What Is Horizontal Gaze Nystagmus?
Horizontal Gaze Nystagmus is simply described as involuntary jerking of the eyes occuring as the eyes track towards the sides. The presence of HGN is considered an indicator of potential alcohol impairment in DUI cases, and police officers are trained to check for the presence of HGN in conducting field sobriety tests prior to arresting a subject for DUI.
In testing for HGN, DUI suspects are instructed to stand with their feet together and hands by their sides and to keep their head still. They are then instructed to focus on the tip of a stimulus which should be held approximately 12 to 15 inches from the suspect's nose and slightly above the eye level. The officer usually uses his finger or a pen for a stimulus. The officer will then move the stimulus to the sides in order to establish the suspects ability to track the stimulus evenly and smoothly.
In addition, the stimulus is moved to the subject's extreme left and extreme right and held for about four seconds to look for distinct nystagmus at the extremes.
Finally, the officer will check for the presence of nystagmus prior to 45 degrees. the officer will slowly move the stimulus to the left and right and stop at an angle of 45 degrees. Jerking of the eye prior to 45 degrees is recorded by the officer.
A lack of smooth pursuit, distinct nystagmus at the extremes or nystagmus prior to 45 degrees are all indicators of potential alcohol impairment.
In cross examining an officer on this test, counsel should focus on a number of issues. Firstly, some medical conditions may result in HGN independant of alcohol impairment. Influenza, vertigo, hypertension, epilepsy and even motion sickness may result in HGN. Some drugs may cause HGN including antihistamines, aspirin and caffeine
A jury should understand that a police officer has very limited training in the field. The officer should be asked to confirm that he has no medical qualifications or training and that the training received was from another police officer and for approximately six hours in total.
The officer should be cross examined on the details of how he performed the testing in the field as many officers do not follow the required protocol. For example, how did the officer estimate an angle of 45 degrees? Was the suspect still seated inside his vehicle instead of standing? Did the officer perform the required sequence of "passes"? Did the officer make sure that the suspect was not wearing contact lenses?
Some officers will testify that a suspect was swaying about while performing other field sobriety tests, but was standing still for the HGN test as this is required for reliability of the test. The officer should be required to explain this inconsistency.