Horizontal Gaze Nystagmus (HGN) Field Sobriety Test
One of the standardized field sobriety tests (FSTs) used by law enforcement for a DUI arrest is the Horizontal Gaze Nystagmus (HGN) Field Sobriety test, often abbreviated as the HGN FST.
What is HGN?
The Nystagmus in the Horizontal Gaze Nystagmus (HGN) Field Sobriety Test is derived from the Greek word, νμσταγμνσ, meaning drowsiness, is derived from νμσταζειν, meaning “to nod in one’s sleep”), the rhythmic to-and-fro oscillation of the eyes, (such as you might see in REM sleep), which was regarded as a mystery throughout history.
Why do the police use HGN?
The manual used to train all officers in the United States, the NHTSA manual, contains the instructions and strict testing protocol for this test because it is believed to be strongly correlated with alcohol impairment at or above a .08% blood alcohol level. The HGN instructions from the manual read as follows:
Horizontal Gaze Nystagmus
The Horizontal Gaze Nystagmus (HGN) Field Sobriety Test checks your eyes for involuntary jerking as you gaze to the side. The test begins by the police officer placing the tested person in the instructional stance where the feet are placed together and the arms are kept to the side. The tested person is told to maintain that position until told otherwise and asked if they understand. Once the tested person is in the in the instructional position the Deputy must qualify the tested individual as a candidate for the test by ensuring that the eyes exhibit equal tracking and have equal pupil size. If the eyes do not track equally or the pupils are of different size the person is not a candidate and the test shall not be conducted. (And those symptoms may have other causes than intoxication).
If found to be qualified, the test is conducted with the Deputy presenting a stimulus for the tested person to fix their vision upon. The stimulus is placed approximately 12-15 inches in front of the individuals face slightly above eyebrow level. The test consists of the Deputy moving the stimulus horizontally at designated speeds. While doing this, the testing Deputy looks for horizontal jerking in the eyes at different positional locations.
HGN Cues/Clues of Intoxication
There are a total of three clues that can be assessed in this test: lack of smooth pursuit, distinct nystagmus at maximum deviation, and the onset of nystagmus prior to 45 degrees. Each eye is scored independently for three clues in each eye giving a total of six indicators/cues.
Horizontal Gaze Nystagmus (HGN) Field Sobriety Test Administration
According to NHTSA, the testing Deputy must provide the instructions for the test to the suspect in a standardized manner covering specific elements.
NHTSA says, “Give the suspect appropriate verbal instructions. Put feet together, hands at your side. Keep head still. Look at the stimulus. Follo the movement of the stimulus with the eyes only. Keep looking at the stimulus until the test is over. (Instructor notes) Emphasize that these are the major points that must be conveyed during the verbal instructions” (P. VIII-13, NHTSA SFST Instructor Manual, 2002).
“Give the suspect the following instructions from a safe position. I am going to check your eyes. Keep your head still and follow the stimulus with your eyes only. Keep following the stimulus with your eyes until I tell you to stop.”
According to NHTSA “Prior to the administration of the HGN, the eyes are checked for equal tracking (can they follow an object together) and equal pupil size. If the eyes do not track together, or if the pupils are noticeably unequal in size, the chance of medical disorders or injuries causing the nystagmus is present” (P. VIII-5, NHTSA SFST student manual, HS 178 R1/02).
NHTSA says, “Position the stimulus approximately 12-15 inches from the suspect’s nose and slightly above eye level. You may observe resting nystagmus at this time. Check the suspect’s eyes for the ability to track together. Move the stimulus smoothly across the suspect’s entire field of vision. Check to see if the eyes track the stimulus together or one lags behind the other. If the eyes don’t track together it could indicate a possible medical disorder, injury or blindness. Next check to see that both pupils are equal in size. If they are not, this may indicate a head injury”.
Movement of the stimulus for the check for the qualification pass check equal tracking should take approximately 2 seconds. NHTSA says, “Check for Equal Tracking: Move the stimulus rapidly from center to far right to far left and back to center (Approximately 2 seconds)” (P.VIII-13, NHTSA SFST Instructor Manual, HS 178 R1/02).
Lack of Smooth Pursuit
According to NHTSA, “Check the suspects left eye by moving the stimulus to your right. Move the stimulus smoothly, at a speed that requires approximately two seconds to bring the suspects eye as far to the side as it can go. While moving the stimulus look at the suspect’s eye and determine whether it is able to pursue smoothly. Now move the stimulus all the way to the left, back across suspects face checking if the right eye pursues smoothly. Movement of the stimulus should take approximately two seconds out and two seconds back for each eye. Repeat the procedure”. (P.VIII-7, NHTSA SFST Student Manual, 2002)
Distinct Nystagmus at Maximum Deviation
NHTSA says, “After you have checked both eyes for lack of smooth pursuit, check the eyes for distinct nystagmus at maximum deviation beginning with the suspects left eye.
Simply move the object to the suspects left side until the eye has gone as far to the side as possible. Usually, no white will be showing in the corner of the eye at maximum deviation.
Hold the eye at that position for a minimum of four seconds, and observe the eye for distinct and sustained nystagmus. Move the stimulus all the way across the suspects face to check the right eye holding that position for a minimum of four seconds. Repeat the procedure”. (p. VIII-7, NHTSA SFST Student Manual, 2002)
“Distinct nystagmus will be evident when the eye is held at maximum deviation for a minimum of four seconds. People exhibit slight jerking of the eye at maximum deviation even when unimpaired, but this will not be evident or sustained for more than a few seconds. When impaired by alcohol, the jerking will be larger, more pronounced, sustained for more than four seconds and easily observable” (P.VIII-5, NHTSA SFST Student Manual, 2002).
Onset of Nystagmus Prior to 45 Degrees
NHTSA says, “Next, check for onset of nystagmus prior to 45 degrees. Start moving the stimulus toward the right (suspects left eye) at a speed that would take approximately four seconds for the stimulus to reach the edge of the suspects shoulder. Watch the eye carefully for any sign of jerking. When you see it, stop and verify that the jerking continues.
Now move the stimulus to the left (suspect’s right eye) at a speed that would take approximately four seconds for the stimulus to reach the edge of the suspects shoulder.
Watch the eye carefully for any sign of jerking. When you see it stop and verify that the jerking continues. Repeating the procedure” (P. VIII-7, NHTSA SFST student manual, HS 178 R1/02).
Horizontal Gaze Nystagmus (HGN) Field Sobriety Test Scoring
The scoring card used in training for officers, approved by NHTSA, states as follows:
Horizontal Gaze Nystagmus (HGN) Field Sobriety Test
• Are you wearing glasses or contacts?
• I’m going to check your eyes.
Stand with your feet together, with your hands by your side.
Follow the stimulus with your eyes, but do not move your head. Focus on the stimulus until I tell you stop.
Hold stimulus approx. 12” to 15” in front of face.
CHECK EQUAL TRACKING & PUPIL SIZE.
Equal Tracking Yes No
Equal Pupil Size Yes No
CLUES: LEFT RIGHT
*Lack of Smooth Pursuit
2 seconds out; 2 seconds back
Hold minimum of 4 seconds
*Onset of Nystagmus
Prior to 45 Degrees
Move at speed taking 4 seconds
*Vertical Nystagmus Yes No
Hold approx. 4 seconds
TOTAL SCORE: (Decision Point: 4; Max: 6)
Horizontal Gaze Nystagmus (HGN) Field Sobriety Test Scoring. With the scoring card, the existence of four cues of intoxication is an indication of a driver being above a 0.08% blood alcohol level.
Note that nystagmus in one eye only is typically an indication of neurological disorders, not intoxication.
Problems with the Horizontal Gaze Nystagmus (HGN) Field Sobriety Test
Unfortunately, that alcohol can produce horizontal gaze-evoked nystagmus has led to a “roadside sobriety” test conducted by law-enforcement officers. The Horizontal Gaze Nystagmus (HGN) Field Sobriety Test as an indicator of alcohol intoxication is fraught with extraordinary pitfalls:
- many normal individuals have physiologic end-point nystagmus; small doses of tranquilizers that wouldn’t interfere with driving ability can also produce nystagmus;
- nystagmus may be congenital or consequent to structural neurologic disease;
- and often a neuro-ophthalmologist or sophisticated oculographer is required to determine whether nystagmus is pathologic.
Such judgments are difficult for DUI experts to make under the best conditions and nearly impossible to make accurately under typical roadside conditions. It is unreasonable to have cursorily trained law officers using the test, no matter how intelligent, perceptive, and well meaning they might be. As noted, medical history and drug-screening blood studies are often essential in evaluating patients with nystagmus.