If you have ever had a concussion or brain trauma, you are not a candidate for the horizontal gaze nystagmus test.  This is the eye test the Officer performs when they are investigating a DUI offense.  If you have ever been knocked out, you are not a candidate for this test.  When I was five, I was hit by a car.  My head hit the pavement and I was knocked unconscious.  This event of having the brain matter in my brain shake in my skull manifests itself as nystagmus in my eyes to this day.  Nystagmus is the bumping of the eye as a stimulus is moved from left to right.  Think about a marble rolling over sand. This movement is not smooth. There is an involuntary jerking movement of the eyes.  An Officer will assume, often incorrectly, that you are under the influence of alcohol.  If you played football, boxed, or had any event where you lost consciousness, even for a very brief period of time at any age in your life, this test will manifest as a false positive.  Remember, the eye test and any other test associated with field testing is optional.  Never agree to perform this optional test.  The Officer will not care if you have natural nystagmus or a bumping of the eye due to head trauma.  They will mark in their police report that you tested positive for alcohol when this may not be the case.  Again, this false positive could erroneously formulate a false basis for probable cause for an arrest.