Figure 5. IR knock-down rods are less sensitive than WT rods.

A, response-intensity functions for WT strain control rods and IR knock-down rods (IR-KD), dark-adapted or at steady state after bleaching 50% of the visual pigment. Data were taken from the same rods as in Fig. 3 and have been fitted with the function r = r_max [1-exp(-kI)], where r is the mean peak amplitude of the response, r_max is the maximum value of r, k is a constant, and I is the flash intensity in units of photons μm⁻². Data points are means ± SEM of 21 dark-adapted WT rods (•) with r_max = 13.6 pA and k = 0.022 photons⁻¹ μm²; 9 bleached WT rods (○) with r_max = 4.7 pA and k = 0.0011 photons⁻¹ μm²; 16 dark-adapted IR knock-down rods (▪) with r_max = 14.7 pA and k = 0.011 photons⁻¹ μm²; and 8 bleached IR knock-down rods (□) with r_max = 5.6 pA and k = 2.4 × 10⁻⁴ photons⁻¹ μm². B, mean sensitivity as a function of background light intensity for 5 WT rods (•), 5 Grb14^−/− rods (□), and 5 IR knock-down rods (Δ). Sensitivity (S_F) has been calculated by dividing the peak amplitude of small-amplitude flash responses by the flash intensity and has been normalized to the dark-adapted sensitivity . Data points give means ± SEM and have been fitted with the Weber-Fechner function, S_F/ = I₀/(I₀ + I_B), where I_B is the intensity of the steady background light in photons μm⁻² s⁻¹ and I₀ is a constant. Best-fitting values for I₀ were 73 photons μm⁻² s⁻¹ (WT), 68 photons μm⁻² s⁻¹ (Grb14^−/−), and 197 photons μm⁻² s⁻¹ (IR knock down); only curves for WT and IR knock down are shown.