vision: how to use event structure from the output of the polling structure.

Hello.

I'm making a video detection program with vision. I'm using "IMAQ count objects" to detect people in movement and identify if a people enters or leaves by going through bounded areas (left center and right). if somebody is detected, the program returns booleans depending on the areas.
this function periodically repeats and verify the image.
To make my work easyier, I'd rather like to use the event programming, which would permit me to check the values of the three areas only when one of them is changing.
problem, the event structure only works with commands and property nodes "value(sign)" and even if I use the "value(sign)" the event structure considers the value changing even if the polling returns the same value without changing

is there another way to adapt an event structure from a boolean output or an IMAQ count objects function when the beginning of the program is doing polling?