On the Variability of Critical Illumination for Flicker Fusion and Intensity Discrimination

From the data of experiments with bees in which threshold response is employed as a means of recognizing visual discrimination between stripes of equal width alternately illuminated by intensities I(1) and I(2), it is shown that the detectable increment of intensity DeltaI, where DeltaI = I(2) - I(1), is directly proportional to sigma(I2) (I(1) being fixed). From tests of visual acuity, where I(1) = 0 and the width of the stripes is varied, sigma(I2) = kI(2) + const.; here I(2) = DeltaI, and DeltaI/I(2) = 1. When the visual excitability of the bee is changed by dark adaptation, lambdaI identical with kDeltaI (= k' sigma(DeltaI)) = k'' I + const. For the measurements of critical illumination at threshold response to flicker, sigma(I2) (= sigma(DeltaI)) = k I(2) = k' DeltaI + const. The data for critical illumination producing threshold response to flicker in the sun-fish Lepomis show for the rods sigma(I2) = K I(2) for the cones sigma(I2) = K'(I(2) + const.). The data thus indicate that in all these experiments essentially the same visual function is being examined, and that the recognition of the production of a difference in effect by alternately illuminated stripes takes place in such a way that d (DeltaI)/d (sigma(I2)) = const., and that DeltaI is directly proportional to I (or "I(2)," depending on the nature of the experiment). It is pointed out that the curve for each of the cases considered can be gotten equally well if mean I or sigma(I) is plotted as a function of the independent variable involved in the experiment. Certain consequences of these and related facts are important for the treatment of the general problem of intensity discrimination.