Figure 4. Light adaptation of control and Nckx2^−/− cones.

Responses were evoked by step of 500 nm light (at t = 0 s) followed by bright saturating test flash in control (A) and Nckx2^−/− (B) cones from Gnat1-deficient mice. Intensity of the step lights was 470, 1.5 × 10³, 4.1 × 10³, 1.3 × 10⁴, 5.6 × 10⁴, 1.7 × 10⁵ photons μm⁻² s⁻¹. The red traces indicate responses to a 13,000 photons μm⁻² step of light. (C) Residual response amplitude 2 s after onset of step light in control (black, n = 8) and Nckx2^−/− (red, n = 7) cones. Fractional residual response, R_res/R_max, was estimated as the amplitude of the residual saturated response R_res evoked 2 s after onset of the background exposure, normalized to the maximal response in darkness R_max. Solid curves indicate the fitting function of Eqn. (2) with I_b of 1.2 × 10⁵ and 6.5 × 10⁴ photons μm⁻² s⁻¹ and k of 0.58 and 0.59 for control and Nckx2^−/− cones, respectively. (D) Comparison of the responses of control (black) and Nckx2^−/− (red) cones to backgrounds of 4.1 × 10³ photons μm⁻² s⁻¹ (dashed lines) and 1.7 × 10⁵ photons μm⁻² s⁻¹ (solid lines), replotted from (A,B). (E) Integration times of saturated responses to identical test flashes elicited in backgrounds of increasing intensity for control (n = 8) and Nckx2^−/− (n = 7) cones. The test flashes were delivered 2 s after the onset of the background as shown in (A,B), respectively. Error bars show SEM. Linear fits to each data set reveal the convergence of the response kinetics for the two genotypes with increasing background light intensities.