Figure 7. Two component analysis of GRK7-tg rod responses.

A different rod from a GRK7-tg zebrafish was stimulated with a series of 76 flashes delivering ∼8 photons μm⁻². A, sample consecutive responses. The amplitude of each individual response was determined by least-squares fitting each trace with eqn (3) (with n = 2.5 and with t_peak constrained to be in the range 1.0–1.5 s). B, amplitude histogram for the 76 responses, plotted using a bin width of 0.077 pA. The smooth curve plots a laterally shifted sum of Poisson-weighted Gaussian components, with an offset of 0.182 pA (see Results), and with remaining parameters: a = 0.23 pA; ϕ= 0.99; σ₀= 0.062 pA; σ₁= 0.045 pA. C and D, analysis of ‘small responses’, defined as those responses with an amplitude lying between the lower and upper threshold levels of 0.08 and 0.28 pA indicated in the amplitude histogram in B. C, ensemble mean μ_T(t) of all 76 responses, together with ensemble mean μ_S(t) of the selected small responses (32 from 76 responses); the curve fitted to μ_S(t) is the Poisson kinetics of eqn (3), with r_peak= 0.18 pA, t_peak= 1.4 s, and n = 2.5. D, ensemble variance for the same selected responses (red trace), compared with the square of the ensemble mean μ_S(t)² (black trace) from C. E and F, analysis of responses to the ‘large events’, defined as the 76 raw traces after subtraction of the mean small response μ_S(t) determined in C. E, ensemble mean of the large responses, μ_L(t). F, ensemble variance of the large responses, (red trace), together with a scaled version of the square of the mean of the large responses, μ_L(t)² (black trace); with a scaling factor of 1/0.65 the two traces coincided, yielding an estimate for the number of photoisomerizations underlying the large responses of ϕ= 0.65; see eqn (4a). G, estimated quantal response of the ‘large events’, a_L(t), obtained by dividing the mean response in E by the estimate for ϕ; see eqn (4b). a_L(t) was fit with the Poisson kinetics of eqn (3), with r_peak= 0.24 pA, t_peak= 1.04 s, and n = 2.5. For this cell, the macroscopic sensitivity was lower than in other GRK7-tg rods, with Q_1/2≈ 100 photons μm⁻² compared with the mean of 55 photons μm⁻² (Table 1).