The Horizontal Gaze Nystagmus Test is one of the three tests approved by the National Highway Traffic Safety Administration for detection of impairment.
This test is used by Police Officers when administering Field Sobriety Tests during a stop for Driving While Intoxicated.
The other two tests, the Walk and Turn Test and the One-Legged Stand Tests are both Balance Tests as well as Divided Attention Tests.
The Horizontal Gaze Nystagmus Test is testing for an involuntary motor reflex normally present when an individual is intoxicated.
For clarification, this article will first explain what this test consists of, then explain what the Police Officers are attempting to observe.
The Horizontal Gaze Nystagmus Test is one where the Police Officer asks the Driver to keep their head straight while looking straight a head.
The Officer will introduce a stimulus for the Driver to follow with their eyes along, normally either their finger or a pen.
The Officer keeps the stimulus between twelve and fifteen inches from the Drivers Face.
The Police Officer will then move the stimulus from left to right, and right to left.
Additionally, they may sometimes move the finger up.
Though many believe they are being tested on their ability to follow the stimulus, this is not completely true.
Though a Driver's inability to follow the stimulus with their eyes will certainly be recorded and perhaps used in the Police Officer's judgement regarding whether to arrest the Driver for Driving While Intoxicated or not, it is not the main purpose of this test.
The Horizontal Gaze Nystagmus Test's main purpose is to observe any Nystagmus of the eyes.
Nystagmus is in an involuntary twitching of the eye which is present when an individual is intoxicated.
Though other factors can cause the appearance of Nystagmus, this is more common in those substantially impaired above the legal limit.
During the Test, the Police Officer is looking for several things with regards to Nystagmus.
First, when moving the stimulus from the center of the face to the left or right, the Police Officer is looking for the onset of Nystamus prior to forty-five degrees.
That is, the eye begins to jerk prior to the stimulus reaching fort-five degrees from the center of the Driver's face.
Second thing the Officer is looking for is Distinct Nystagmus at maximum deviation.
During this test, the stimulus is moved to the towards the outside of the face such that the Driver's eye is as far over as it will go.
The Police Officer will hold the stimulus there for at least four seconds.
They are looking for Nystagmus that is distinct and sustained.
Lastly, the Officer is looking for a lack of smooth pursuit.
During the test, as the stimulus is moved left to right and back, the Police Officer is looking to see whether the eyes track the stimulus smoothly, or do they stop and "catch up" as they try and follow the stimulus.
Sometimes the Police Officers will also check vertical Nystagmus.
During this test, the stimulus is raised several inches above the eye line of the Driver to see if Nystagmus is present.
Though this test is not necessarily a part of Standard Field Sobriety Tests, it is supposed to be a good indicator of severe and substantial impairment.
This concludes a review of the Horizontal Gaze Nystagmus Test performed during Field Sobriety Tests.
This test, along with the Walk and Turn Test and the One-Legged Stand Test are voluntary tests designed to not only detect impairment, but also to gather evidence used against the Driver if they are arrested for Driving While Intoxicated.
For this reason, it is recommended that you politely and respectfully refuse to perform these tests.
If you have already been arrested for Driving While Intoxicated, it is advisable to speak with a DWI Attorney in your area for specific advice on your case.