HGN attempts to measure the amount of involuntary twitching in the eye. A police officer waves his finger in front of the suspect’s face and the cop watches the eye. Although most experts agree that there is usually some kind of twitching in the eye, there is no consensus as to how much twitching will occur in an intoxicated person’s eye.

In Pennsylvania, HGN test results have been inadmissible in DUI trials since 1992 — however, police may still use HGN to determine probable cause (although very rarely — in seven years I have seen it happen once). HGN results may also be used in civil license suspension proceedings, if there is an expert witness available. In other words, the Pennsylvania courts have throw up roadblocks to the use of junk science. It’s too bad that other states haven’t followed suit.