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What Happens In The Horizontal Gaze Nystagmus Test?


This first test is sometimes given while the person is still in their car. When given properly, the test should be done with the driver out of the car looking directly at the officer, not with the person’s head turned to the side as when they are in the car looking out the window. This test would be to check for the involuntary jerking of the eye, or nystagmus, so if there was nystagmus present at certain angles then it would tell the officer there was a high percentage, not 100%, but closer to 80%, that the person may be at or over a certain limit. This test was designed when the limit was a 0.10. People may have seen this happen on TV and it is a similar test that doctors give to see if someone has a concussion because there are other things that can cause nystagmus.

What Issues Can Affect The Accuracy Of The HGN Test?

They would have the person look at their finger or a pen or a flashlight and they would hold it above 12 to 15 inches from the driver’s nose and they would go back and forth doing a number of passes. If this test is done correctly, it should take about 90 seconds and they would first have to see if the person’s eyes were tracking properly and if there was an eye injury because if there is, then the person should not be given this test. They would see whether it looked like the eye was tracking smoothly while going back and forth. A good way to compare would be to have a marble rolling on 60-grit sandpaper and somewhat balancing a little bit. It would be very hard to detect, and an officer would have to do this test over and over, and do a lot of practice to be good at it because it can be hard to pick that up. Officers often claim to see this movement when it is not really there. Another thing they also do is that they would take the pen or finger out to a point with each eye as far as the person can really see with, so there would be no white in the far left corner of the left eye and no white in the far right corner of the right eye when the finger or pen was going in either of those directions. The officer would want to see if the eye was jerking back and forth so they would hold the finger, pen, or flashlight out for a moment. They would have already told the person to keep their head still and just follow the object they were using or the stimulus. They would hold it for a few seconds and observe and then they would do this over and over. They would continue to go out to the side, each side, with whatever object they were using and the person would have to follow it with their eyes. They would take it to about a 45-degree angle which would usually be the person’s shoulder width and if they saw that involuntary jerking of the eye at that point, then that would be when they could come up with a number and try and make a prediction regarding whether or not the person was at or over a 0.08. Some officers feel they can actually predict what the person’s actual blood alcohol level would be depending on at what angle they noticed the onset of nystagmus.

For more information on Hoizontal Gaze Nystagmus Test, a free initial consultation is your next best step. Get the information and legal answers you’re seeking by calling (415) 523-7878 today.

Aaron Bortel

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