The horizontal gaze nystagmus or HGN test works under the principle that there’s a condition in our eyes known as nystagmus; horizontal as well as vertical nystagmus. Our eyeballs are in effect twitching on a sideward movement at all times. It’s just a natural condition. Under normal conditions, the sideward movement cannot be observed because it’s so minute, a very small sideward movement. Alcohol or other central nervous system depressants will accentuate the movement and make it much more of a jerking movement and very noticeable.
A common analogy is to think about if a person hasn’t consumed a central nervous system depressant, and they’re asked with their eyes to go out to the side, it will go very smoothly like a windshield wiper going across a wet windshield because the sideward movement is not visible. It’s not very noticeable. Alcohol, in effect, slows down the movement of the eyeball and so it’s analogous to a windshield wiper going across a dry windshield, namely very jerky as it goes out to the side.
The condition as it relates to DUI enforcement, if the officer is looking for a central nervous system depressant, alcohol or others, HGN will manifest itself in three different ways. There are six possible cues of impairment, three in each eye. The first thing the officer would do, is he would take or she would take some stimulus; a pen, a light, even a finger if they had to, and hold it roughly 12 to 18 inches from the person’s face and go out to the side and ask the person to track the movement of the stimulus out to the side as the officer moves the stimulus to the side. What the officer is looking for initially is, is there any jerky sideward movement? If that is present in either eye, and usually it’s present in both if at all, that would be a lack of smooth pursuit. That would be one indicator. A second indicator is to make sure that the condition that the officer is observing is, in fact, nystagmus. The officer would then ask the person to have the eyeball go out to roughly a 90-degree angle, or as far out as the person can see to their side, and then look to see if there a bouncing movement that’s sustained, meaning it continues noticeably for a period of 4 seconds. If that’s present, that’s the second cue or indicator that nystagmus is present at maximum deviation. The third condition would be where the condition becomes pronounced, or more visible. The officer would roughly determine what would be a 45-degree angle looking at the person straight on and then off to the side what would be 45 degrees out. If the nystagmus becomes very noticeable prior to the 45 degrees, that would be an indication of the presence of the cue angle of onset; onset meaning becomes noticeable at or prior to 45 degrees. The presence of the angle of onset prior to 45 degrees then that would be the third indicator. In terms of allowing an officer to make a conclusion as to his or her belief what the alcohol level is, if four of the possible six cues are present, that would be an indication that the person’s alcohol level is at or above .08%. The angle of onset prior to 45 degrees can also be used, at least scientifically, to predict what the alcohol level is. The closer the angle of onset is to zero, straight ahead, the higher the alcohol level is. For example, if the angle of onset is 35 degrees, that would indicate a person’s well above a .08%. If it was at 25 degrees when it first became noticeable, again it would be a higher indication or an indication that the alcohol level is even higher.
The initial training is 16 hours where they’re taught how to do it. At the conclusion of that training, they go out into the street and they do it on subjects 35 times. In order to be proficient to do this test, they have to be correct 80% of the time. What correct means is they come to their conclusion the persons alcohol level is above or below .08%. They then get in the context of the test a subsequent breath or a blood test. If they thought the person would be over a .08%, that’s considered to be correct in their assessment. If they thought the person would be over a 08%and it turns out the subsequent breath or blood test turns out to be less than a .08%, then that’s considered to be incorrect. During this training phase to become eligible to get certified, they have to be correct in their assessment 80% of the time on these 35 tests. Once they’ve done that and they are correct 80% of the time, they go back to another training session, which is euphemistically referred to as a wet workshop where they do the test again under controlled circumstances.
At what us euphuistically describes as a “wet workshop”, people have consumed alcohol at the workshop and the officer then has to do the test on these people and then come to a conclusion whether they think it’s above or below .08% It has to be done at least five times at the wet workshop, and they have to be correct four out of five times. If they’re correct on four out of the five, then they go back out and they’re certified, and they have to maintain an 80% proficiency in order to stay certified.
One of my beliefs about the HGN test and why it’s well overdone in terms of its reliability and its accuracy is the test is not done in a vacuum, meaning it’s done on a person who has been drinking. It’s done on people with an alcohol level that turns out to be a .07% or a .08% or a .09%, .20% or .30%. If it’s done on a person whose alcohol level is it turns out to be a .20%, I could do that without much difficulty because I would not only be looking at the eye movement, but the person’s general appearance, and looking at such things as their sway and posture and I could guess the person’s over .08%.
What boosts the accuracy, an officer’s accuracy, is that these tests that are done at extreme alcohol levels are also included. Where it can be fallacious is it doesn’t measure the officer’s proficiency when talking about close tests,, where the alcohol level could be .07%, .08% or .09%, where it could be plus or minus. Those are the important ones. The testing criteria is designed to make an officer sound far more accurate than he or she really is. However, I’ve never seen an officer lose their certification and frankly it’s based on self-reporting. I do not believe there’s any oversight or review, so to say that the officer has maintained certification frankly I think is quite meaningless. In Arizona, officers are limited though to say that it’s present and it could be indicated the person’s alcohol level is over .08% In Arizona, in order for an officer to testify as to the results of the HGN in terms of an alcohol level, there has to be a subsequent breath or blood test to correlate that to see if they’re correct or not. If there isn’t, for whatever reason, then the officers cannot correlate or come to a conclusion or at least testify as to the conclusion as to what the alcohol level is above or below a .08%. They’re left to being able to say that they observed a neurological dysfunction, one cause of which could be alcohol. That’s been true here for at least 25 years. That greatly minimizes the impact of the HGN test on a jury trial, because most people have no idea what that means.
For more information on the Horizontal Gaze Nystagmus Test, a free initial consultation is your next best step. Get the information and legal answers you’re seeking by calling [number type=”1″] today.