If you find yourself facing a charge of drunk driving, the officer might ask you to take sobriety tests at the side of the road. These tests are meant to determine if it’s possible that you’re under the influence of alcohol or over the legal limit. There are several different types of sobriety tests that you may be asked to perform. One of them is called the horizontal gaze nystagmus test or HGN test.
The horizontal gaze nystagmus test is a test that measures eye movements. A nystagmus is an involuntary movement in the eye’s iris and pupil. Scientists have learned that the eye has a horizontal nystagmus when a person has a significant bodily alcohol content. Looking at a person’s eye movements can be an indicator of whether a person has a bodily alcohol content and what their alcohol content might be. The horizontal gaze nystagmus test is just one of several ways that an officer might assess a driver’s level of intoxication.
When the officer performs an HGN test, they’re checking to see if the subject’s eyes move normally. Specifically, they’re checking to see if the person’s pupils jerk back and forth when the person looks out of the corner of their eyes. As a person’s blood alcohol level grows, their pupils jerk back and forth more and more when they look out of the corners of their eyes. When a person has a high alcohol level, these movements are noticeable.
The jerking movement is involuntary. It’s not something an intoxicated person can control. The officer is looking for how much the eyes move in the corner. They’re also looking for how soon the eyes start to flutter back and forth as the person moves their gaze towards the corners of their eyes. When the movement starts farther away from the corners of the eyes, it’s a sign of high intoxication.
The officer is also looking to see whether the test subject follows instructions. If the test subject purposefully evades the test or refuses to follow instructions to keep their head still, the officer may interpret it as a sign that the person is trying to cheat the test or that they have something to hide. The officer checks to see if you move your entire head or just your eyes as instructed.
To perform the HGN test, the officer starts by giving the test subject instructions. They instruct the test subject to watch a specific object in front of them. The officer may use their finger or they may use a pen or pencil. They hold the object about a foot from the test subject at eye level.
The officer instructs the test subject that the officer is going to move the object to the right and to the left. The person must keep their eye focused on the object. While the officer moves the object, they look at the corner of the test subject’s eyes. They’re looking for jerking movements when the pupils of the eyes are in the corners.
Once the officer moves the object from side to side, they may also move the object up and down. The officer moves the object high and low in order to see if the test subject has a nystagmus when looking up or down. If the test subject’s eyes jerk when they look up or down, the person has a high level of intoxication compared to their usual levels of intoxication. When a person has a high blood alcohol level but little or no horizontal nystagmus, that means they’re used to having a high blood alcohol level.
The horizontal gaze nystagmus test isn’t a perfect test. It’s only one tool that an officer can use to gauge a driver’s sobriety. The test only produces reliable results if the officer performs it property.
If the officer fails to give the test subject instructions, the test may not produce good results. In addition, holding the object too close or too far from the test subject can skew the test results. Moving the object too fast can also produce inaccurate results.
The officer must be trained to properly interpret a horizontal gaze nystagmus test. If the officer doesn’t know how to identify eye movements that indicate intoxication, the results aren’t going to help the officer honestly determine a subject’s sobriety. An inexperienced officer, an officer who lacks training or an officer whose training has lapsed may not have the skills needed to appropriately interpret the test. Finally, the HGN test is only one of three standardized field sobriety tests. The test must be combined with a complete and thorough investigation in order to provide a reliable estimation of a driver’s level of intoxication.
The information on this website is for general information purposes only. Nothing on this site should be taken as legal advice for any individual case or situation. This information is not intended to create, and receipt or viewing does not constitute, an attorney-client relationship. Learn more about DUI by reading this wikipedia page.