When a law enforcement officer believes that a person they have stopped has been drinking they will ask that person to submit to a number of tests. One of those tests is the horizontal gaze nystagmus test. Most people recognize this test when they see a police officer moving a pen back and forth while the suspect follows it with their eyes. Understanding how this test is administered and interpreted can help you determine whether to take it and how to fight it if you are accused of a DWI.

The horizontal gaze nystagmus test is an attempt by the police to measure the involuntary jerking of the eye that is often associated with intoxication. This involuntary movement may show alcohol intoxication but can also be caused by the presence of PCP, certain inhalants, brain damage, diseases of the inner ear, and some people just 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 results of the test to be admissible at trial. The five stages are:

  1. Instruction
  2. Check eye for signs of brain damage
  3. Smooth pursuit
  4. Maximum deviation and
  5. Forty-five degree angle.

When the officer administers the HGN test to a person accused of DWI or DUI he or she must give very specific instructions to the person they are giving the test to. If they fail to give the proper instructions the test may be invalid and should not be presented to a jury. The instructions do not have to be word for word but the general idea must be clearly conveyed. The officer must tell the person taking the HGN test that they must keep their head still and follow the pen with their eyes only. They must tell the test taker at they must focus on the pen until told to stop. At this point the officer should position the pen twelve to fifteen inches from the test taker’s nose and slightly higher than their eyes. The person being accused of driving under the influence should then be asked to touch the top of the pen. Once this has been done the police officer should begin the test.

At the beginning of the HGN test the officer should move the pen smoothly from side to side while observing the suspect’s eye movement. At this stage the office is looking to see if the eyes track the pen easily together. If they do not track along together there is a chance that the test taker has suffered from brain damage. The office should also check to make sure that the person’s pupils are the same size. After checking for signs of brain damage the actual test will being.

On the first pass the officer is looking for smooth pursuit. Eyes should move easily from side to side with minimal jerk 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 be looking to see if the eye twitches when there is no white eye between eyeball and end of eye. The idea here is that when the eye is at it’s maximum deviation or as far to one side as it can go it should not jerk. If there is movement in the eye the person may be intoxicated. This is generally where most people who have been drinking fail the test.

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

For each stage of the test each eye is awarded a point for failing up to a maximum of six points. It is possible to score one point per pass as one eye might twitch and the other might not. Generally a score of 4 or more is assumed to mean intoxication, though it is possible for a lesser score to still be considered grounds for arrest for driving under the influence of alcohol.