Nystagmus is defined as an involuntary bouncing or jerking of the eye caused by any number of vestibular, neurological or physiological disturbances. In DUI parlance, leo, the law enforcement officer is looking for nystagmus because he is trained to believe that is an indicator of impairment. Well, at least that was true in the "old days." In the present day, thanks to case law, the presence of nystagmus is nothing more than an indicator that, allegedly that the person consumed some alcohol. That is a big difference from the old days when they thought nystagmus was an indicator that person's BAC was a .10. It's like a witch hunt to think because somebody exhibited clues of nystagmus that they must be impaired, thankfully science has caught up with the law a little bit and dispelled these beliefs.
Were are not totally where we belong yet because the courts still rely on these silly field sobriety tests to determine if somebody is impaired. The concept being that if you cannot follow multiple directions, and remember what you are told and then balance on the street like a pet monkey, then you must be impaired by alcohol. Naturally, this is the dumbest concept around but the courts are still placing value on those tests of this point. Hopefully somebody will be able to show how stupid that is because when people are nervous, old, young, excited, sick, heavy, scared, etc., they will not perform adequately on the tests. However, prosecutors and judges put little value on those conditions; rather they assume that if you cannot walk 9 straight steps on the street and follow their directions you must be impaired. Kinda silly, but one day they will be retired like nystagmus has.
So regarding nystagmus, it has now been reduced to simply tell if a person has consumed alcohol, and since most people admit that they drank two beers before stopped by the police, this is rarely an issue. Even so, the courts have found this test to be scientific in nature and thus if the cop wants to come in and talk about this test, the cop has to be identified as an expert if the defense has properly filed discovery.
In the old days, curbing this testimony was more important than it is today. Today, as indicated the test has little meaning in court and thus the fight is not worth the effort to exclude the testimony in court.