There are several tests that are part of the standardized field sobriety test that are used to help police officers determine the impairment of a driver. One of these tests is the horizontal gaze nystagmus test. This one has to do with how your eyes react when you are looking at a moving object.
Remember, this is only one component of the test. There are two others – the one-leg-stand and the walk-and-turn. Together, these provide a glimpse into your impairment level. The test is highly subjective, so there is a chance that these points might be called into question as part of your drunk driving defense.
What is the horizontal gaze nystagmus?
One impact that impairment has is that it will cause your eyes to involuntarily jerk when you look side to side. While some jerking is normal, it becomes more pronounced when you are impaired. This is due to the nervous system becoming depressed by the alcohol. You won't notice this even though it is noticeable to others. You might think that your eyes are moving just as smoothly as normal.
How is this tested?
The police officer will use a light or another object. You will have to follow the movements of the item using your eyes only. It is imperative that you aren't facing the flashing lights of the police vehicle because those lights can impact the results of the test.
The officer doing the test should be properly trained and able to make accurate determination about the impairment of individuals. Calling the officer to the stand in a trial to question their methodology and training might be one defense option.