The stimulus-response functions of visual systems.

Two commonly used forms of presenting photoreceptor functions are compared: the incident light function, which represents photoresponses as a function of the total light intensity (stimulus plus background), and the increment stimulus function, which represents photoresponses as a function of light increments. The changes in these two functions with increasing background light are analyzed. If the receptor adapts to light according to Weber's law, the changes are very similar. The increment stimulus functions are often adequate in studies of excitation and adaptation mechanisms, while the incident light functions are more explicit when interest is focused on contrast sensitivity.