Demonstration 1a (1a.swf: 3 kbytes)
The luminance levels of the disks in the top row modulate from light to dark; the luminance levels of the disks in the bottom row modulate from dark to light (sawtooth waveform).
The red buttons change the background: when the background is grey, the disks group together by rows; when the background is split light/dark (or coloured), the disks primarily group along the diagonals. Therefore, even though the disks are physically the same, the background changes the gestaltian groupings. This suggests that grouping processes can use the contrast information that cuts across the figure-ground border.
Some observers can group by diagonals and by rows, suggesting that both luminance and contrast can be used in gestalt processes.
Demonstration 1b (1b.swf: 3 kbytes)
Same as demonstration 1a, except that the luminance of the disks modulates with a triangle waveform.
Demonstration 2 (2.swf: 3 kbytes)
The luminance levels of the disks in the top row modulate from light to dark; the luminance levels of the disks in the bottom row modulate from dark to light (sawtooth waveform).
The red button changes the background. When the background is grey, the disks group together by rows. When the background consists of two gradients, apparent motion sweeps from right to left. The motion is defined by the contrast of the disks relative to the background.
The disks are physically the same in both conditions, but the perceived gestaltian groups are not the same. The gestalt, therefore, must be determined by contrast information that crosses the figure-ground border.