Understanding the Horizontal Gaze Nystagmus Test

The “horizontal gaze nystagmus” or “HGN” test is just one of the field sobriety tests a Colorado police officer may expect a driver to submit to when the driver is pulled over on suspicion of driving under the influence of alcohol (DUI). The test requires the driver to follow a small moving object with his or her eyes. By watching the eye movements, the police officer is supposed to be able to determine whether or not the driver is intoxicated.

Understanding the Horizontal Gaze Nystagmus TestWhile this test has an impressive, scientific-sounding name, the principles on which it is based should be analyzed carefully in any DUI criminal charge that depends on the results of the test.

As many experienced Colorado DUI defense attorneys know, the HGN test is not foolproof. Many factors can increase the rate of nystagmus, which can lead to a test “result” that makes the officer believe the driver is driving while intoxicated when this is not the case.

First, the circumstances of the test can affect the results. If the driver is looking toward a bright or flashing light, such as the sun or the flashing lights of a police car, the test may give a false positive. Second, many drivers have medical conditions that can cause the same type of nystagmus that intoxication causes. A brain injury or brain tumor, an inner ear disorder, neurological conditions like epilepsy, or a naturally-occurring case of nystagmus can all create the same “results” in an HGN test as being intoxicated.

Any driver arrested on suspicion of drunk driving should speak to an experienced criminal defense lawyer as soon as possible. Working with an attorney helps you fully protect your legal rights and pursue the best possible outcome in your case with all available resources.