When a police officer suspects a person has been driving under the influence (DUI), they will likely try to administer field sobriety tests. One of the most common field sobriety tests is known as the Horizontal Gaze Nystagmus (HGN) test.
Nystagmus is basically involuntary eye movement. It is a normal occurrence but is often intensified after alcohol consumption. Before conducting the HGN test, an officer is trained to look for conditions or reasons other than alcohol that could cause the driver to fail the test.
Nystagmus can be caused by a number of factors, such as:
- Head injuries
- Rotation of the body
- Temperature differences from one ear to the other
- A foreign fluid in the blood
- Objects moving in and out of sight
- Brain tumors/damage
- Inner ear disorders
If the officer determines, to the best of their knowledge, that the driver does not have a medical condition that would impact the eyes, an HGN test follows. The officer administers the test by tracking a penlight or finger in front of the driver’s eyes at about one foot distance from the face. The driver is asked to follow the light or finger with their eyes.
The examining officer watches for these signs that indicate possible intoxication:
- The eye cannot follow a moving object smoothly (or jerking of the eyeball at rest)
- Jerking is distinct when the eye is at maximum deviation
- Jerking as the eyeball moves to the side before it gets to a 45 degree angle
It is important to note that Nystagmus is a naturally occurring sensation of the eyes. Drugs and/or alcohol only increase or exaggerate the twitching or jerking effects. Because of this, HGN is a very subjective test. The results can be altered by a wide array of factors, and it is ultimately up to the police officer to make a subjective judgment. HGN tests are often used and abused by police and prosecutors in DWI cases. That is just one reason why a skilled DUI attorney is essential in defending a drunk driving case.