Circular boundary for mouse follower

All,

I’m a bit stuck. I’m creating a series of mouse followers, with various
movie clips and other effects following the mouse or the mouse movement altering a 3-d object.

What I need to know is:
Is there a way to define a circular boundary for the mouse. So that
the mouse follower effect stays constricted to that circular area?
I can do sguare sure…but circular?

cheers, arcticacre