As stated in this paper, Laughlin’s 1981 data provided a poor fit to the measured response nonlinearity. But Laughlin made more accurate measurements in 1987, which provided an excellent fit to the measured response nonlinearity. See:

Laughlin, SB, Howard, J, and Blakeslee, B. Synaptic limitations to contrast coding in the retina of the blowfly Calliphora. Proc Royal Society London B, 231(1265):437–467, 1987.