 These two mechanisms give ways to register and measure spatial and temporal change. Now it is clear that these two mechanisms, when coupled together, are exactly the original Reichardt detector. But now we can see exactly what that measures. Calculus allows us to write \(dx/dt =(dI/dt)/(dI/dx)\). But \(dx/dt\) is the rate change of position over time, which is speed. Importantly, it is the intensity variable \(I\) which is fixed here. Thus what this measures is how a single intensity, such as that of a moving dot, an edge, or a shadow, is varying its spatial position over time. This is motion. For a given pair of input cells, the direction of the motion is fixed (different detectors for different directions), so we have a register for velocity.