[Ed. note - Horizontal gaze nystagmus testing is part 3 of a DUI Training series.  Other articles cover Walk and Turn, One Leg StandVehicle in MotionPersonal Contact and specific safety concerns when conducting DUI investigations.]

The next step in the investigation of impaired drivers is to have them exit the vehicle for the Standardized Field Sobriety Tests (SFST).

The Frye standard, Frye test, or general acceptance test is a test to determine the admissibility of scientific evidence. It provides that expert opinion based on a scientific technique is admissible only where the technique is generally accepted as reliable in the relevant scientific community.

To overcome the scientific requirements of United States v. Frye, the National Highway Transportation and Safety Administration (NHTSA) funded scientific research on sobriety tests starting back in the 1970’s. The results were the three “standardized and validated” field sobriety tests that most of us in police work have become the most familiar with. “Validated” means that a scientific study confirmed that the tests do in fact work to identify impaired drivers.

The (3) sobriety tests that rose to the top during the NHTSA studies are:

  1. Horizontal Gaze Nystagmus (HGN)
  2. Walk-and-Turn
  3. One-Leg-Stand

DUI Training - HGNPlease note:  The intent of this article is simply to refresh previously trained officer’s on techniques and methods used for the SFST, and to encourage those who have not been properly trained to obtain such required training.  This article does not replace your state’s required training, nor certify you in anything.

When instructing officers on the SFST I developed a pattern that separated the test instructions into four easy to remember parts:

Starting position Basic information on test Demonstration of test Final instructions on body requirements during test (the head to toe instructions).

After each section I would ask the driver “Do you understand”. I would only continue when I obtained a verbal affirmative, which was conveniently recorded on audio and video equipment in the car. This played out extremely well because on each test the subject would have stated their understanding a minimum of four times! And if the subject says “no” to understanding I only repeat those parts that they say they did not understand. This break down also made it easy for me to ensure that all of the instructions from the NHTSA manual were provided.

This article will focus on the administration of the Horizontal Gaze Nystagmus (HGN) test. This is probably the most difficult test to administer because the emphasis is on the officer, not the test subject, to get the test correct. The officer must follow the NHTSA standards of spacing and timing in order to get this test admitted in court. The positive side is that most courts around the country now accept HGN results as a very strong indicator of impairment. The Appellate Courts in my State have ruled, “…four clues on HGN is indicative of impairment”. That is a very strong statement from the courts and makes the administration of this test that much more important to our case.

HGN – the involuntary jerking of the eyes as the eyes track horizontally. NOTE: All people have nystagmus (attorney question), but that nystagmus is not visible to the naked eye and requires doctor’s instruments to see. Alcohol and drugs enhance (not cause) nystagmus allowing it to be visible. Think of it this way – the eye is a ball sitting in a fluid socket. Like any ball it naturally wants to rotate and move around. The eye muscles keep the eye on track, but the eye is always making small adjustments to accommodate for lighting and distance. Alcohol and other drugs that enhance nystagmus weaken the eye muscles from doing what they need to do to control the eye, bringing nystagmus to the visible point. And don’t you agree that an eye bouncing around would prevent someone from driving safely?

  • Starting Position – Standing with feet together, heels and toes, with arms down to side.
  • Basic Information – Explain that you’re going to move your finger left to right and they are expected to follow your finger with their eyes only.
  • Demonstration – Actually position you finger and give an example pass left and right so they clearly understand your movements and intentions.
  • Final Instructions – Watch your finger with their eyes only, do not move their head. Keep their arms down to their side, and do not move out of the starting position until told to do so. (Remember to ask “do you understand” after each step)

To be “standardized” the officer should start each examination with his finger in the center of the subject’s face. Then move to the right (subject’s left) to start each test. This is the way NHTSA teaches and should be followed by officers.

OFFICER SAFETY ISSUE – I am going to go on record here to advocate that the officer use his/her finger as the test stimulus, and NOT some foreign object like a pen light. I’ve never heard of an officer having his finger pulled off and used as a weapon against him, but I have heard of pens or other objects being used in that manner. In one demonstration at an SFST conference or training seminar a petite female officer was able to grab a metal pen from another officer “administering” HGN and stab that pen about 3-4 inches into a mannequin chest – through a ballistic vest! No thank you – my finger is just fine for me.


Equal Tracking and Equal Pupil Size (4 seconds required) This step is a 2-second pass. The 2-seconds is from center of the subject’s face

to their left maximum deviation, back all the way to their right maximum deviation, and back to center. This is a fairly quick movement, but not too fast that a sober person could not follow your finger. This should be repeated twice to ensure proper observations.

***This step is critical because lack of either equal tracking or equal pupil size could indicate a serious brain injury/trauma that needs immediate medical attention. Ask the subject about any such events. I processed a guy who had been hit with a baseball bat 14 years earlier when he was a Catcher. Pupils were way off in size. But he’d been functioning fine since. Note this observation and explanation in the report and move on. Don’t get a good reason from the subject? Call for paramedics and have them transported to a hospital where you can draw blood and have them medically examined. Protect yourself and the subject!

Lack of Smooth Pursuit (16 seconds required – first scoring clues observed) The next step is to determine if the subject’s eyes move “smoothly” while

following your finger. This is a slower movement that takes “approximately” 2-seconds from center to to left maximum deviation, then about 4-seconds to get to right maximum deviation (2 seconds to center, 2 seconds to max. deviation), and then another 2- seconds back to center. This should be repeated at least twice for proper observation.

If the eyes are jerky when following then you score lack of smooth pursuit. Each eye gets its own score. Maximum clues to be scored is 2 – 1 for each eye.

Distinct (and sustained) Nystagmus at Maximum Deviation (minimum 24 seconds) Nystagmus at maximum deviation is observing the “bouncing” or jerking of the

eyes when the Iris (colored portion of the eye) is taken to the far corner of the eye with none of the white part of the eye showing in that corner.

VERY IMPORTANT – This is the only part of the HGN test that requires a minimum of 4 seconds of observation at maximum deviation. All of the other NHTSA time instructions are “approximately”, but maximum deviation is clear – “minimum”. The HGN test also requires the officer to obtain a minimum of 8-hours of instruction and training in order to be able to testify to the results of the HGN test in court.

So you take the subject’s eye to maximum deviation in a movement that takes about 1 second from center, wait a minimum of 4 seconds, move across to the subject’s right side maximum deviation in about a 2-second movement (just slow enough for them to follow your finger), hold for a minimum of 4 seconds, and then about 1 second to center. Repeat the same process and you can be done. You do not need to stop at center to start the next pass, but if you’re a little weak on your tests it’s a good place to get your timing in balance.

Here’s a hint – during this test you will count fast to yourself (trust me), so when counting by “one thousand one”, etc., count to about “1006” to make sure you get the full 4-seconds. Each eye is scored independently. Maximum clues to be scored is 2 – 1 clue per eye.

There’s good, scientific reasons we hold the eye a minimum of 4-seconds. As a person’s Blood Alcohol Concentration (BAC) goes up they will start to show signs of nystagmus at maximum deviation. However, a person at say a .06% or .07% may only show that nystagmus for 2-3 seconds and then it will stop completely. This is why failing to hold the subject’s eye at maximum deviation for a minimum of 4-seconds will definitely cause the test to be tossed in court, and could cause you embarrassment when you put that subject on the BAC instrument at the station. Remember, we arrest for impairment – not a BAC score.

Many officers see this phenomenon happen and then begin to doubt the accuracy of the tests. That’s why we do at least two passes to ensure what we are seeing. And on that note – the NHTSA manual says to do a “minimum” of two passes, but doesn’t say you can’t do more passes if something doesn’t feel right and you want to check things more closely.

Dr. Karl Citek, who has done extensive work with NHTSA on the eye examinations, advises that you can hold the eye at maximum deviation for up to 15 seconds for observation before worrying about causing a fatigue nystagmus – where the eye muscles just get too tired to hold the eye in that position and start to show nystagmus for that reason alone.

Remember – we are impartial investigators of the truth. Sometimes we will test people who are under the statutory legal alcohol limit. When you see this you’ve actually done a great job, and should start to realize this person is borderline impaired. They may still be too impaired to drive based upon your other observations because not all people handle alcohol or drugs the same. A person who takes their first “two drinks” ever might only get to a .05% but have difficulty standing, where a professional drinker could be a .150% and almost nail the other two tests in the SFST.

That’s why HGN is so very important, because “the eyes are the windows to the soul” and they don’t lie. And regardless, of drinking ability, age, size or anything, HGN will show up and has been shown to be over 90% effective at finding the .08% “drunk” and above.

Onset of Nystagmus Prior to 45 Degrees (20 seconds required) This is probably the most tricky of the HGN tests and requires the most practice.

This is because it requires the officer to move 12-15 inches from center of subject to maximum deviation, and time the movement to occur in “approximately” 4-seconds. During that time the officer must look for the first signs of jerking (nystagmus). Once observed the officer pauses there for at least one second to ensure what they observed. If nystagmus is still there the officer can move back to center and start the process for the other eye. If the nystagmus is not there, the officer continues the process until observing nystagmus or reaching maximum deviation.

This is a very slow movement, that requires skill to observe correctly, but has a great opportunity to determine greater impairment. Think of it this way – the sooner the nystagmus is observed, the greater the eye muscles are effected by the alcohol or drugs, meaning the greatest level of impairment.

Taking the example of an officer reaching maximum deviation with no signs of nystagmus prior to, the officer should take about one second to get to center again, and then start the 4-second process on the other eye, with a one second return to center. This entire process should be repeated again for at least two passes.

Attorney Question – why do we move our finger Maximum clues to be scored is 2 – 1 per eye.

NHTSA does allow the officer to conduct more than two passes per examination. If something happens and you’re not comfortable with what you saw, simply conduct another pass. This comes about quite frequently when the subject is uncooperative with doing the test. Document any reasons for more than two passes, and don’t sit there and do 12 passes or you’ll be accused of being on a fishing expedition.

Quick Guide for HGN: Maximum Clues Possible – 6 (3 separate clues (Lack of smooth pursuit,

Nystagmus at Maximum Deviation, and Onset of Nystagmus prior to 45 degrees. (1 clue per eye or 2 possible each clue tested)

Maximum Clues for Impairment – 4

Clues should appear in order – meaning you should not observe nystagmus at maximum deviation and not see lack of smooth pursuit.

VERTICAL GAZE NYSTAGMUS (VGN)After completing the HGN tests keep the subject in the same position and ask them to follow your finger as you track in an upward motion. Keeping the 12-15″ spacing, position your finger in a horizontal manner in front of their face and slightly elevated. In a movement that takes approximately one second move your finger up about 12-15″ and hold that position for approximately 4-seconds. Repeat this process twice to ensure your findings.

If there is a noticeable bouncing of the eyes then the person is exhibiting VGN. If there is none, or it starts and then stops before the 4-seconds is complete, then that subject does not have VGN.

Although not a clue that is scored, VGN provides invaluable evidence of impairment. Through training with Dr. Citek I was taught that the presence of VGN means that the amount of alcohol or drug in the subject’s system indicates a high level of impairement for that particular subject’s circumstances. What that means is that the 100 pound woman or the guy that just had his first few drinks, who may only have a BAC of .08%, may still exhibit VGN because for them the alcohol or drug is causing greater impairment. The heavy drinker on the other hand, may have a BAC of .180% and not exhibit any VGN.

VGN is, therefore, a great indicator of that person’s impairment. VGN, like HGN can be enhanced by alcohol, other CNS Depressants, Inhalants, and Dissociative Anesthetics like PCP. It is NOT a drug detecting test, or a test for marijuana use as some have misinterpreted about the test.

FINAL NOTE: Dr. Citek conducted a NHTSA funded scientific study on the ability to perform HGN on subjects who were seated or laying down. His results are now published in the scientific journals. Officers CAN perform HGN on seated and prone subjects so long as the subject’s head is in-line with the body. So if you want to do a preliminary HGN test on a subject who is still in their car you can, so long as you get them to turn slightly so their body and head are facing in the same direction. Same goes for that crash subject on the backboard. Remember, keep the subject’s head straight in-line with their body (no tilting, or turning of the head) and you can get your HGN test in.

I hope this information clears up any confusion you may have had, educates you on some of the intricacies of HGN, and encourages you to use this skill to remove impaired drivers. If you’ve already been trained – happy hunting! If not, you still must go through a NHTSA approved SFST course to properly administer the tests and testify in court. Don’t be the officer that sets bad case law precedent!

Aaron is a sergeant with a midwestern police department, where he serves as a trainer, supervisor and SWAT sniper. In addition to his broad tactical knowledge, Aaron has experience in DUI, DRE and undercover narcotics investigations.

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Richard Johnson is a gun writer, police trainer and really bad joke teller. Check out his other writing on sites like Human Events, The Firearm Blog and Police & Security News.

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