The “Horizontal Gaze Nystagmus” (HGN) test is one of the three standardized NH field sobriety tests that a police officer might give to a driver, if the officer suspects the driver is driving under the influence of alcohol. HGN occurs when the eyes jerk involuntarily from left to right when trying to see something that is traveling sideways out of the field of vision.
Many factors can cause horizontal gaze nystagmus. For instance, medical conditions such as a concussion or neurological disorders will cause HGN. Some prescription medications will also increase the chances of its occurrence, whether or not the person taking the medication is fully capable of driving a car. Finally, horizontal gaze nystagmus will occur even in healthy people if they are facing a strobe light or other rapidly flashing light or object.
In order to determine whether HGN is the result of alcohol intoxication or another medical condition, NH police officers are trained to check for equal pupil sizes and whether the eyes can â€œtrackâ€ the same object together before administering the test. However, since so many factors other than intoxication can cause or aggravate the condition, even a carefully-given test cannot conclusively prove that a driver is too drunk to drive.