What is an Officer Looking for During the Horizontal Gaze Nystagmus Test?

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.

What is the Horizontal Gaze Nystagmus 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.

What Does the Officer Look for During the Test?

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.

How Does the Officer Perform the Test?

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.

Is the Test Full Proof?

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.

FAQ: Horizontal Gaze Nystagmus (HGN) Test in a DUI Stop

1) What is the HGN test?

The Horizontal Gaze Nystagmus (HGN) test is a roadside sobriety test used during DUI investigations. It measures a driver’s eye movements to help officers determine whether alcohol impairment may be present.

2) What does “nystagmus” mean?

Nystagmus is an involuntary movement in the eye’s iris and pupil. It can cause the eyes to jerk or flutter, especially when looking to the side.

3) Why do police use the horizontal gaze nystagmus test?

Police use the HGN test because research shows the eye may have horizontal nystagmus when a person has a significant bodily alcohol content. Officers can use eye movement as one indicator of possible intoxication.

4) What is the officer looking for during the HGN test?

During the HGN test, the officer checks whether the subject’s eyes move normally. Specifically, they look for:

• Jerking or fluttering of the pupils when the eyes move toward the corners

• How soon the eyes begin to jerk as the gaze moves outward

• Whether the movement starts earlier (farther away from the corner), which can indicate higher intoxication

5) Can a person control the eye jerking in an HGN test?

No. The jerking movement is involuntary and cannot be controlled by an intoxicated person.

6) Does the officer also judge how well you follow instructions?

Yes. Officers also look at whether the subject follows instructions, including keeping their head still and moving only their eyes. If someone refuses or appears to evade the test, the officer may interpret that as an attempt to cheat or hide something.

7) How is the HGN test performed?

The officer typically:

• Gives instructions first

• Holds an object (finger, pen, or pencil) about a foot away at eye level

• Moves the object left and right

• Watches the corners of the subject’s eyes for jerking movements

• Checks if the subject tracks the object properly

8) Can the officer move the object up and down too?

Yes. After moving side to side, the officer may move the object up and down to check for nystagmus when looking upward or downward.

9) What does it mean if someone shows nystagmus when looking up or down?

The content explains that if a person’s eyes jerk when looking up or down, it can suggest a higher level of intoxication compared to their usual levels.

10) Is the HGN test “foolproof”?

No. The HGN test is not perfect and is only one tool used in DUI investigations. It is only reliable if performed properly by the officer.

11) What can make the HGN test results inaccurate?

The HGN test may produce unreliable results if:

• The officer fails to give proper instructions

• The object is held too close or too far

• The object is moved too quickly

• The officer is not properly trained or their training has lapsed

12) Does officer training matter for the HGN test?

Yes. The officer must be trained to interpret the eye movements correctly. An inexperienced officer or one lacking proper training may not accurately identify signs of intoxication.

13) Is the HGN test the only sobriety test police use?

No. The HGN test is only one of three standardized field sobriety tests. It should be combined with a complete investigation to estimate intoxication more reliably.