The Horizontal Gaze Nystagmus (HGN) test is a type of field sobriety test used by law enforcement officers in New York and around the country to determine whether a person is impaired by alcohol or drugs while driving. It is one of several tests that may be administered during a traffic stop, and it is considered to be a reliable and accurate way to detect driver impairment.
What is nystagmus?
Nystagmus is an involuntary movement of the eyes, where the eyes move back and forth or up and down repeatedly. There are many different types of nystagmus, and it can be caused by various factors such as neurological conditions, brain damage or certain medications.
However, in the case of the HGN test, the type of nystagmus that is being observed is called “alcohol-induced nystagmus.” This is a type of nystagmus that occurs as a result of alcohol consumption and is a common sign of a DUI.
How the HGN test is conducted
During the HGN test, the officer will ask the driver to follow a stimulus with their eyes, such as a pen or a small flashlight. The stimulus is moved from side to side in front of the driver’s face, and the officer observes the driver’s eyes for any signs of nystagmus.
There are three different types of nystagmus that the officer will look for during the HGN test:
The officer will look for any jerky or uncontrolled eye movements as the driver follows the stimulus, jerky or uncontrolled eye movements when the driver’s eyes are looking as far to the side as possible and any uncontrolled eye movements when the driver’s eyes are looking to the side before they reach a 45-degree angle.
If the officer observes any of these three types of nystagmus, it is considered a sign of impairment and may be used as evidence against the driver in court.
Limitations of the HGN test
While the HGN test is considered to be reliable and accurate, it is not foolproof. Several factors can affect the results of the test, including some medications, medical conditions and environmental factors such as flashing lights or uneven terrain.
Furthermore, some people may naturally have nystagmus or other eye conditions that could affect the results of the test. Therefore, officers need to be properly trained in administering the test and interpreting the results.