A police officer with adequate training and experience in performing the horizontal gaze nystagmus test
(HGN) may testify as to the significance of a defendant’s performance on an HGN test, according to a recent California Court of Appeal decision. Although HGN testing alone cannot determine whether a driver is under the influence of alcohol (nor determine a blood alcohol level), the HGN test is part of an officer’s total observations of a suspect. The horizontal gaze nystagmus is one basis for an officer’s opinion concerning intoxication.
A police officer can use findings from horizontal gaze nystagmus (HGN) testing as a basis for an opinion that the defendant was driving under the influence of alcohol. The prosecution is not required to submit expert testimony to confirm the officer’s evaluation of the HGN test.
Field Sobriety Tests
The three-test battery of standardized field sobriety tests have been shown to accurately determine blood-alcohol concentration (BAC) of 0.10 percent or higher 83 percent of the time, according to the National Highway Traffic Safety Administration (NHTSA). The other two components of field sobriety tests are the one-leg stand (OLS) and walk-and-turn (WAT) tests.
The HGN screening has been shown to be the most accurate indicator of alcohol impairment and the most accurate of the three-part field sobriety test, according to NHTSA studies.
How Police Conduct the HGN Test
Police Officers conduct the HGN test either in a well-lit area or with the use of a flashlight, but with the the driver facing away from the police cruiser’s flashing lights (and the lights from passing cars) which could skew results. The officer then tells the driver that she is going to check their eyes and asks whether they wear contact lenses or have any medical issues that would affect the results. The officer then asks the driver to follow an object (usually the tip of a pen or a pen light), placed 12-15 inches away, with only their eyes.
The Officer is looking for the following three clues in each eye:
1. Lack of smooth pursuit:
Stimulus is placed 12-15 inches from the subject’s nose and slightly elevated. The stimulus is moved across the field of vision two seconds over and two seconds back for each eye.
2. Distinct and Sustained Nystagmus at Maximum Deviation:
Start with the left eye. Move the stimulus to the extreme side so that no white is showing in the corner of the eye. Hold this for a minimum of four seconds. Look for a distinct and pronounced bounce which is sustained. Repeat this for the other eye.
3. Onset of Nystagmus Prior to 45 Degrees:
Beginning with the left eye, move the stimulus to the right slowly so that it takes approximately 4 seconds to reach the edge of the shoulder. Stop when jerking is observed and verify that the jerking continues. If the onset is before the shoulder and there is still white present on the edge of the eye, the onset is prior to 45 degrees. No white appearing on the edge of the eye would be greater than 45 degrees.
The horizontal gaze nystagmus is not the only tool police employ to establish probable cause for a DUI arrest. Officers typically require drivers whom they suspect to be impaired to take the three-part sobriety test and then test the individual’s BAC using a portable breath alcohol monitor (or “Breathalyzer”) before making an arrest. A second, more conclusive BAC test is taken with a stationary breath alcohol monitor or blood draw at the police station for use at trial.
Admissibility of HGN Test Results at Trial
Evidence of nystagmus does not always signify impairment. Nystagmus, in rare instances can also be caused by the movement of inner ear fluid, eye strains, neural activity, brain damage, or just as a natural occurrence, according to the NHTSA. Drivers who can prove they have these conditions may be able to have the evidence excluded before trial.