Rapid Changes in Foveal Sensitivity Resulting from Direct and Indirect Adapting Stimuli*

Abstract

It is well known that an intense stimulus imaged upon one part of the retina has the capacity to reduce the sensitivity of a spatially remote area. Controversy has long existed regarding the interpretation of this effect—whether it is the result of stray light in the eye or of some kind of indirect inhibition. A method first used by Crawford makes it possible to measure sensitivity changes associated with the onset of an adapting stimulus by determining the threshold of a test flash at various times near the onset of the adapting stimulus. Two adapting stimuli were used alternately within each experimental session. The “direct” adapting stimulus was concentric with and larger than the foveal test flash; the “indirect” or glare-adapting stimulus was presented at a glare angle of 18°. When the luminances of these radically different adapting stimuli were suitably adjusted, the resulting foveal sensitivity changes, as a function of time after onset of adapting stimulus, were found to be identical. The equivalence held when the luminance of each adapting stimulus was reduced 1 log unit. The results are interpreted as favoring the stray-light hypothesis.