Pupil test for marijuana?

[Nature Boy]

I was watching an old repeat of Cops tonight on TV and saw a police officer performing tests on a man suspected of being on illegal narcotics. The dude was on meth and was scrutinized by the officer by matching his pupil size to a chart and testing the reaction of his pupils to light. However the cop mentioned a test they perform to find out whether someone is under the influence of marijuana (the suspect claimed he was just stoned, not on meth). The test involves pointing a finger straight at the suspected marijuana smokers' eyes in which he's expected to follow the finger with his vision, coming closer until the finger touches the patch between the eyes. The officer claimed that if the pupils do not cross over, as they normally would, as the finger comes in closer, that the suspect is under the influence of marijuana.

This seemed odd to me. Could this possibly be true?

[Solidly-here]

The quickie sobriety tests (especially individually) do NOT show that a person is under the influence. They will perform several tests, and see how many the possible-Pot-smoker can pass.

The finger-in-the-eye test is hard for some Pot smokers to do. I, however, can pass that one. If you are worried, have a friend do it to you one day when you're blazed 9and see how you do on it).

Another test is reciting the ABCs. Many Pot smokers will sing it abcdefg, hijk... (this is also a flunk).

[Forthesevenlakes]

swim always figured the cops were looking for nystygmus with the pupil test, that is, the involuntary movements of the eyes from side to side that occur under the influence of many drugs. people may think they are following the finger smoothly, but their eyes will rapidly saccade to one side or the other. swim had this administered to him before, but he was sober, and thus was fine. however none of these tests are really accurate, swim doesnt think...in fact swim (definitely wasnt me!) had an officer tell him, "we know you're on something from the test, but we cant prove what, so we cant do anything. just make someone else drive."