A common test used by police officers conducting field sobriety tests is to have a suspect hold his/her head still and follow a pen tip with their eyes. What are the officers looking for? What do your eyes do while you’re under the influence that they don’t do when you’re not?
It’s the “Horizontal Gaze Nystagmus” test. Basically, your eye movements react differently under the influence of alcohol. Seasoned cops can get a pretty accurate guage of one’s BAC level with this test. (so I’m told) There is also a “Vertical Gaze” test for a few types of narcotics.
A person’s BAC can be determined by this formula
50 - angle of onset = B.A.C.
This can also be used to support the breathalyzer reading. Vertical gaze nystagmus typically occurs with very high blood alcohol levels, CNS depressant use, inhalant use, and PCP use.
Hmm, maybe I’ll actually finish that post. So if someone had an onset of nystagmus at 40 degrees, their blood alcohol would be estimated at .10