Hubel & Wiesel 's (1962) description of the receptive fields of cat visual corti cal neurones suggests that fiimple cells may behave linearly, whereas complex cells are clearly non-linear. But laffei & Fiorentini (1973) have reported that the modulation depth of the response of simple cells to drifting gratings is proportional to the logarithm of the grating's contrast. If simple cells were linear, the response would be directly proportional to the contrast.