When a law enforcement officer believes that a person they have arrested has been drinking, they will ask that person to undergo a series of tests. One such test is the horizontal gaze nystagmus test. Most people recognize this test when they see a police officer waving a pen around while the suspect follows him with his eyes. Understanding how this test is administered and interpreted can help you determine if he should take it and how to combat it if you are charged with a DWI.

The horizontal gauze nystagmus test is an attempt by police to measure the involuntary twitching of the eye that is often associated with intoxication. This involuntary movement can show alcohol intoxication, but it can also be caused by the presence of PCP, certain inhalants, brain damage, inner ear disease, and some people naturally have some movement even when sober.

The HGN or horizontal gaze nystagmus test has five stages. Each stage must be completed perfectly for the test results to be admissible at trial. The five stages are

1. Education

2. Check the eye for signs of brain damage

3. Smooth Chase

4. Maximum deviation and

5. Angle of forty-five degrees.

When the officer administers the HGN test to a person charged with DWI or DUI, they must give very specific instructions to the person they are giving the test to. If they do not give the proper instructions, the evidence may not be valid and should not be presented to a jury. Instructions do not have to be word for word, but the general idea should be clearly conveyed. The officer should tell the person taking the HGN test that they should keep their head still and follow the pen with their eyes only. They should tell the examinee that he is to concentrate on the pen until told to stop. At this point, the officer should place the pen twelve to fifteen inches from the test taker’s nose and slightly higher than the test taker’s eyes. The person charged with drink driving should be asked to touch the top of the pen. Once this is done, the police officer must begin the test.

At the beginning of the HGN test, the officer should gently move the pen from side to side while watching the suspect’s eye movement. At this stage, the bureau is looking to see if the eyes follow the feather easily together. If they do not scan together, there is a possibility that the examinee has suffered brain damage. The office should also verify that the person’s pupils are the same size. After checking for signs of brain damage, the actual test will be done.

On the first pass, the officer looks for a smooth chase. Eyes should move easily from side to side with minimal jerky movement. If the eye jerks or twitches as it moves from side to side, it is a sign that the person may be intoxicated.

On the second pass, the officer should look to see if the eye twitches when there is no white eye between the eyeball and the end of the eye. The idea here is that when the eye is at its maximum deviation or as far as it can go to the side, it should not jerk. If there is movement in the eye, the person may be intoxicated. This is usually where most people who have been drinking fail the test.

Lastly, the officer checks to see if the eyeball twitches when the eye is looking at a forty-five degree angle. This is a spot that is roughly halfway between as lateral as possible and as straight as possible. The eye should not twitch or jerk at a forty-five degree angle.

For each stage of the test, each eye receives one point for failing up to a maximum of six points. It is possible to score one point per pass, since one eye may twitch and the other may not. In general, a score of 4 or higher is assumed to mean intoxication, although a lower score may be considered grounds for a DUI arrest.