Horizontal Gaze Nystagmus is the first field sobriety test that an officer will generally administer to someone the officer suspects of Driving while intoxicated. What the officer is looking for is the involuntary jerking of the eyes of the suspect. Involuntary jerking of the eyes becomes readily noticeable when a person is impaired. As a person’s blood alcohol concentration increases, the eyes will begin to jerk sooner as they move to the side. The suspect that is being tested is usually unaware that the jerking is occurring. The exact same nystagmus can be exhibited when a person has used central nervous system depressants, inhalants and dissociative anesthetics.
Before an officer can validly administer the test the officer must check for resting nystagmus and equal tracking of both eyes. Some people have a medical condition that causes their eyes to exhibit nystagmus naturally, these people should not be tested. Also, some suspect’s eyes will not track equally, they should not be tested.
What the officer is looking for during the HGN test is the following three clues.
1. Lack of smooth pursuit: As a person becomes impaired by alcohol their eyes will exhibit a jerking or inability to maintain a constant rate of pursuit to an observed stimulus.
2. Distinct and Sustained Nystagmus at Maximum Deviation: When the eye moves as far to the side as possible and is kept at that position for four (4) seconds, if it jerks after four seconds then this is an indicator of impairment.
3. Onset of Nystagmus prior to 45 degrees: As the eye moves from the center position to the side, if it jerks before the stimulus is at a 45-degree angle to the eye this is an indicator of impairment.
There are six possible clues that an officer can claim indicate impairment on the HGN test. (Three clues per eye) It only takes two clues out of the six possible to “fail” the HGN test.
Observations that are not clues that officers sometimes score by mistake.
Failure to maintain balance while taking the test. This is a clue on another field sobriety test it is not a valid clue when scoring the HGN test. The officer will bring up failure to maintain balance as an indicator of impairment but cannot clue the suspect for poor balance.
Not keeping the head stationary while taking the test. This is not a clue in any standard field sobriety test. It is common for an officer to forget to tell a suspect not to move their head during the instructional phase of the HGN test, then “correct” the person during the test. The officer may then bring up at trial that the suspect kept moving their head during the test to try to imply that the suspect was impaired and could not follow instructions or that the suspect may have been trying to circumvent the test.
Closing your eyes during the test. This is not a clue in any standard field sobriety test. Occasionally an officer will indicate that the suspect closed their eyes during the test to imply that the suspect was either not cooperating or was otherwise impaired. If a suspect closes their eyes during the test it can be an indicator that they are medically unqualified to take the HGN test or that there is some other stimulus causing unrelated nystagmus or eye fatigue.
Not keeping hands at sides during the test. This is a clue on another field sobriety test. It is not a valid clue for the HGN test. The officer may try to introduce that the person did not hold their hands down to their side like the test requires. When a suspect does anything other than have their hands at their sides, (most commonly the suspect puts them in their pockets or crossed in front of them) it is usually due to poor instruction by the officer.
Vertical Nystagmus as a clue. Before the conclusion of the HGN test the officer will test for Vertical Nystagmus, Vertical Nystagmus is an involuntary jerking of the eyes occurring as the eyes are held at maximum vertical elevation. Vertical nystagmus cannot be recorded unless it is distinct and sustained for a minimum of four seconds. It is not a clue for the HGN test and does not count toward the two clues necessary to fail the test
Problems with the Horizontal Gaze Nystagmus
Admissibility in Court: The HGN test is not admissible in Kansas Courts and has been equated to the “Magic 8 ball” and the “Ouija Board” by the Kansas Supreme Court.
(See the Case law section of this website to see the Supreme Court case)
Optokinetic Nystagmus: This is another type of nystagmus that is quite common. Optokinetic Nystagmus can give an officer a “false positive” for clues during the scoring of the HGN test. Optokinetic Nystagmus in a person’s eye can be caused by flashing lights or objects moving quickly across the field of view of the suspect. Because most police cars have flashing lights on during a traffic stop and usually fast moving cars are passing in the field of vision of the suspect during the administration of the test, Optokinetic Nystagmus can cause real problems.
Reliability: There are Five studies that are cited to back up the HGN .
Southern California Research Institute Study: The original research conducted by the Southern California Research Institute that was used to develop the Standard Field Sobriety Test curriculum indicated that the HGN test was 77% accurate at detecting subjects at or above a .10 BAC. This study is aimed at detecting an alcohol concentration that is higher than the legal limit. It was performed nearly 40 years ago and has an abysmal accuracy rating. (This study is not available online)
Validation of SFST at BAC below 0.10: This study was finalized in 1998. It is a relatively small sample size of 297 motorists. The study claims an 88% accuracy rate for arrests decisions based on the HGN test.
(Click to see Standardized Field Sobriety Test (SFST) Validated At Bacs Below 0.10 Percent)
A Colorado Validation Study of the SFST Battery: This study was finalized in 1995. This study had a sample size of 305 participants. The study claims an 86% accuracy when administering all three tests.
(Click to see the Colorado Validation Study of Standardized Field Sobriety Battery – Burns Article)
A Florida Validation Study of the SFST Battery: This study was finalized in 1997. This study had a sample size of 256 breath tests. The study claims 95% accuracy when administering all three tests.
(Click to see the Florida Validation Study of the Standardised Field Sobriety Test Battery -Burns Article)
The Robustness of the Horizontal Gaze Nystagmus Test: This scholarly article was published in 2007. It is not a lab/test study like the others. The ultimate conclusions from the study is that, “It is concluded that HGN is a robust phenomenon.”