The Horizontal Gaze Nystagmus test is usually the first test an officer will conduct on someone they suspect of a DUI. They are looking for involuntary jerking of the suspect’s eyes. When an individual is impaired, this involuntary jerking becomes very apparent. As an individual’s BAC increases, as their eyes begin moving to the side they will begin jerking sooner. Usually a person being testing is completely unaware of this jerking occurring. The same nystagmus is present with those individuals who have used inhalants, dissociative anesthetics, and central nervous system depressants.
Before a person can have this test conducted on them, the officer must check for equal tracking of both eyes and resting nystagmus. Some individuals have medical conditions that naturally causes their eyes to exhibit nystagmus, and therefore they should not be tested. The same goes for those individuals whose eyes don’t track equally.
Officers are looking for three clues during the HGN test
Lack of smooth pursuit
As an individual become impaired by consuming alcohol, their eyes will display a jerking or an inability to sustain a constant rate of pursuit to a detected stimulus.
Distinct and Sustained Nystagmus at Maximum Deviation
When the eye moves as far as possible to the side and is held at that position for four seconds, if after four seconds it jerks, that is an indicator of impairment.
Onset of Nystagmus prior to 45 degrees
When the eye moves to the side from the center position, if it jerks before the stimulus reaches a 45-degree angle to the eye, it is an indicator of impairment.
There are six possible clues (three per eye) that law enforcement can claim demonstrate impairment for the HGN test. To “fail” the HGN test, it only takes two out of the six clues.
Observations that are not clues that law enforcement sometimes mistakenly score
Failure to maintain balance while taking the test: This is a clue on other field sobriety tests, but not on the HGN test. Law enforcement can use the failure to maintain balance as an indicator of impairment, but they cannot clue the individual for their poor balance.
Not keeping the head stationary while taking the test: This isn’t a clue on any field sobriety test. An officer will commonly forget to tell a person to keep their head still during the instructional phase and then “correct” them during the test. The cop will then bring this up at trial by saying the person couldn’t follow instructions properly or was trying to cheat the test and thus indicates impairment.
Closing your eyes during the test: This is also not a clue for any field sobriety tests. Sometimes an officer will state that the suspect was either impaired or otherwise not cooperating because they closed their eyes during the test. A person closing their eyes could be an indicator that they are not medically qualified to take the test or that some other stimulus was causing eye fatigue or unrelated nystagmus.
Not keeping hands at sides during the test.: This is a clue for a different field sobriety test, not the HGN test. Officers will usually try to state that the suspect didn’t keep their hands at their sides like the test requires. Usually whenever a person moves their hands (i.e. crosses their arms in front of them or puts hands in their pockets) it is due to the officer’s poor instructions.
Vertical Nystagmus as a clue: Before concluding the HGN test, law enforcement will test for Vertical Nystagmus. This is where an eye will involuntarily jerk when the eyes are at a maximum vertical elevation. It cannot be recorded unless it is obvious and maintained for four seconds minimum. A clue for vertical nystagmus is not a clue for the HGN test and nor does it count toward the two clue “failure” requirement.
Problems with the HGN test
Admissibility in Court: This test is not admissible in Kansas Courts. Further, the Kansas Supreme Court has likened this test to the “magic 8 ball” and the “ouiji board.”
Optokinetic Nystagmus: This is a different, yet common, type of nystagmus. Optokinetic Nystagmus will give the offer a “false positive” for clues when they score the HGN test. This nystagmus is caused by flashing lights or other object moving quickly across the suspect’s field of view. Because the officer’s car has flashing lights on it and there are usually fast traveling cars moving past the suspect’s field of vision during the HGN test, optokinetic nystagmus can be prevalent.
Reliability: There are only five studies which back up this test.
Southern California Research Institute Study: This was the original research conducted and used to develop the Standard Field Sobriety Tests and it indicated that the HGN test was 77% accurate at determining if a suspect was at or above 0.10 BAC. This study detects an alcohol concentration which is higher than the current legal limit of 0.08 BAC. This study is also over 40 years old and the approval rating is terrible. Further, it is not available online.
Validation of SFST at BAC below 0.10: This study was in 1998 and had a small sample size of only 297 motorists. It claimed an 88% accuracy rate for arrests based on the HGN test.
A Colorado Validation Study of the SFST Battery: This study was in 1995 and had a sample size of 305. This study asserts an 86% accuracy when all three tests are administered.
A Florida Validation Study of the SFST Battery: This study was in 1997 and had a sample size of 256 breath tests. It asserts a 95% accuracy when all three tests are administered.
The Robustness of the HGN Test: This scholarly article was published in 2007 and was is not a test/lab study like the other studies. The conclusions of this study was that, “It is concluded that HGN is a robust phenomenon.”