Horizontal Gaze Nystagmus
You may have heard of the "eye test" in DUI investigations. This is the nystagmus field sobriety test or, more accurately (there are 47 different kinds of nystagmus), the horizontal gaze nystagmus test. It is one of the most commonly used field sobriety tests, as it is one of three which make up the federally-approved "standardized battery" of tests, or SFSTs.
The Nature of the Test
The test is essentially a measurement of the movement of the eye. Simply stated, "nystagmus" refers to a distinctive involuntary jerking of the eyes; horizontal gaze nystagmus is a pendular (back and forth) movement. This type of nystagmus is commonly measured by the officer in three three different ways, each time using an object such as a pencil, penlight or finger placed a foot or so in front of the suspect's nose and moving it slowly to the left and then to the right.
Measuring "Angle of Onset"
The first part of the test is to determine the angle of onset of nystagmus - that is, the angle at which the moving eye begins the jerking motion. The suspect looks straight ahead and, without moving his head, moves his eyes slowly to the right or left. The officer is supposedly able to detect when the nystagmus begins and is supposedly able to estimate the angle from straight ahead at the point where it begins. If the onset is prior to 45 degrees, in theory, the blood alcohol level will be over .05%.
Measuring "Distinct Nystagmus"
The second part of the test is to note whether the jerking becomes more "distinct" when the eye is moved to the lateral extreme - that is, when there is no longer any white of the eye visible at the outside of the eye.
Measuring "Smooth Pursuit"
The third part is to determine whether there is a lack of smooth pursuit: rather than following a moving object smoothly from the beginning, the eye jumps or "tugs".
Validity of the Nystagmus Test in DUI Cases
Under federal standards, the officer is supposed to use an objective scoring criteria for each of the three tests, and the total score determines whether the suspect passed or failed. In reality, few officers understand the test, administer it correctly, or use objective scoring. Many simply report that they "detected the presence of nystagmus", and subjectively count that as a failure. It is, however, the characteristics of nystagmus, not the simple presence, which is relevant to determining possible impairment. And, unfortunately, many things cause nystagmus and some of us have it under normal conditions.