A DUI investigation often begins with observations made during a traffic stop. If an officer suspects alcohol or drug impairment, you may be asked to complete a series of field sobriety tests before any arrest is made.
One of the most common is the horizontal gaze nystagmus test. Understanding what this test measures and what it can and cannot prove may help you better understand how it fits into a DUI investigation.
How does the horizontal gaze nystagmus test work?
Horizontal gaze nystagmus refers to an involuntary jerking of the eyes as they move from side to side. While some eye movement is normal, alcohol and certain drugs can make the jerking more pronounced, allowing trained officers to look for signs that may suggest impairment.
During the test, you are typically asked to keep your head still while following a small object, such as a pen or light, using only your eyes. The officer watches each eye for several indicators that have been recognized through standardized field sobriety testing procedures.
These observations generally include:
- Whether your eyes move smoothly while tracking the object
- Whether noticeable jerking occurs when your eyes reach their outer position
- Whether the jerking begins before your eyes reach a certain angle
The horizontal gaze nystagmus test does not measure your blood alcohol concentration. Instead, it provides one piece of information that officers may consider when deciding whether additional testing is appropriate.
The results should also be interpreted carefully. Certain medical conditions, neurological disorders, eye conditions and other non alcohol related factors may also affect eye movement. Officers are trained to check for some of these issues before beginning the evaluation, but the test is not considered conclusive on its own.
If impairment is still suspected, the investigation often continues with other field sobriety tests or chemical testing, such as breath, blood or urine analysis. Courts may also examine whether the officer followed standardized testing procedures before deciding how much weight to give the results.
A horizontal gaze nystagmus test can become an important part of a DUI case, but it is only one part of the overall evidence. If you have questions about your rights during a traffic stop or Tennessee criminal defense, seek legal guidance as soon as possible.