 The second of these simpler mechanisms is a  temporal change detector. It consists of a single input cell, whose output \(I_{1}\) at \(t_{1}\) is split into two, one of which passes through a delay, then compared with the undelayed signal \(I_{2}\) from the next moment \(t_{2}\). The quotient \((I_{2}-I_{1}) / (t_{2}-t_{1})\) measures change in I at a single input over the time period. This can be written \(dI / dt\). This, and its numerator, are zero iff there is no change in the intensities presented to the same cell at different times. The spatial variable \(x\) is fixed here.