The relation between the critical duration and intensity in brightness discrimination.

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Abstract
A method is described for examining the influence on foveal intensity discrimination of variations in the duration ([tau]) of [image], the increment added to the basic intensity I. In foveal intensity discrimination the Bunsen-Roscoe law ([image] [tau] = constant) holds for exposures of [image] up to the limit of a critical duration ([tau]c) at all levels of the basic intensity, I. Beyond the critical duration, intensity discrimination is independent of duration and [image]= constant. The critical duration decreases as the basic intensity increases according to an empirical relation of the form [tau]c[long dash]kl-[rho], where k and [rho] are constants. A further analysis shows that, with durations of [image] greater than [tau]e, failure to consider the influence of the critical duration may alter the form of the intensity discrimination function in such a way as to produce a rise at high intensities. This observation may partially resolve some disagreements in the work of earlier investigators.

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