Horizontal Gaze Nystagmus
Horizontal Gaze Nystagmus is the involuntary jerking of the eye as they track from one side to the other. So the law enforcement officer following a stop for perceived drunk driving will ask the driver to get out of his automobile and walk to the back of the car. While the defendant is walking to the back of his car the officer will monitor how the defendant gets out of the car and how he walks and stands when he reaches the rear of the car. This evidence, if useful to the police officer for example if the driver is swaying or stumbling, will be used against them in court. Once at the rear of the car, the defendant will be asked to track the officers pen or finger while the officer slowly and gently moves it from one side of the defendants head to the other side.
Before the officer can begin this test he should ask the driver if he has any head injuries or if he has taken any medication that evening which may cause my nystagmus. once those issues are ruled out the officer will make one or two quick passes with his finger to see if the eyes have the ability to track equally the officer will also check for resting nystagmus as well to rule out any innate conditions that the defendant may have. Once the officer is convinced that the defendant driver is in good health and that his eyes can track equally he will begin the test. The test consists of several passes of the finger in front of each individual I to check for the presence of involuntary jerking in the viewing plane. The test is essentially ridiculous because most of the officers rushed the test and do not even do it properly number one. Number two, the test is completely subjective and on top of not knowing how to do the test properly, the officer can say the defendant performed any which way the officer wants to say and nobody can question that. And finally and most interestingly, an officer can actually cause nystagmus by taking his finger to the furthest point that the eye can travel and simply holding it there too long thereby actually causing nystagmus to appear.
So in addition to most officers not performing the test accurately, and the fact that officers can bring one the condition by performing the test a certain way, there are 37 different known causes for nice stag miss as documented in Maryland case law not the least of which is the emergency signals that the police officers use in their cars. Thus if you are asked to perform the HGN test and the officers emergency rotational lights are activated on his lightbar on his hood this very fact alone has been proven to cause HGN. There are 37 other causes such as lack of sleep, medications, head injuries, caffeine, to name but a few.
Thankfully the legislature and the courts have come to the conclusion that this test is beyond silly and non-effective in the proper determination of impairment by alcohol such they had they have limited its usefulness to only say that a person has consumed alcohol, a fact which the majority of defendants will admit when asked.
Thus when an officer tells you that you have failed and HGN test, you do not need to place too much weight on that particular test as it has very little significance in the realm of things.