Three normative models based on the Change Detector analogies of Shallice (1964) are investigated to see how well they account for the integration of input over time that occurs in absolute brightness threshold experiments. Of the three, the geometric moving average receives no support. The perceptual moment and moving average models give about equally satisfactory but not perfect fits for short flash results, but the perceptual moment model is much more satisfactory for long flash results. The analysis takes into account variations in information transmission times, noise in the visual system and the quantal nature of the light input. The locus of temporal integration is discussed and a central locus preferred, following Treisman (1966).