officer observes whether both eyes move in sync. Two passes for all stimulus movements apply to every HGN step.
Understanding horizontal gaze nystagmus clues is important. The first scored test is lack of smooth pursuit, followed by checking for distinct and sustained nystagmus at maximum deviation. The distinct nystagmus at maximum deviation must be continuous for four seconds or longer at each endpoint.
Checking Each Eye for Lack of Smooth Pursuit
The first scored pass of the HGN test evaluates smooth pursuit. The officer moves the stimulus smoothly from the center of the subject's face toward one side over roughly two seconds, then back to center over another two seconds. Each complete pass takes about four seconds total. Two passes per eye are required under NHTSA standards.
The officer watches to see whether the eye follows the stimulus in a fluid, continuous motion – like a marble rolling across a sheet of glass. If the eye bounces or jerks during tracking – like a windshield wiper stuttering across a dry windshield – the officer may score a lack of smooth pursuit clue.
Moving the stimulus too fast is one of the most frequent officer errors. A rushed pass can cause the eye to appear jerky even in a completely sober person. Dashcam and body camera footage often reveals this mistake.
Checking for Distinct and Sustained Nystagmus at Maximum Deviation
The second scored pass measures nystagmus at the extreme edges of the subject's gaze. The officer moves the stimulus to the far corner of each eye until no white sclera is visible on the outside edge. The officer holds it there for a minimum of four seconds. Two passes per eye are required.
The officer looks for distinct, sustained jerking that continues throughout the entire hold. Brief, fleeting movement does not count.
Fatigue nystagmus is a natural reflex that can appear when the eye is held at maximum deviation for too long. If the officer holds the stimulus beyond four seconds, this natural response may appear. It could be mistaken for alcohol-related nystagmus. On the other hand, holding the position for fewer than four seconds means the officer did not complete the pass correctly under NHTSA standards. This is another basis for challenging the results.
Onset of Nystagmus Before 45 Degrees
Finally, the third set of passes looks for the onset of nystagmus prior to 45 degrees. This is the pass that 90 percent of officers perform incorrectly, even when they substantially comply with the first two sets.
What Is Vertical Gaze Nystagmus (VGN)?
In recent years, the IACP added a step of conducting an unscored vertical gaze nystagmus test. The officer looks for signs of VGN by moving the stimulus up and down, twice.
What does vertical nystagmus indicate? If VGN is present, it is supposed to mean either a high dose of alcohol or that PCP or dissociative anesthetics are in the person's system.
The VGN test is not scored. It was never included in Dr. Burns' original research studies and was not scientifically validated.
The 3 Biggest Police Errors That Create False Positives in HGN Tests
Police officers in Georgia use this field sobriety test at the roadside. They use it to bolster their probable cause to arrest suspected drunk drivers.
Holding the target too high. The most common police mistake is holding the target too high. The stimulus should be at eye level or slightly above. Holding it too high engages the wrong eye muscles. The examination becomes compromised for accuracy. The muscles that control side-to-side eye movements will not pull back and forth smoothly.
Speed of the stimulus. Another common error is moving the pen, flashlight, or fingertip too fast. When moved too quickly, these faster passes trigger nystagmus. This can happen even if the person has not consumed alcohol.
Height and distance of the stimulus. Holding the stimulus too close or too far compromises test reliability. The validation studies used 12 to 15 inches from the face as the proper distance and positioning.
Video: Watch the Officer Perform the HGN DUI Test
The video below shows a Georgia State Patrol officer demonstrating how to properly perform the HGN DUI eye test and the signs officers analyze during the HGN.
Why the HGN Test Alone Cannot Prove DUI
Many officers fail to adhere to the precise testing procedures needed to ensure reliable HGN test results. They may not hold the pen at the right height, move it too fast, or misjudge the angles. This makes the HGN test prone to false positives. Because of these factors, the HGN test alone is not enough to prove intoxication in court. The state still has to prove impairment beyond a reasonable doubt, and the penalties that follow a conviction under Georgia DUI law attach only if it carries that burden.
Talk to a Georgia DUI Lawyer About Your HGN Test
A defense attorney who understands the NHTSA HGN test steps and clue scoring criteria can review your police report and any available video footage. An attorney can identify specific officer errors. Deviations from the manual include holding the stimulus too high, moving a pass too fast, or holding a maximum deviation position for fewer than four seconds. Each deviation becomes a point of cross-examination. This may weaken the prosecution's case.
The attorneys at Kohn & Yager LLC are NHTSA field sobriety test instructors. They know precisely how the HGN test should be administered. They have taught the procedure themselves. That training-level knowledge allows them to question arresting officers on the exact deviations from the standardized protocol. Those deviations could have produced unreliable results in your case.
In Georgia, HGN evidence is admissible without expert testimony to lay a foundation first. The Georgia Court of Appeals settled that in Hawkins v. State, 223 Ga. App. 34 (1996). The same decision also marks the limits of the evidence. The court treated the tests' lack of specific, quantitative value about the extent of a driver's impairment as a question of weight for the jury, not admissibility. It confirmed that officers must administer the tests properly under law enforcement guidelines, and that an accused may always introduce evidence of the possibility of error. A defense lawyer can use that framework to attack the weight prosecutors place on HGN results.
Every DUI case is different. Results depend on the specific facts. If you were arrested after an HGN evaluation in Georgia, speak with a licensed DUI attorney about your options. You can reach Kohn & Yager partners Larry Kohn and Cory Yager at 404-567-5515 for a free consultation.
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Frequently Asked Questions
What Are the 6 Clues for HGN in a DUI Test?
The six clues for HGN are scored three per eye. They consist of lack of smooth pursuit in the left eye, lack of smooth pursuit in the right eye, distinct and sustained nystagmus at maximum deviation in the left eye, distinct and sustained nystagmus at maximum deviation in the right eye, onset of nystagmus prior to 45 degrees in the left eye, and onset of nystagmus prior to 45 degrees in the right eye. Under NHTSA guidelines, four or more clues may indicate a BAC at or above 0.08. This conclusion is valid only when the test is performed correctly.
What Are the Steps of the HGN Test That Georgia Officers Must Follow?
Georgia officers must follow the NHTSA DWI Detection and Standardized Field Sobriety Testing manual. The HGN test steps include pre-test medical screening, positioning the stimulus 12 to 15 inches from the nose, checking for resting nystagmus and equal pupil size, and verifying equal tracking. Officers then score all three clue categories with two passes per eye for each category. An optional vertical gaze nystagmus check may follow.
How Long Should an Officer Hold the Stimulus at Maximum Deviation During the HGN Test?
The NHTSA manual requires the officer to hold the stimulus at maximum deviation for a minimum of four seconds per pass. Holding for fewer than four seconds means the officer did not complete the pass correctly. Holding for significantly longer may cause fatigue nystagmus – a natural response that could be mistaken for alcohol-related impairment.
Can the HGN Test Be Wrong if I Have Not Been Drinking?
Yes. Many non-alcohol factors can cause nystagmus and produce a false positive. These include neurological conditions like multiple sclerosis, inner ear disorders, certain medications (anti-seizure drugs, antidepressants, antihistamines), fatigue, and flashing lights. Officer errors such as holding the stimulus too high or moving it too fast can also induce nystagmus in a sober person.
Do I Have to Take the HGN Test if a Georgia Officer Asks Me to?
No. The HGN test is entirely voluntary in Georgia. You have the right to refuse all field sobriety tests, including the HGN. Refusing the roadside eye test is not the same as refusing a breath or blood test under Georgia's implied consent law. Consult a Georgia DUI attorney to understand how refusal may affect your specific situation.