Families of b-wave responses to the five dimmest flash intensities from Fig. 1, for subject A.M.C., in a single experimental session. A, in dark-adapted conditions; B, in the presence of a very dim background of 0.015 Td. The traces in B are comparable to the five smallest responses in Fig. 1A, but were obtained in a single session. Flashes delivered 0.0027, 0.0054, 0.011, 0.022 and 0.043 Td s. Each trace has been averaged from between 106 and 140 responses. C and D plot the response versus intensity relations for the traces in A and B. The curves are as follows: C, dotted curve is eqn (2) with rmax= 95 μV and Q0= 0.05 Td s; dashed curve is an exponential saturation, rmax (1 − exp(–Q/Q0)), with rmax=−5 μV and Q0= 0.002 Td s; continuous curve is the sum of the dotted and dashed curves. D, continuous curve is eqn (2) with rmax= 90 μV and Q0= 0.07 Td s.