Along with the breathalyser test, the horizontal gaze nystagmus test has been treated by law enforcement officials and judges as providing conclusive proof of an individual’s consumption of alcohol. For years, judges and lawyers have allowed police officers to testify about not only their observations but also their conclusions.

Facing the HGN Test

Horizontal gaze nystagmus refers to the jerking movement of the eye as it follows an object across a horizontal plane. According to the National Highway Traffic Safety Administration (NHTSA), this uncontrollable jerking movement is evidence of alcohol consumption because alcohol inhibits coordination.

In performing the test on a suspected drunk driver, a police officer holds a pen, or similar object, about fifteen inches from the driver at eye level. As the officer gradually moves the pen horizontally, the test is whether the driver’s eyeballs begin to jerk (“nystagmus”) . Nystagmus is present if the jerking occurs before the pen is at a 45 degree angle relative to the driver’s eyeball.

Defeating the HGN Test

Maryland law recognizes that the HGN test is a scientific test. The courts do not require that a police officer possess a medical degree to testify about the HGN, but some level of expertise is needed to explain the physiology of the nystagmus, to identify the presence of nystagmus, and to make sufficiently precise measurements to insure the validity of the test.

Determining whether a police officer has these foundational requirement is a matter for a trial judge. Whether the officer correctly tested the defendant is yet another area to challenge the prosecution.

Ultimately, accepting the validity of a test, without understanding the underlying science, is the same as pleading guilty without investigating the evidence