I saw two cops questioning two guys today. I heard afterward that they were caught smoking pot. Or one of them was, anyway.
At one point, the cop asked one of the guys to close his eyes and lean his head back. Then the cop shined his flashlight on the guy’s neck and throat. (This was early-ish evening, starting to get dark.)
At little later, he asked the other guy to do the same, only he didn’t shine his flashlight at the guys neck. Just asked him to lean his head back, for about 30 seconds.
From that entry:
The exam is based on the premise that a person requires at least two of the three following senses to maintain balance while standing: proprioception (the ability to know one’s body in space); vestibular function (the ability to know one’s head position in space); and vision (which can be used to monitor [and adjust for] changes in body position)…
When used to test impaired driving, the test is performed with the subject estimating 30 seconds in his head. This is used to gauge the subject’s internal clock and can be an indicator of stimulant or depressant use.