The quantal source of area supralinearity of flash responses in Limulus photoreceptors

The time integrals of the responses of dark-adapted Limulus ventral photoreceptors to flashes exhibit a supralinear dependence on intensity at intermediate intensities. By decomposing the responses into their elementary single-photon components ("bumps"), we are able to calculate the overall quantum efficiency and to display the time courses of the bump amplitude and rate of appearance. Since the time course of the flash response is not slow compared with that of the bump, it was necessary, in order to carry out the decomposition, to develop a new technique for noise analysis of dynamic signals. This new technique should have wide applications. Our main finding is that the supralinearity of the flash responses corresponds to an increase in bump amplitude, with little change in bump duration or quantum efficiency. The time courses of the bump rate and of the change in bump amplitude are peaked and have widths similar to that of the response itself. The peaks of the time courses of the bump rate and amplitude displayed against the starting times of the bumps do not coincide and occur approximately 80 and approximately 40 ms, respectively, before the peak of the response. The time from the start of a bump to its centroid is approximately 70 ms, which means that the time at which the bump centroid reaches its maximum follows the response peak by 30 ms. These results impose constraints on possible mechanisms for the amplitude enhancement.