The horizontal gaze nystagmus (HGN), sometimes referred to by DUI defendants as the “pen test,” is frequently an important part of Georgia DUI cases. Nystagmus is described as a “bouncing” or “involuntary jerking” of the eye. Normally when the eyes follow an object moving left and right, the lateral rectus muscle (contraction pulls the eye away from the nose) contract and relax appropriately so the movement is smooth. If the concentration and relaxation of these two muscles is not coordinated, then the eye movement is jerky and results in nystagmus.

 

There are numerous types of nystagmus. However, in a DUI investigation, officers should be concerned with alcohol gaze nystagmus (AGN) and more specifically horizontal gaze nystagmus (HGN). HGN refers to jerky eye movement from left to right to left. Vertical gaze nystagmus (VGN) is also frequently a part of Georgia DUI cases. However, vertical gaze nystagmus is not a part of the National Highway Traffic Safety Administration’s (NHTSA) standardized field sobriety battery of tests. The theory underlying testing for horizontal gaze nystagmus (HGN) is that alcohol, a central nervous system depressant, affects motor control systems for the eye and produces the uncoordinated concentration and relaxation of the two muscles. HGN represents involuntary movement, and the individual exhibiting HGN cannot control eye movement. According to the NHTSA validation studies, the horizontal gaze nystagmus test is the most powerful of the three tests which comprise the standardized field sobriety test battery.