>>(4) describing a circle with a first finger at one's temple, to indicate a belief that another suffers insanity

the only ASL gesture i know is a variation of this-- If you fold down all your fingers except the pinkie, and they use your thumb to point to the first joint (top of the finger) this is ASL for a little or very little. If you now use your pinking to make circle at the temple, (standard crazy sign) you get a little crazy-- or wacky-- not as insulting and could be complimentary--
(some one was talking about me...)