HGN Horizontal Gaze Nystagmus -- what it looks like
HGN is one of the three standardized field sobriety tests developed by NHTSA in the early 1980s. These tests are not designed to determine impairment by alcohol. The test designers conceded that these tests do not show impaired driving. Instead, a certain number of clues on these tests show a probability that the subject has a blood alcohol content of .08 or better.