Improvements in visual performance following a pulsed field of light: a test of the equivalent-background principle.

The offset of a pulsed conditioning field of light has recently been shown to produce enhancements of temporal resolution and brightness discrimination. These enhancements are similar to those that are due to imposing a high level of light adaptation on the visual system. Here we analyze whether there is a true equivalence of adaptive state between some level of steady light adaptation and the offset of a conditioning field. The enhancements of visual function at field offset were replicated by using a suprathreshold two-pulse discrimination task and a task requiring detection of an incremental probe stimulus superimposed upon a suprathreshold pulse. These effects are shown to be qualitatively but not quantitatively similar to those produced by an equivalent background selected on the basis of its ability to raise threshold to the same degree as field offset. We conclude that the equivalent-background principle cannot be supported for our measures of foveal visual function.