White noise analysis of Phycomyces light growth response system. II. Extended intensity ranges.

By means of white gaussian noise stimulation, the Wiener kernels are derived for the Phycomyces light growth response for a variety of intensity conditions. In one experiment the intensity I, rather than log I, is used as the input variable. Under the very limited dynamic range of that experiment, the response is fairly linear. To examine the dependence of the kernels on dynamic range, a series of experiments were performed in which the range of log I was halved and doubled relative to normal. The amplitude of the kernels, but not the time course, is affected strongly by the choice of dynamic range, and the dependence reveals large-scale nonlinearities not evident in the kernels themselves. In addition kernels are evaluated for experiments at a number of absolute intensity levels ranging from 10(-12) to 10(-3) W/cm2. The kernel amplitudes are maximal at about 10(-6) W/cm2. At 10(-12) W/cm2, just above the absolute threshold, the respond is very small. The falloff at high intensity, attributable to inactivation of the photoreceptor, is analyzed in the framework of a first-order pigment kinetics model, yielding estimates for the partial extinction coefficient for inactivation epsilonI455 = (1.5 +/- 0.2) X 10(4) liter/mol-cm and a regeneration time constant of tau = (2.7 +/- 0.6) min. A model is introduced which associates the processes of adaptation and photoreceptor inactivation. The model predicts that the time constants for adaptation and pigment should be identical. This prediction is consistent with values in this and the preceding paper. The effects of pigment inactivation are simulated by a linear electronic analog circuit element, which may be cascaded with the linear simulator circuit in the preceding paper.