The officer moves a penlight in front of the driver’s eyes – about one foot distant –and asks that the driver follow the light with the eyes. The officer estimates the angle at which the eye twitches; if this happens at less than 45 degrees, it is a sign of high blood-alcohol concentration (BAC).
This is the ‘horizontal gaze nystagmus test’ (HGN), which is relatively new to field sobriety tests and is fairly accurate if administered properly.
Nystagmus – an involuntary twitching of the eye – typically occurs when a person uses their peripheral vision by looking sharply to the side – at about 45 degrees – with the head held in the forward position. Under the influence of alcohol, this twitching occurs at a lesser angle and the driver has difficulty following the object with his or her eyes.
The results indicate to the officer if the suspect has a BAC of .10 or higher or if the driver has ingested a wide variety of drugs. NHTSA studies indicate that the test results will be correct approximately 77 percent of the time, so you have to retain a veteran DUI/DWI lawyer to fight for your rights.