This is a graphical representation of the simplest unit of the motion detector, consisting of two intensity detectors spaced some distance apart, a delay process and a multiplier.  The two semi-circles at the top represent detectors that read image intensity at two separate points in space continuously through time.  The circle labeled m is a multiplier function and the box labeled T is a delay function that delays the input from the first detector in a specific way so that the highest response will be to objects moving from left to right at a speed specified by the detector.  The design of this model suggests parameters that may be used to tune the detector’s response: The spacing between the two detectors and the length of the delay.