03.13.09
What the heck is Horizontal Gaze Nystagmus?
Perhaps the most curious of the field sobriety exams in DUI investigations is the HGN (horizontal gaze nystagmus) test. Few drivers have any idea of what it is or why officers will check their eyes in a DUI investigation. It is not uncommon for an attorney to hear from their client the statement “I know I passed the eye exam” without the client having any understanding of what the officer was actually checking.
The National Highway Traffic Safety Administration (NHTSA) describes nystagmus as “an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye,” and horizontal gaze nystagmus (HGN) as “a lateral or horizontal jerking when the eye gazes to the side.” In essence, HGN is a type of eye twitch which can be caused by many things, including central nervous system depressants such as alcohol. Many, if not most, officers have been trained to look for this jerking as a sign of alcohol impairment.
The HGN test is the standardized method of detecting HGN developed by NHTSA as part of the standardized field sobriety test battery for use by law enforcement in DUI investigations. Officers are trained to looked for 6 clues by moving a pen or other stimulus back-and-forth across a subject’s eyes. An observation of 4 of 6 or 6 of 6 clues is considered an indicator of impariment according to this test. Some states accept the HGN test as a valid tool, while others question the dubious scientific nature of this exam. In Georgia, however, it is accepted by the courts as a valid scientific indicator of impairment due to alcohol. Essentially, the only thing a prosecutor must do to admit the officer’s observations and test results into evidence against a DUI defendant is sufficient proof that the officer administered the HGN exam consistently with his training. The jury will also be instructed that the HGN test is based on accepted scientific principles.
Since Georgia law places such significance on this strange “test”, it is important that a driver in this state understand at least the basics about it. While nystagmus may be caused by alcohol impairment, it can also be caused by dozens of other factors including head injuries, blood pressure medication, flashing lights, inner ear problems, etc. Some people even have it naturally! A driver must be aware of these other causes of nystagmus as law enforcement may consider the resulting nystagmus to be caused by too much alcohol, rather than by, for example, medication for hypertension (Lasix, Vasotec, Adalat, etc.), which will lead to an arrest for DUI. Of course, like all field sobriety tests in Georgia, participation in an HGN test is strictly voluntary. It would be wise to know it before you do it!
k & b, llc
