The horizontal gaze nystagmus test (HGN), commonly called the eye test or the pen test, is when the subject is asked to look at a pen or other object while it is moved from side to side. The purpose of this test is to determine if there is “nystagmus,” or the involuntary jerking or vibrating of the eyeball as it tries to follow the pen to the side. These movements of the eyeballs occur due to alcohol’s effect on the central nervous system.
The subject is not usually aware of the jerking motion in their eyes. That is why researchers believe it is a useful test — because the driver that is taking the test cannot hide his nystagmus, he is unable to control it. Some degree of nystagmus occurs in most people. Alcohol impairment exaggerates the presence of nystagmus.
How the Test Works:
Performance is measured on a point scoring system, with 3 possible points assessed for each of the two eyes. A point will be counted against the driver for each of the following:
The eyeball cannot stay focused on an object passing in front of it.
The eyeball jerks “distinctly” when it is all the way to the side.
The jerking begins in the eyeball before it turns 45 degrees away from center.
The officer is supposed to hold the pen 12 to 15 inches away from the face, and move the pen to the side slowly enough that it should take two seconds for it to pass from directly in front of the driver — all the way to the side.
There are causes of nystagmus other than alcohol or drug impairment. Taking medications such as seizure medications, barbiturates, and other depressants can result in nystagmus. You should inform your DUI lawyer if you take any of these medications. Nystagmus can occur simply as a result of being very tired. In cases where a failed HGN exam will play a critical role in your DUI trial, we may advise you to hire a neurologist to testify as an expert witness in your case. Such an expert may be able to testify that alcohol consumption was not responsible for the nystagmus observed by the officer.
