Figure 7.

A, Normalized maximum rod responses as a function of background intensity for control (black circles, n = 10) and GCAPs^−/− (open circles, n = 7) retina. The smooth curves represent best–fitted Hill equation with k = 1.20 and 1.29, I_R = 756 and 27 photons μm⁻² s⁻¹ for control and GCAPs^−/− rods, respectively. Error bars show SEM. The dotted curve represents the expected change in maximum response of GCAPs^−/− rods in the absence of adaptation (Eq. 4). B, Relative sensitivity as a function of background intensity. Flash sensitivity (S_F) was determined for each retina by dividing the peak amplitude of dim flash response by the flash intensity and the maximal amplitude for each retina. S_F^D is flash sensitivity in darkness. Averages of control (filled circles, n = 10) and GCAPs^−/− (open circles, n = 7) retinas. Solid lines are best-fitting Weber–Fechner functions with I_S = 272 [wild type (WT)] and 14 (GCAPs^−/−) photons μm⁻² s⁻¹. The dotted curve represents the expected change in sensitivity of GCAPs^−/− rods in the absence of adaptation (Eq. 4). C, Relative sensitivity (S_F/S_F^D) as a function of residual amplitude (R_max/R_max,dark). The decline in sensitivity against the reduction of residual amplitude (indicative of calcium level) for both rods and cones was substantially shallower in GCAPs^−/− retinas than in control retinas. The parameters of the best-fitting power functions are 2.30 for control rods (solid black line) and 1.69 for GCAPs^−/− rods (solid red line). The dashed lines are the fitting functions for control cones (black) and GCAPs^−/− cones (red) from Figure 6D. The blue dotted line represents the expected change in sensitivity in the absence of Ca²⁺ modulation. Error bars show SEM. D, Single-photon responses of rods (top) and cones (bottom) normalized by dividing them by their respective control peak amplitudes. The time of divergence of the GCAPs^−/− (red) responses from control (black) responses was 120 ms for rods and 85 ms for cones.