The Horizontal Gaze Nystagmus Eye Test

One of the standardized field sobriety tests which officers typically employ to determine if someone is above the legal limit is the HGN Horizontal Gaze Nystagmus eye test.
The Horizontal Gaze Nystagmus standardized field sobriety test iinvolves the officer moving a stimulus, usually the tip of his finger, back and forth in front of the driver’s eyes to see if the driver’s eyes are jerking while following his finger. “Nystagmus” is the “involuntary jerking” of the driver’s eyes that the officer is looking for when scoring this test.

NHTSA, The National Highway Traffic Safety Administration, indicates that the HGN Horizontal Gaze Nystagmus test is the most reliable of the three standardized tests (the other two being the walk-and-turn test and the one-leg stand test) in determining whether or not the driver is above the legal limit.

There are, of course, things other than alcohol, or other central nervous system depressants, which can cause this nystagmus or jerking in the driver’s eyes. Three to four percent of the general population has nystagmus at all times for reasons other than the consumption of alcohol.

Other causes of nystagmus or jerking of the driver’s eyes besides alcohol include:

(1) problems with the inner ear labyrinth; (2) irrigating the ears with warm or cold water under peculiar weather conditions; (3) influenza; (4) streptococcus infection; (5) vertigo; (6) measles; (7) syphilis; (8) arteriosclerosis; (9) muscular dystrophy; (10) multiple sclerosis; (11) Korchaff’s syndrome; (12) brain hemorrhage; (13) epilepsy; (14) hypertension; (15) motion sickness; (16) sunstroke; (17) eye strain; (18) eye muscle fatigue; (19) glaucoma; (20) changes in atmospheric pressure; (21) consumption of excessive amounts of caffeine; (22) excessive exposure to nicotine; (23) aspirin; (24) circadian rhythms; (25) acute trauma to the head; (26) chronic trauma to the head; (27) some prescription drugs, tranquilizers, pain medications, anti-convulsants; (28) barbiturates; (29) disorders of the vestibular apparatus and brain stem; (30) cerebellum dysfunction; (31) heredity; (32) diet; (33) toxins; (34) exposure to solvents, PCBS, dry cleaning fumes, carbon monoxide; (34) [sic] extreme chilling; (35) eye muscle imbalance; (36) lesions; (37) continuous movement of the visual field past the eyes . . .; (38) antihistamine use.

There are also 46 types of nystagmus in individuals, separate from horizontal nystagmus.

There are three clues that the officer is looking for in each of the driver’s eyes:

  1. Lack of Smooth Pursuit
  2. Distinct Nystagmus at Maximum Deviation
  3. Onset of Nystagmus Prior to 45 Degrees.

The officer tests for these three clues in each of the driver’s eyes by moving the stimulus (his finger) back and forth in front of the driver’s eyes. Each of the clues requires the officer to move the stimulus at different speeds, and holding the stimulus different amounts of time when testing for each of the three clues.

An experienced defense attorney can readily identify an improperly administered HGN horizontal gaze nystagmus test through cross examination of the arresting officer, even in cases where a videotape does not exist.

The HGN Horizontal Gaze Nystagmus test is a very specific test, in that it requires the officer to do very certain things, in specific order, for the test to be considered valid and properly administered. More often than not, the officer improperly waives his finger around 90 miles per hour in front of the driver’s face, and then marks down all six marks against the driver (all three clues in both eyes), and the test comes into evidence as being valid because the defense attorney did not know how to properly challenge the results of the HGN test.

For the first clue, lack of smooth pursuit, the officer is looking to see whether or not the driver’s eyes can smoothly follow his fingertip while it moves back and forth in front of the eyes, and whether or not the eyes jerk or bounce.

For the second clue, distinct nystagmus at maximum deviation, the officer is supposed to take his finger to the extreme side of the driver’s vision in each eye, and hold it there for a fixed amount of time to determine if the eyes jerk when held to the side.

For the third clue, onset of nystagmus prior to 45 degrees, the officer is attempting to measure the angle at which the eyes begin to jerk by moving his finger from the center of the driver’s face out to the edge of the driver’s shoulder, seeing at what point, if any, the driver’s eyes begin to jerk.

Prior to administering the HGN Horizontal Gaze Nystagmus test, the officer is supposed to pretest the driver to make sure that he is an eligible candidate for the test. He does this by a quick pass or two of his finger in front of the driver’s face prior to checking for the three clues to see if the driver’s eyes have the same pupil size and if they track equally. If they do not, the HGN test is not supposed to be given, but I am sure you can guess how often that happens in the real world, where an officer pretests the individual and chooses not to continue on giving the test. Our office has seen HGN tests administered improperly to people with glass eyes, people strapped to a gurney with head injuries after an accident, the list is endless.

Conditions in the field also affect the driver’s ability to take the test. One of the biggest mistakes officers make is not cutting off their flashing lights prior to giving the HGN test, and then face the driver directly into the lights while giving the HGN. The lights often cause the driver’s eyes to jerk, and then the officer will mark them down as being above the legal limit.

The officer must always face the subject away from rotating lights or flashing or strobe lights while performing the HGN test to avoid stimulation of artificial jerking of the eyes from the light. 1995 NHTSA Instructor Manual, VIII-65; 2000 NHTSA Instructor Manual, VIII-56; 2002 NHTSA Student Manual, VIII-15. The jerking of the eyes stimulated by this strobe effect is referred to as optokinetic nystagmus.

Prosecutors and attorneys for Missouri Director of Revenue will go to great lengths to discount the effect that flashing lights causing optokinetic nystagmus has on the validity of the HGN test.

One Missouri Court even recently noted in Porter v. Director of Revenue, 168 S.W.3d 147 (Mo. App. S.D. 2005), that NHTSA has indicated that optokinetic nystagmus will not affect the HGN test. Of course this is nonsense. They specifically noted in the original validation studies and the first drafts of the student and instructor manuals warnings about optokinetic nystagmus. 1995 NHTSA Instructor Manual, VIII-65; 2000 NHTSA Instructor Manual, VIII-56; 2002 NHTSA Student Manual, VIII-15.

These lawyers will also argue that even if the flashing lights do cause an artificial jerking of the eyes, that because the driver is instructed to be watching the officer’s finger and not the lights that the artificial jerking if the driver sees the lights is the fault of the driver for not following instructions! Of course, it would be too much to bother to make the officer just turn off the lights.

This, of course, is also nonsense. I have seen numerous demonstrations at defense attorney seminars where the subject’s eyes jerked when facing a flashing light regardless of whether or not the subject stared at the finger moving in front of him or whether the eyes went out of focus by looking directly at the flashing lights.

It would be important to note that no matter how poorly the officer administered these tests in your Missouri DUI / DWI or other drunk driving case, that the prosecutor or attorney for Missouri Director of Revenue will go to great lengths to say “close enough.” It is important to consider bringing in an expert witness to challenge the administration, interpretation and scoring of these tests, or else courts will go to great lengths to let them into evidence.

You should also be aware that these manuals are constantly revised by NHTSA, and always in favor of the officer and against you, the driver. There certainly isn’t any scientific basis for the changes. NHTSA spent an immense amount of the taxpayers dollars attempting to standardize these tests, and the original validation research indicated a very specific way that these tests must be given.

When defense attorneys started attacking the officers when they did not do the tests correctly, NHTSA began immediately backpeddling, and meeting every couple of years to determine which elements of the rules were causing the officer to lose these DWI cases, and then they just take that out of the next version of the manual. We of course would not want to have one set of rules that everyone must abide by. It is a lot easier to take that item out than insist on the officer doing the test correctly.

Missouri courts rely heavily on the HGN Horizontal Gaze Nystagmus test in Missouri DUI / DWI or other drunk driving cases. It is imperative that you vigorously challenge it’s admission into evidence in your case. You had better make sure that the attorney you hire for your case knows how to handle HGN.

Mr. Guilfoil has completed the International Association of Chiefs of Police (IACP)/National Highway Traffic Safety Administration (NHTSA) approved student course in administration of standardized field sobriety tests, (NHTSA Certified Field Sobriety Test Examiner) in Fort Worth, Texas (Summer, 2003); and is one of a very few IACP/NHTSA Certified Field Sobriety Instructors defense attorneys in the State of Missouri, having received the Field Sobriety Instructor distinction under Defense Consultants Walden, Platt and Associates, in Atlanta, Georgia (March, 2004). He also has completed the 24-hour Drug Evaluation and Classification Overview Course, also given by Walden, Platt and Associates. (May, 2004).


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