What is the best way to access parent scope object from multi-instance directive

I have directive which can have many instances created in the same parent controller. Directive uses isolated scope with some properties taken from parent scope. One of the directive property can be the callback function which is invoked from directive when directive internal model has changed. If the callback function will change also the model from parent scope which is also assigned to other instance of directive I'm loosing the "connection" between parent scope and directive for this changed model.I have directive which can have many instances