Standardized Field Sobriety Tests: Horizontal Gaze Nystagmus Test - California DUI Law and Legal Information

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California law enforcement officers including members of the California Highway Patrol (CHP), local police officers, or sheriff's deputies who suspect a driver of driving under the influence (DUI), or driving while intoxicated (DWI) usually conduct a field sobriety test to gather evidence to support the charge. The Horizontal Gaze Nystagmus test is one of the three standardized field sobriety tests recognized by the National Highway Transportation Safety Administration (NHTSA).

Many drivers who take field sobriety test test during DUI investigation fear that poor performance means an automatic conviction, but that's not the case. The experienced San Francisco DUI attorneys at the Law Office of Robert Tayac will challenge the outcome of the field sobriety tests at the criminal trial and the California Department of Motor Vehicles (DMV) hearing.

Horizontal gaze nystagmus describes an involuntary jerking of the eyes as they move from side to side. The theory behind the horizontal gaze nystagmus test is the higher the blood alcohol concentration, the sooner the eyes will begin jerking as they move from side to side.

California police officers conduct the test by asking the driver to use only the eyes to follow a small stimulus, such as the tip of a pen, without moving the head. The officer watches for a lack of smooth tracking as the eyes move, and sustained jerking when the eye reaches the furthest point. The officer will also look for the onset of jerking prior to the eye reaching a 45-degree angle.

The officer counts the number of these "clues" in each eye. If the officer spots a total of four out of the six clues, he or she will conclude that there is a 77 percent chance that the driver's blood alcohol content (BAC) is above .10 percent, and the motorist will be arrested for driving under the influence.

Police officer is looking for signs of mental and physical impairment to support a driving under the influence (DUI), or driving while intoxicated (DWI) charge. However, even though police rely heavily on the horizontal gaze nystagmus test, it is not foolproof. Involuntary jerking of the eye, or nystagmus, occurs in everyone, even if no alcohol has been ingested. Alcohol and drugs magnify the nystagmus effect, but so do many other factors, including illness or injury.

Experts agree that when it comes to alcohol intoxication, mental impairment always occurs before physical impairment. Therefore, a physical impairment such as horizontal gaze nystagmus doesn't necessarily support a charge of drunk driving if it can be attributed to another source.

In addition, many police officers do noteven administer the test correctly. The driver's head and body must be facing the object of stimulus for the test to be valid. If the motorist performs the test while sitting in the car, the test will not be valid, because the driver's head will be turned at a 45-degree angle.

A criminal defense attorney with experience defending California DUI cases can determine whether causes other than alcohol may have contributed to any impairment, and challenge the results of any field sobriety test.