Seeing the flashing lights of a police vehicle behind you after you have left a party and you know you had a couple of drinks can make your heart drop. If the officer has a reason to think that you have been drinking, you might asked be asked to take a field sobriety test. One of these tests is the horizontal gaze nystagmus test.
In this test, the police officer is looking at the way your eyes react to specific movements. Typically, you will have to follow an object, such as a pen or the officer's finger while the officer watches what goes on with your eyes.
When you look around, your eyes have small involuntary movements that are jerky, but this movement is usually large enough to be seen. When you aren't impaired, the nystagmus, or involuntary movements are usually smooth. If you are impaired by drugs or alcohol, this involuntary movement is more pronounced.
There are some points that are very important to note when you are asked to take this test. One is that you should be facing away from the flashing lights on the officer's vehicle, as well as from the lights of passing vehicles because these can affect the results since your eyes would try to adjust to the changes in light as you take the test.
As is the case with every component of the field sobriety test, the result are largely dependent upon how the officer interprets the test. This could become a point to focus on as part of your defense, so you should explore this possibility if you are facing a DUI.
Source: FindLaw, "What Is Horizontal Gaze Nystagmus (HGN)?," accessed Oct. 28, 2016