 The original Reichardt detector involved two spatially-separated retinal transducers responding to intensities \(I_{1}\), \(I_{2}\) of incident light respectively at times \(t_{1}\), \(t_{2}\). Essentially, I measures the number of incident photons on a single retinal cell at a time. (For convenience we can regard the input into a given retinal cell as the same as its output, though the output is electrochemical, as befits a transducer.) One of the two inputs, say \(I_{1}\), undergoes a delay, and so is compared at a later time \(t_{2}\) with the other input \(I_{2}\) undelayed. There are different ways to make the comparison: Reichardt suggested both addition and multiplication, but it is more reasonable to consider subtraction of one from the other (a fixed operation for a given Reichardt detector). The difference between \(I_{1}\) and \(I_{2}\) is then propagated further into the CNS. Thus we have a change detector, there is no change in intensity from \(I_{1}\) at \(t_{1}\) to \(I_{2}\) at \(t_{2}\) iff \(I_{2} - I_{1}=0\).