The National Highway Traffic and Safety Administration (NHTSA) conducted research to validate research concerning several field sobriety tests. Please note: all information referenced to “NHTSA” is from their February 2006 Edition, DWI Detection and Standardized Field Sobriety Testing.
I am using the language from NHTSA and DWI Detection and Standardized Field Sobriety Testing, because law enforcement is trained with this information and book. I want you, the reader, to see what law enforcement if being told about DUI investigations, because you have more likely than not, been subjected to a DUI related investigation conducted in part based on techniques and procedures attributed to NHTSA.
NHTSA directs the officer to administer three scientifically validated field sobriety tests. Based on the performance on these tests, as well as the observations made concerning the driving and face to face contact, the officer must then decide whether to arrest the driver for a suspected DUI.
One of the standardized field sobriety tests is horizontal gaze nystagmus, abbreviated as “HGN.” “Nystagmus” means an involuntary jerking of the eyes. Alcohol can cause nystagmus. (Page VIII-3)
There are three categories of nystagmus (Page VIII-3).
One of the three categories is neural nystagmus which includes
a. optokinetic nystagmus (Page VIII-3): when the eyes fixate on an object that suddenly moves out of sight or when the eyes watch sharply contrasting moving images
Examples of optokinetic nystagmus include watching strobe lights or rotating lights, or rapidly moving traffic in close proximity.
b. physiological nystagmus Page VIII-4) is the most common kind of nystagmus; it keeps the eye cells from tiring, but its jerking is too minor to be seen by the naked eye
c. gaze nystagmus (Page VIII-4) occurs when the eyes move from the center position; there are three kinds of gaze nystagmus:
horizontal gaze nystagmus (HGN): its presence is the best indicator of alcohol impairment; however, its presence may also indicate the use of certain drugs
vertical gaze nystagmus (VGN): an “involuntary jerking of the eyes (up and down) that occurs when the eyes gaze upward at maximum elevation”
resting nystagmus is a jerking of the eyes as they look straight ahead (indicative of PCP use).
NHTSA at pages VIII-4 and VIII-5 recognizes that nystagmus may also be caused by medical conditions that include brain tumors and other brain damage or some diseases of the inner ear. NHTSA directs the officer to rule out medical causes of nystagmus by assessing
NHTSA at Page VIII-5 requires that before administering HGN, the officer must check the person’s eyes for equal pupil size, resting nystagmus and equal tracking (can the eyes follow an object together). NHTSA warns the office that “if the eyes do not track together, or if the pupils are noticeably unequal in size, the chance of medical disorder or injuries causing the nystagmus is present.”
HGN refers at Page VIII-5 to an involuntary jerking of the eyes when the eyes gaze to the side. A person is unaware if and when HGN occurs. Horizontal Gaze Nystagmus is, per the NHTSA, the most reliable field sobriety test if performed correctly (emphasis on correct administration). As a person’s blood level alcohol concentration increases, the person’s eyes will begin to jerk involuntarily sooner as they look to the side.
The officer will administer the HGN test by having the driver follow or track the motion of a small stimulus with only the driver’s eyes. The driver will not be allowed to move his or her head (ie turn the head) to track the movement. The officer will use as the small stimulus the tip of a pen or penlight, an eraser on a pencil or a fingertip, whichever contrasts with the background.
The officer must always begin administering the HGN test with the driver’s left eye. In his or her examination, the officer is looking for three specific clues (Page VIII-5):
1. Lack of smooth pursuit per NHTSA
As the eye moves from side to side as it tracks the stimulus, does it move smoothly or jerk noticeably? The eyes can be seen “to jerk or bounce as they follow a smoothly moving stimulus.” As drivers become more impaired by alcohol, their eyes track less smoothly. NHTSA states at page VIII-5 that an unimpaired person’s eyes will smoothly track a moving stimulus such as a marble rolling across a flat, smooth pane of glass or a windshield wiper moving across a wet windshield.
2. Distinct and sustained nystagmus at maximum deviation per NHTSA
When the eye moves as far to the side as possible and is kept at that position (looking as far away from the nose as possible yet with the head and nose pointing forward) , does the eye jerk distinctively? Distinct and sustained nystagmus at maximum deviation (looking as far away from the nose as possible yet keeping the head and nose pointed forward) is another clue of impairment. However, the eye must be held at maximum deviation for four seconds and, when the person is impaired, “the jerking will be larger, more pronounced, sustained for more than four seconds and easily observable.” (All information is from Page VIII-5).
3. Onset of nystagmus prior to 45 degrees per NHTSA
As the eye moves toward the side, does it begin to jerk before it reaches a 45 degree angle to the nose? The “onset” of nystagmus is when the eye is first seen to jerk. If it jerks before it reaches a 45 degree angle, NHTSA claims this means the person has a BAC above .08% and is another clue of impairment. “The higher the degree of impairment, the sooner the nystagmus will be observable.” (All information is from Page VIII-5).
NHTSA indicates at Page VIII-8 that the maximum number of clues per eye is three and the total maximum is six. Research, per NHTSA, shows that if four or more clues are present, then it is likely that the driver’s blood level alcohol concentration is above .10%. With four or more clues present, HGN is accurate 77% of the time if administered correctly.
Estimating 45 degree angles per NHTSA
Officers must administer the HGN correctly for it to have any meaningful result. It is critical therefore that they know how to estimate a 45 degree angle. If the stimulus is held at 12 inches in front of the person’s nose, it must be moved in an imaginary straight line of 12 inches distance from the person to the side to reach 45 degrees.
At page VIII-6, NHTSA requires the use of specific HGN procedures. If a person is wearing glasses, the person must remove his or her glasses before HGN may be administered. Before administering HGN, the officer must instruct the driver:
“I am going to check your eyes”
“Keep your head still and follow this stimulus with your eyes only”
“Keep following the stimulus with your eyes until I tell you to stop”
The officer must position the stimulus 12 to 15 inches from the driver’s nose and slightly above eye level. The officer must check to see that both pupils are equal in size. Unequal size may indicate a head injury. The officer must also check for resting nystagmus and for the person’s eyes to track together. The officer must “move the stimulus smoothly across the person’s entire field of vision.” The officer can then “check to see if the eyes track the stimulus together or if one lags behind the other. If the eyes do not track the stimulus together, it could indicate a possible medical disorder, injury or blindness.” (All information is from Page VIII-6)
The officer will first check for the lack of smooth pursuit.
The officer must begin with the person’s left eye by moving the stimulus to the officer’s right. The officer must move the stimulus smoothly, at a speed that requires approximately two seconds to bring the person’s eye as far to the side as it can go. While moving the stimulus, the officer must look at the person’s eye to determine if it is able to pursue the stimulus smoothly. Then the officer must move the stimulus all the way back to the left, back across the person’s face and check if the right eye pursues it smoothly. Movement of the stimulus should take approximately two seconds out and two seconds back for each eye. The officer must repeat the complete procedure one more time. (All information is from Page VIII-7)
Next, the officer will then check for distinct and sustained nystagmus at maximum deviation.
To do so, the officer will begin with the person’s left eye. The officer will move the stimulus to the officer’s right until the person’s eye has gone as far to the side as possible. Usually, no white will show in the corner of the eye at maximum deviation. The officer must hold the eye at that position for a minimum of four seconds, and observe the eye for distinct and sustained nystagmus at maximum deviation. Then the officer must move the stimulus all the way across the person’s face to check the right eye, again holding that eye in that position (maximum deviation) for a minimum of four seconds. People exhibit slight jerking of 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. The officer must repeat the complete procedure one more time. (All information is from Page VIII-7).
Finally, the officer checks for onset of nystagmus prior to 45 degrees.
The officer will start moving the stimulus toward the right (the person’s left eye) at a speed that would take approximately four seconds for the stimulus to reach the person’s shoulder. NHTSA instructs the officer to watch the left eye carefully for any sign of jerking. As soon as the officer sees any sign of jerking, the officer must stop moving the stimulus and verify that the jerking continues. (All information is from Page VIII-7).
Then, the officer must move the stimulus to the left (the person’s right eye) at a speed that would it approximately four seconds to reach the person’s shoulder. NHTSA instructs the officer to watch the right eye carefully for any sign of jerking. As soon as the officer sees any sign of jerking, the officer must stop moving the stimulus and verify that the jerking continues. The officer must repeat the complete procedure one more time. NHTSA warns: “NOTE: it is important to use the full four seconds when checking for onset of nystagmus. If you move the stimulus too fast, you may go past the point of onset or miss it all together.” (All information is from Page VIII-7).
If a person’s eyes start jerking before they reach 45 degrees, the officer must check to see that some white of the eye is still showing on the side closet the ear. If no white of the eye is showing, the officer has taken the eye too far to that side (that is more than 45 degrees) or the person has unusual eyes that will not deviate very far to the side. (All information is from Page VIII-7).
NHTSA warns that nystagmus may be due to causes other than alcohol that include seizure medications and some other drugs. A large disparity between the performance of the right and left eye may indicate a medical condition. (All information is from Page VIII-8).
Examples of additional evidence of impairment during HGN:
The person is unable to keep his or her head still.
The person is swaying noticeably.
The persons utters incriminating statements. (All information is from Page VIII-15).
Examples of conditions that may interfere with the person’s performance on HGN:
Wind, dust, etc irritating the person’s eyes
Visual or other distractions, impeding the test (the officer, per NHTSA, at VII-15, must ALWAYS face the person away from rotating lights, strobe lights, and traffic passing close proximity).
Test interpretation of the HGN results
The officer should look for three clues of nystagmus in each eye:
1. The eye cannot follow a moving object smoothly
2. Nystagmus is distinct and sustained when the eye is held at maximum deviation for a minimum of four seconds
3. The angle of onset of nystagmus is prior to 45 degrees
If the officer observes four or more clues, there is a 77% likelihood that the person’s BAC is above .10%.(All information is from Page VIII-8).
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