Alcohol Gaze Nystagmus (AGN)

Gaze nystagmus generally means an involuntary twitching of the eye when focused fixedly upon an object or attempting to follow the object with the eyes. The focus lags, and the eye twitches in an attempt to ‘catch up’ with the object.

Specifically, alcohol gaze nystagmus indicates jerking of the eye at a lesser angle than a sober person would experience because alcohol has a depressing, or slowing, effect upon the central nervous system. In horizontal gaze nystagmus (HGN), the eyes are turned horizontally with the head held still in the forward position.

If the eyes are turned upwards, the effect is vertical gaze nystagmus (VGN). Officers usually check for VGN because it is a good indicator if the subject is under the influence of inhalants, depressants, and several widely used drugs. VGN is not part of the NHSTA’s battery of standardized field sobriety tests, but it is a useful indicator for the officer as to what the driver might have ingested.