HGN in the Courtroom

The horizontal gaze nystagmus test, as well as the other standardized field sobriety tests – the walk-and-turn and one-leg stand – are intended to be used by police officers to determine probable cause that a driver is under the influence of alcohol. After determining per se intoxication – that a driver is probably under the influence as indicated by these test results – the next test should be a breathalyzer, blood, or urine test to determine blood-alcohol content.

In the courtroom, defense attorneys will argue vigorously against the use of HGN test results as evidence. The court usually allows the HGN test results because of general acceptance of its reliability in this area of law.

The defense attorney may try to prove that the driver was experiencing one or more of the following when detained to such a degree that the test results were incorrect and therefore the driver was illegally determined to be intoxicated:

- car or motion sickness or other illnesses such as a cold or flu that affect the sinuses
- eye disease or abnormalities
- contact lenses
- ear infections
- sunstroke
- eye muscle fatigue, such as created by long hours of driving
- vertigo
- medications or ingredients that are acceptable to use while driving, including caffeine, nicotine, aspirin, and cold remedies
- congenital defects
- as a side-effect of neurologic disease.

However, in most cases the arresting officer will have followed the standardized field sobriety tests with a breathalyzer, blood, or urine test to determine the actual BAC.