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The Horizontal Gaze Nystagmus Test in DUI Cases

One of the field sobriety tests typically given during a DUI investigation is the Horizontal Gaze Nystagmus (HGN) test. The officer has the DUI suspect follow an object (usually a pen or the officer's finger) from side to side with his eyes. The officer notes whether and when "nystagmus" occurs. Nystagmus is an involuntary bouncing of the eyes that is associated with alcohol impairment.

An experienced Riverside DUI defense lawyer can point out the many problems with using the horizontal gaze nystagmus test as evidence of alcohol impairment.

Three Clues The Officer Looks For With The Nystagmus Test

1. Lack of smooth pursuit
In theory, a sober person's pupils should glide from side to side smoothly. If the pupils track side to side in a jerky manner, such as a marble rolling across sand paper, this is believed to be a sign of DUI intoxication.

2. Sustained and distinct nystagmus at maximum deviation
The DUI officer guides the suspect's pupil to the far outer edge of the eye, and looks for nystagmus. If a distinct nystagmus is observed for more than four seconds, this is believed to be a sign of DUI impairment.

3. Angle of onset prior to 45 degrees
The officer starts with the DUI suspect's eye at center and then slowly guides it to the outside until a 45 degree angle is reached. If nystagmus appears prior to the pupil reaching the 45 degree angle, this is believed to be a sign of DUI impairment.

How The Nystagmus Test is Scored

The DUI officer looks for six clues: each of the above three clues with both the left and the right eye. According to NHTSA, a DUI suspect who exhibits at least four of the six clues has a 77% chance of having a BAC of 0.10 or higher. What this means is that if people were convicted of DUI based on this evidence, approximately a quarter would be wrongfully convicted.

Problems With The Horizontal Gaze Nystagmus Test

To begin with, the horizontal gaze nystagmus test is administered by officers with little to no understanding of the physiological processes that produce nystagmus. Few of the officers are trained to administer the test properly according to NHTSA's guidelines. Few officers do it correctly.

Even if the nystagmus test is administered correctly, there are many causes of nystagmus other than alcohol. Some people have natural nystagmus. Other causes of nystagmus (other than alcohol) can include streptococcus infection, hypertension, motion sickness, inner ear infections, epilepsy, antihistamines, anticonvulsants, and even caffeine and nicotine.

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