We report that when the luminance of a homogeneous spot of light is gradually increased or decreased, there are conditions in which the brightness of the spot is spatially nonuniform. When the spot luminance is increased, brightness changes in the spot's center lag behind changes at the edge and brightness appears to sweep inward, Conversely, if the luminance of the spot is decreased, there is a relative lag in the darkening toward the center of the spot and darkness seems to spread inward. In Experiment 1 we found that with both increasing and decreasing luminance sweeps, the strength of the brightness filling effects was strongest with luminance sweep durations of 0.25-0.5 sec. In Experiment 2, the sweep duration was held constant at 0.5 sec; the filling effect was seen when the dwell time spent at each luminance step was less than about 100 msec, but nonuniformities were not observed at longer dwell times. In Experiment 3, a spot of light was positioned to surround the optic disk in one eye. Surprisingly, when the spot was luminance modulated from bright to dark, darkness appeared to sweep from the edge to the center of the modulated disk, even though most of the disk's interior was imaged on a portion of the retina devoid of photoreceptors. These findings are consistent with the hypothesis that a neural filling-in mechanism in visual cortex plays a key role in brightness perception.
Paradiso, M. A., & Hahn, S. (1996). Filling-in percepts produced by luminance modulation. Vision Research, 36(17), 2657–2663. https://doi.org/10.1016/0042-6989(96)00033-8