One way police officers determine that a driver might be drunk is to use the horizontal gaze nystagmus (HGN) test. Usually, an officer will ask a Massachusetts driver to follow an object, and while the driver does so, the officer watches to see if the driver’s eyes start jerking if the driver looks off to the side. This kind of test is considered more accurate than other field sobriety tests. However, this does not mean there are not valid defenses against this test.
FindLaw explains that while many state courts consider a HGN test to be credible, the test still should be conducted under proper guidelines, namely the Standardized Field Sobriety Test (SFST) guidelines established by the NHTSA. A defense attorney may assert that the officer in question did not implement the HGN test as these guidelines dictate. The judge hearing the case will then determine if the test was conducted appropriately or if the results are not admissible.
The medical condition of the driver at the time the HGN test was conducted can also be a major factor. Sometimes a person may experience excessive jerking of the eye because of inner ear fluid or strain on the eyes. Some people have experienced injuries to the head that result in brain damage or neural irregularities that can produce HGN false positives. There are also individuals who naturally jerk their eyes in this fashion without any alcoholic drink as a contributing factor.
If medical or physical issues are in play, a defense attorney will probably seek to have the HGN results thrown out before the trial even begins. There may be other defenses relating to how the test was conducted, which could involve bringing in expert witnesses to testify to the veracity of the test, or perhaps the police officer may testify about what he or she observed while testing the driver.
There are many different scenarios that can result if someone is given a field sobriety test. Because of the sheer variety of possibilities, you should only read this article as a reference on this topic. Do not consider it as legal counsel for your particular situation.