What is Nystagmus?
Nystagmus is a term that describes a "bouncing" eye motion that is seen in two ways -- pendular nystagmus, where the eye wavers equally in two directions (like a pendulum), and jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a fast movement. Horizontal gaze nystagmus is a type of jerk nystagmus and is an involuntary motion, meaning that the person showing it cannot control it, and in fact, is unaware that it is happening. When impaired, a person's nervous system displays a breakdown in the smooth and accurate control of eye movements, resulting in a number of observable changes.
It is these observable changes that prompted NHTSA to research and standardize HGN for field sobriety tests in the 1970s. The Standardized Field Sobriety Tests (SFSTs) are used across the country whenever a law enforcement officer has reason to believe a motorist is driving impaired.
HGN in the Courtroom
Despite the proven correlation between alcohol consumption and HGN, many trial courts across the country still do not admit the results of the HGN test into evidence. Prosecutors and law enforcement have lacked either the knowledge or the https://one.nhtsa.gov/About-NHTSA/Traffic-Techs/current/ci.A-Resource-Guide-Describes-The-Science-And-The-Law-About-Horizontal-Gaze-Nystagmus.printresources, or both, to lay an adequate foundation for the admissibility of HGN.
