 The first of these is a spatial change or spatial difference detector. This consists of two transducers whose outputs without a delay are compared by subtraction. If the two input cells have positions \(x_{1}\), \(x_{2}\), then the quotient \((I_{2} - I_{1}) / (x_{2} - x_{1})\) measures the difference in simultaneous values of I over the spatial distance \(x_{2}-x_{1}\). This can be written as the derivative \(dI / dx\). It, and its numerator, are zero iff there is no difference in \(I\) measured by the two input cells at the same time. The temporal variable \(t\) is fixed here. In passing, we can also see a mechanism here for the perception of spatial relations as emphasised by Gestalt psychologists.