During the last two decades the presumption that sensitivity regulation is basically controlled by photochemical processes located in the outer segment of the receptors has been replaced by much more complex theoretical conceptions. A recent review paper on gain controls in the retina by Dunn and Rieke (2006) gives an excellent illustration of this development. The authors argue that there are multiple retinal gain controls (i.e. adaptation mechanisms) that adjust sensitivity to different aspects of the light stimulus such as changes in mean intensity, variability about the mean (i.e. contrast variability) and spectral composition, and that the gain controls have diverse temporal and spatial properties, serve different functional roles and are located at different sites in the retina. Indeed, an additional dimension of complexity is introduced in that the gain controls are assumed to interact with other computations carried out in the retinal pathways.

In support of their suggestion that gain controls of the cone system may operate at different sites in the retina, they present evidence that both small and large steps in mean intensity and contrast may alter the gain adaptation level of ganglion cells, while only large steps change the gain of the receptors. Strong support in favour of the suggestion that gain controls may operate at different sites had previously been provided by Ahn and MacLeod (1993) and Yeh et al. (1996).