Limiting time constant (τ_D) as function of light intensity and circulating current. (A) Mean values of the τ_D as a function of the intensity of the background light (I_B) in photons µm⁻² s⁻¹ for 12 GAPux (□) and 6 GAPux;Rv−/− (■) rods (only 2 GAPux;Rv−/− rods at the brightest background intensity). Curve through GAPux data points is of the form τ_D = τ_D0 + A[exp(−I_B/k)], with τ_D0, A, and k constants whose best-fitting values were τ_D0 = 69 ms, A = 160 ms, and k = 538 photons µm⁻² s⁻¹. Dashed line is linear regression for data from GAPux;Rv−/− rods with the slope constrained to be zero. The best-fitting value of τ_D was 191 ms. (B) Values of the τ_D for the same GAPux rods as in A (□) but plotted as a function of circulating current (r) normalized to its maximum value before presentation of the backgrounds (r_max). Circulating current was estimated rod by rod from the saturating value of the response to flashes in each of the backgrounds. Straight line through data are best fit of linear straight line with coefficient of determination r² = 0.76. Data for GAPux;Rv−/− rods (■) is also given as a function of circulating current, and the dashed line is again linear regression with the slope constrained to be zero and with a best-fitting value of τ_D of 191 ms. Error bars in A and B are SEMs.