Simultaneous contrast phenomena typically contain test patches that have identical luminance levels but different perceived brightness levels. We have examined a variety of simultaneous contrast illusions and have shown that when low-spatial frequencies are removed from the displays, the physical values of the test patches in the resulting images follow the same pattern as the perceived brightness levels (see "http://www.shapirolab.net/fvm2006"). Furthermore, if a wide enough range of higher spatial frequencies remains in the filtered image, the test patches do not have the scalloped pattern that has been problematic for many lateral-inhibition type models.

Here we examine two different paradigms with this approach: 1) Gilchrist has shown that equiluminant test disks placed on a photograph are perceived to have different lightness values (i.e., test disks in shadows appear bright; those in the illuminated areas appear dark). We placed seven test disks on a natural image and had observers rank the disks from brightest to darkest.  The observer rankings were nearly identical to the physical luminance values of the disks following the removal of the low spatial frequencies from the image. 2) Anderson and Winawer have shown that gray objects viewed through layers of rendered clouds take on dramatic differences in lightness. We show that the test patches are physically different when the low spatial frequencies are removed from these images. We compare the results to psychophysical estimates of the reduction in contrast sensitivity produced by the top layers. We conclude that the visual system may eliminate spatial frequencies lower than the fundamental frequency of the area of interest. The appropriate scale may be influenced by edges, grouping, scission and attentional factors.