Flash enhancement of isoluminant chromatic discrimination

Abstract

We have observed that when two isoluminant fields, which are juxtaposed with no dividing line between them, are adjusted from an initial match to differ slightly along a tritanopic confusion line, the initially apparent chromatic difference fades away over a brief but perceptible interval. A larger difference is required to reach a stable chromatic threshold. This effect was explored by flashing the fields simultaneously for various durations, and having the observer adjust the chromatic difference to threshold on the basis of the appearance of the fields at the end of the flash. As predicted, there is a duration-dependent enhancement of sensitivity for flashed relative to steadily viewed fields. A similar but smaller effect was found for fields differing along a deuteranopic confusion line, but no effect was found for a luminance discrimination. The time course of the enhancement is slow, peaking at between 0.2 and 0.4 s and ending by 1.6 s. Control experiments suggest that the effect does not depend on second site adaptation, and that it is dependent on the fields being juxtaposed. The observed fading of chromatic differences may be the result of a time-dependent averaging process which assigns a hue to a given region; when the fields are juxtaposed at equal luminance, the averaging process spills over into the adjacent region, reducing the apparent difference between the two fields over time.