Change saturation dynamically around the Player object?

The idea I am trying to implement is to change the saturation around the player object(second person view) as it moves in the game screen. However, the saturation of the entire screen changes. I want the saturation to change only in a certain radius around the player game object. How do I achieve this?

My shader is only a slight modification in : http://armedunity.com/topic/4950-brightnesscontrastsaturation-shader/