Horizontal Gaze Nystagmus
Over 20 years ago, several states began implementing a new field sobriety test for use in DUI investigations. The test was still in its trial stages but the test quickly became popular with many police agencies around the county. The test is referred to as the “horizontal gaze nystagmus” test (HGN). Even though the HGN test is overseen by a police officer at the traffic stop, the test itself is only a surface test when determining the blood-alcohol content of a suspected drunk driver. However, evidence from the test can greatly affect the jury during the trial. A well experienced DUI attorney will have the knowledge to attempt to discredit the results of the HGN test. Even if the test results are admitted as evidence, a DUI attorney will have a chance to give the jury some doubt in the test’s results.
A well-regarded expert on the science of nystagmus is Dr. L. F. Dell’Osso. Dr. Dell’Osso is a professor at Case Western Reseve University, School of Medicine. He is also the director of the Ocular Motor Neurophysiology Laboratory located at the Veterna’s Administration Medical Center on Cleveland. As the use of the HGN test spread in the 1980s, Dr. Dell’Osso was quoted in the article “Nystagmus, Saccadic Intrusions/ Oscillations and Oscillopsia”:
“Using nystagmus as an indicator of alcohol intoxication is an unfortunate choice, since many normal individuals have physiologic end-point nystagmus; small doses of tranquilizers that would not interfere with driving can produce nystagmus; nystagmus may be congenital or consequent to neurologic disease; and without a neuro-opthalmologist or someone knowledgeable about sophisticated methods of eye movement recordings, it is difficult to determine whether the nystagmus is pathologic. It is unreasonable that such difficult judgments have been placed in the hands of minimally trained officers.”
In the same article, Dr. Dell’Osso also discussed over 40 other types of nystagums.
The test measures the movement of the eye to a stimulus, whether it’s a smooth movement or an erratic jolt. Even with the different types of nystagmus, the HGN test is what is commonly used in the field to measure the back and forth movement of the eye. Police officers can measure this type of nystagmus in one of several ways. In contrast to horizontal gaze nystagmus, vertical nystagmus is a vastly different situation caused by symptoms that are not common with horizontal nystagmus. Vertical nsytragmus is typically used to measure the potential presence of drugs in the system.
There are three parts to the HGN test which ultimately give the officer an estimate of the individual’s blood-alcohol level.
- The first step helps the officer establish an angle as a point of reference for nystagmus. When measuring the angle the eye jolts, the interrogating officer can approximate a blood-alcohol level.
- The next step checks to see if there are any jolting movements when moving the eye laterally. Essentially, this step looks for the extent of whiteness on each side of the eye.
- The third step look specifically for eye jerking when following an object such as a light or pen.
As the officer is about to begin the HGN test, he instructs the individual to maintain you head in a forward position while tracking an object with their eyes. After the instructions are given, then test commences. With their object in hand (usually a pen, light, or even their finger), the officer will place the object 12 to 15 inches in front of the suspect’s face and about 2 to 3 inches above the suspect’s eye level. The officer will then move the object across the suspect’s face at a pace of about 3 to 4 seconds. As the officer begins testing the suspected drunk driver, he takes note of the angle of the eye. Even in a standing position the jolting eye movement should continue. The officer will then continue the test with the other eye.
Even when testing a sober individual there is a slight jolting of the eye. However, an intoxicated individual’s eyes will jerk sooner during a test when compared to a sober test taker. A higher intoxication level also means the eyes will jolt faster. Theoretically speaking, an officer can estimate the blood-alcohol level of a suspect based on the angel of the eye and the jerking motions. The National Highway Traffic and Safety Administration conducted a study that concluded that eye jerking at a 40 degree angle is approximately equivalent to a blood-alcohol level of .10%. The study also stated that eye jerking at an angle of 35 degrees compares to a blood-alcohol level of .15%. The same result at a 30 degree is linked to about a .20% (at this level the subject may not even be following the moving object). A patter was then determined; when subtracting the angle used from 50, the approximate blood-alcohol level is determined. If the officer used an angel for 42 degrees the blood-alcohol level was .08%. (See V. Tharp, et al., Psychophysical Tests for DUI Arrest, DOT-HS-8-01970, 1981).
A variation of the HGN test is sometimes used by officers. This variation merely looks for the eye jolting before the eye reaches a 45 degree angle. Typically, this is not even done with an object for the suspect to follow. The lack of properly administering the HGN test is open for attack by a DUI attorney during questioning of the officer during trial. The defense could easily argue that if the eye jerking happened at 45 degrees that would results in only a blood-alcohol level of .05%, below the legal limit. That could result in the evidence being thrown out.