When police pull over a person for suspicion of drunk driving, they generally perform a series of field sobriety tests. One of these looks at horizontal gaze nystagmus. HGN is an involuntary eye motion that occurs when someone's eye looks to the side.
If a person has consumed alcohol or some other drugs, particularly depressants, that jerking increases. The impact isn't noticeable to the person impacted by it, and it has no impact on their vision.
When trained officers perform the HGN test, during which they ask a suspect to follow an object such as a pen with their eyes, they look at:
-- Whether the eye jerks after staying at the outermost edge for several seconds
-- If the eye can smoothly follow the object without jerking
-- If and when the eye jerks when the object is moved towards the person's shoulder.
The HGN test is just one three standard field sobriety tests performed at the scene. The other two are the walk and turn and one leg stand tests. The HGN test has been found to be the most accurate indicator of alcohol impairment of the three tests.
Additionally, police officers often perform a Breathalyzer test on suspected drunk drivers. They may also take blood from a suspect. The results of all of these tests, as well as police officers' observations, can be used as grounds for a DUI arrest and as evidence to charge and convict a person for the crime.
Even if a person's results on an HGN tests aren't good, they can be disputed in court. For example, the test must have been properly conducted by an officer with the training to do so. Further, there are certain medical conditions that can cause a poor result even if a person was not impaired by alcohol.
As noted, law enforcement, prosecutors and judges generally look at the totality of all test results. However, if you believe that one or more of the tests was improperly performed or there were other factors that need to be considered, it's essential to inform your attorney so that he or she can determine whether there is an opportunity to have any evidence against you thrown out.
Source: FindLaw, "What Is Horizontal Gaze Nystagmus (HGN)?," accessed May 25, 2017