Intensity and the Process of Photoreception

Photoreception in animals like Mya and Ciona is essentially a twofold process. This is evidenced by the duality of the reaction time in its division in to an initial exposure period and a subsequent latent period. The underlying mechanism of photoreceptlon follows this composition of the reaction time. During the exposure or sensitization period a photochemical reaction is initiated in which a photosensitive substance S is decomposed into its precursors P and A. • In the subsequent latent period these freshly formed precursors serve to catalyze an independent reaction involving the transformation of an indifferent material L into an active substance T. This active material T sets off the nervous impulse for a response (Hecht, 1918-19, a, b). The exact interrelation between the two reactions of photoreception and the external energy is of importance in a complete understanding of the nature of the receptor mechanism. Of the two processes only the initial photochemical reaction is amenable to direct experimental modification. However, the duration of the subsequent latent period reaction is dependent on the products of the photochemical reaction. In addition, variations in the velocity of the latent period reaction are easily measureable with a stop-watch. These circumstances enable one to study quantitatively the effect of changes in the photochemical reaction on the properties of the latent period reaction. The amount of fresh precursors produced by the initial photochemical reaction is a function of the available energy. Both components of this energy can be controlled independently, and their actions investigated. The effect of changes in the duration of exposure at con537