HORIZONTAL GAZE NYSTAGMUS
The "horizontal gaze nystagmus" field sobriety test, is a relatively recent development in DUI investigation. The officer attempts to estimate the angle at which the eye begins to jerk ("nystagmus" is medical jargon for a distinctive eye oscillation); if this occurs sooner than 45 degrees, it theoretically indicates a blood-alcohol concentration over .05%. The smoothness of the eye's tracking the penlight (or finger or pencil) is also a factor, as is the type of jerking when the eye is as far to the side as it can go.
This field sobriety test has proven to be subject to a number of different problems, not the least of which is the non-medically trained officer's ability to recognize nystagmus and estimate the angle of onset. Because of this and the fact that the test is not accepted by the medical community, it is not admissible as evidence in many states; it continues, however, to be widely used by law enforcement.
If the weakness of the validity of this test is the only defense that you have-don't count on it helping you to avoid a conviction. If you have issues-were not under the influence- You are going to need an attorney who has experience with the court in the city where you were stopped.