Horizontal Gaze Nystagmus Testing Law and Legal Definition

Horizontal Gaze Nystagmus (HGN) Testing is a field sobriety test recognized by the National Highway Traffic & Safety Administration of the federal government. The HGN test involves the police officer having the driver follow a pen with his eyes. The police officer is looking for specific responses from the eyes that show the driver is under the influence.

Nystagmus is the involuntary rapid movement of an eye either horizontally or vertically. It can be brought on by disease, head trauma, and many other physiological conditions. Alcohol consumption also causes this condition. To conduct the test, the police officer will hold a pen or finger approximately 12 inches from the driver’s face at eye level and move it back and forth parallel to the ground in front of the driver. The police officer is looks for three indicators, namely lack of smooth pursuit, onset of nystagmus before 45 degrees and distinct nystagmus at maximum deviation. Each of the above three indicators counts as one point in the test and a total of four points is considered as indication of impairment.

The results of the HGN may be excluded from evidence where it is shown that the test was not performed according to standards accepted by NHTSA. A health condition affecting the eyes such as an astigmatism or lazy eye can cause the results to be inaccurate.

The HGN test is based on the observation of three different physical manifestations which occur when a person is under the influence of alcohol: (1) the inability of a person to follow, visually, in a smooth way, an object that is moved laterally in front of the person's eyes; (2) the inability to retain focus and the likelihood of jerking of the eyeball when a person has moved his or her eye to the extreme range of peripheral vision; and (3) the reported observation that this “jerking” of the eyeball begins before the eye has moved 45 degrees from forward gaze if the individual's BAC [(Blood Alcohol Content)] is .10 [percent] or higher. [State v. Doriguzzi, 334 N.J. Super. 530, 536 (App.Div. 2000)]