English: Two different mechanisms for motion detection. Left) A "Reichardt detector" consists of two mirror-symmetrical subunits. In each subunit, the luminance values as measured in two adjacent points become multiplied (M) with each other after one of them is delayed by a low-pass filter with time-constant "tau". The resulting output signals of the multipliers become finally subtracted. Right) In the gradient detector, the temporal luminance gradient as measured after one photoreceptor (δI/δt, Left) is divided by the spatial luminance gradient (δI/δx). Here, the spatial gradient is approximated by the difference between the luminance values in two adjacent points.