Even when they are under the influence, many people can handle a single, focused attention task fairly well. For example, a driver may be able to keep the vehicle well within the proper traffic lane, as long as the road remains fairly straight. However, most people when under the influence cannot satisfactorily divide their attention to handle multiple tasks at once. The concept of divided attention has been applied to psychophysical testing. Field sobriety tests that simulate the divided attention characteristics of driving have been developed and are being used by police departments nationwide.
Simplicity is the key to divided attention field sobriety testing.
It is not enough to select a test that just divides the person's attention. The test also must be one that is reasonably simple for the average person to perform when sober. Tests that are difficult for a sober person to perform have little or no evidentiary value. In addition to being involuntary the person experiencing the nystagmus ordinarily is unaware that the jerking is happening, and is powerless to stop or control it. Under the influence of alcohol or certain other drugs, the involuntary jerking of the eyes becomes much more distinct, and readily noticeable.