Register optical flow moving over certain pixels

So I'm working on a project in which i want to register a count when a nonzero optical flow vector passes by a specific section of a video feed. Is there a built in function in the image processing toolbox that would allow me to say count up if optical flow present. I have the video feed working and have applied optical flow onto it, this is the final step but struggling to extract data form the video feed and specifically target one area to check for optical flow.