Adaptation to background lights. Ordinate plots sensitivity S_F in the presence of steady background light divided by sensitivity in the absence of background, $SFD$⁠; abscissa gives intensity of background in photons µm⁻² s⁻¹. Sensitivity was calculated as the peak response amplitude for small-amplitude responses divided by the flash intensity in photons µm⁻². Data points give means and error bars give SEMs for 20 WT rods (●), 6 GAPux;Rv−/− rods (□), and 16 GAPux rods (◆). Means have been fitted with the Weber–Fechner equation, S_F/$SFD$ = I₀/(I₀ + I_B), where I₀ is a constant and I_B is the intensity of the background light. The curve in the middle is the best-fitting curve for WT rods with I₀ = 77 photons µm⁻² s⁻¹. The curve to the left is for GAPux rods with I₀ = 20 photons µm⁻² s⁻¹, and the curve to the right is for GAPux;Rv−/− rods with I₀ = 154 photons µm⁻² s⁻¹. (Inset) Superimposed normalized responses for GAPux and GAPux;Rv−/− rods to 20-ms flashes at 238 photons µm⁻² in dark-adapted rods and in the presence of various background lights. Mean responses have been calculated from 11 GAPux rods at backgrounds of 8, 21, 75, and 204 photons µm⁻² s⁻¹, and 6 GAPux;Rv−/− rods at background light intensities of 21, 75, 204, and 760 photons µm⁻² s⁻¹.