Changes of brightness sensation (fading) that occur when images are stabilized on the retina were measured with a matching technique. Increment thresholds were measured under the same conditions. Under stabilization, the increment threshold is proportional to stimulus level (Weber’s law). Increment thresholds change at a different rate than does brightness; this suggests that these mechanisms behave independently.
J. M. B. SPARROCK, "Stabilized Images: Increment Thresholds and Subjective Brightness," J. Opt. Soc. Am. 59, 872-873 (1969)