The horizontal gaze nystagmus ("HGN") test is one used by police officers to establish probable cause to arrest a person for DWI. In theory, when used in combination with other field sobriety tests, HGN supposedly helps police officers correctly distinguish suspects who are under the influence of alcohol from those who are not. The test is based on the fact that alcohol affects the automatic tracking mechanisms of the eyes. Alcohol supposedly slows down the eyes' ability to rapidly track objects and causes the eyes to oscillate, or "jerk", before they normally would in a sober person. Alcohol supposedly stimulates the nerve endings, making nystagmus more pronounced in intoxicated persons. As a person's blood alcohol concentration increases, the eyes will "jerk" sooner as they move to the side. The HGN test claims to gauge intoxication by measuring the involuntary oscillation of the eyes.
However, nystagmus can be caused by problems in an individual's inner ear labyrinth. In fact, irrigating the ears with warm water or cold water...is a source of error. Physiological problems such as certain kinds of diseases may also result in gaze nystagmus. Influenza, streptococcus infections, vertigo, measles, syphilis, arteriosclerosis, muscular dystrophy, multiple sclerosis, Korsakoff's Syndrome, brain hemorrhage, epilepsy, and other psychogenic disorders all have been shown to cause nystagmus. Furthermore, conditions such as hypertension, motion sickness, sunstroke, eyestrain, eye muscle fatigue, glaucoma, and changes in atmospheric pressure may result in gaze nystagmus. The consumption of common substances such as caffeine, nicotine, or aspirin also lead to nystagmus almost identical to that caused by alcohol consumption. (Quoting Pangman, Horizontal Gaze Nystagmus: Voodoo Science, 2 DWI J. 1, 3-4 )
The aggressive DWI Defense Lawyer must challenge the ability of trial courts to take judicial notice of the reliability of HGN testing as well its application in the specific case on trial. The defense lawyer must consider presenting expert testimony challenging the use of HGN by showing it is not scientifically validated. Furthermore, the lawyer must attempt to show at best, the HGN is only circumstantial evidence that a person may have used alcohol or may be intoxicated.