Motorists under investigation for DUI / DWI in Florida often are asked to take field sobriety tests before being arrested. The horizontal gaze nystagmus test is one of three standardized field sobriety tests recognized by the National Highway Traffic Safety Administration (NHTSA).

Horizontal gaze nystagmus is an involuntary jerking of the eyes as they move from side to side. The test is based on the theory that the higher a person’s blood alcohol concentration, the sooner his or her eyes will begin jerking as they move from side to side.

The driver is instructed to use only his or her eyes to follow a small stimulus, such as the tip of a pen, while keeping the head stationary. The officer is looking for a lack of smooth tracking as the eyes move, and sustained jerking when the eye reaches the furthest point. The officer also will watch for any jerking that occurs before the eye reaches a 45-degree angle.

Any of these occurrences in either eye is considered a “clue.”  If the officer notes 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. A drunk driving arrest will follow.

Even though police rely on the horizontal gaze nystagmus test as an indicator of physical and mental impairment from alcohol intoxication, it’s far from foolproof. Everyone experiences nystagmus, or involuntary jerking of the eye, regardless of whether alcohol as been consumed.  Alcohol and drugs magnify the nystagmus effect, but so a number of other factors, including illness, fatigue, and injury.

To understand how a skilled defense attorney attacks the results of a field sobriety test, it’s useful to understand how alcohol affects the central nervous system. When an individual consumes alcohol, mental impairment always occurs before physical impairment takes place. If a driver exhibits physical difficulties but no mental impairment, it must stem from a source other than alcohol.

The problems with the horizontal gaze nystagmus test are compounded by the fact that many police officers don’t even conduct the test properly. In order for the test to be valid, the driver’s head and body must be facing the object of stimulus. If the driver is sitting in a car when the test is performed, the results will not be meaningful, because the driver’s head will be turned at a 45-degree angle.