So What is That Eye Test the Police Do During a DUI Arrest?

You have seen it a number of times on TV, or if you’re real unlucky, maybe you have even seen it in person.  They are the Standardized Field Sobriety Tests.  They consist of the “eye test,” the walk and turn and the one leg stand.  The “eye test” is actually the Horizontal Gaze Nystagmus (HGN) test.
Sounds like a funny name for a test that just sees if you can follow the tip of a pen as an officer moves it back and forth right? Well that is because there is actually a lot more to the HGN then it first appears.  The HGN test gives the officer the opportunity to see how your eye tracks the pen, not how well it tracks it.
So lets start off with what is Nystagmus?  When an eye is gazing in the direction of an object and follows it, normally it rolls along following the object very smoothly like a marble rolling across a glass top.  However, when someone is under the influence of alcohol in a significant enough level, this normally smooth eye movement becomes jerky and the eye now looks like a marble rolling along sandpaper.
It is this jerky eye movement that the police are looking for.  Nystagmus is completely involuntary and the subject has no idea whether or not they are displaying nystagmus.  The fact that nystagmus is involuntary is one of the reasons it is the most accurate of the field sobriety tests.  So, when the officer is moving the pen all around what exactly are they looking for?  They are looking for 3 different things in each eye.  Each of those three things is a “clue” for a total of 6 clues.  If you display 4 of the 6 clues it is considered a failure of the test.
The first thing the officers are looking for is lack of smooth pursuit.  The officer passes the pen back and forth from one side of the head to the other.  As he is doing this he is looking to see if the eyes are following the pen smoothly, if they are jerking as the pen goes by, then the officer will mark that as a clue.
The second thing the officer is looking for is distinct nystagmus at maximum deviation.  During this part of the test, the officer moves the pen as far to the outside as needed to bring the eye to its furthest most point it can go to the side.  Once the officer has the pen to this point, they will hold the pen at this point for at least 4 seconds to see if the nystagmus is continuous and noticeable at this point.  If it is present in both eyes that is considered 2 more clues.
The last thing the officer is looking for is onset of nystagmus prior to 45 degrees.  A 45 degree angle is determined by moving the pen out to approximately the edge of the shoulder.  As the officer moves the pen out toward the shoulder, if he sees nystagmus prior to that point, then the officer will score that as a clue.
If performed correctly by the officer, the HGN can be a good indicator as to whether someone is under the influence at a point that will test over a .08.  However, there are some shortcomings of the test.  One, nystagmus can occur naturally in some people, 2) some medications can cause nystagmus, 3) some head injuries can cause permanent or temporary nystagmus, and 4) if the officer does not perform the test correctly, that can effect the accuracy and reliability of the test.
So, now you know what the officer is looking for when he moves that pen around in front of people’s faces.  Next time you are drinking with your friends, try the test out so you can see nystagmus for yourself.
Banks & Brower is a law firm with offices in Indianapolis and Fishers and represents clients in all criminal matters including DUI.  If you have been arrested, contact us today at 317-870-0019.