Horizontal Gaze Nystagmus
A recently developed and popular field sobriety test used by police agencies is the "horizontal gaze nstagmus" test. When administering this test, the officer estimates the angle of eye jerking when following a standing point (such as a finger or light). Nystagmus is the medical term given for eye oscillation. If the eye jerking motion occurs prior to an angle of 45 degrees, it is an indication of a blood-alcohol level of approximately .05%. The smooth motion of the eye tracking the standing point is also an important factor. Another factor is the eye jerking when aimed fully to one side.
This test, however, is susceptible to several problems. For example, the officer is not medically, or even properly, trained to recognize a proper nystagmus or to properly estimate the angles. In addition, the medical community has yet to fully accept this test as an indicator of intoxication. Also, some states have yet to allow this sobriety test to be admissible as evidence in trial (this test is admissible as evidence in the state of California). Regardless of these pitfalls, police agencies continue to use this test.
For an excellent example of how the fallacies of horizontal gaze nystagmus can be exposed, see the following cross-examination of a police officer by a prominent Los Angeles DUI lawyer.