Examine these figures closely. Note the yellow areas on the left and on the right. Now step back from your computer screen about 6 or 8 feet and look at them again. Do they still appear as they did close up? Most people would probably see the shape of the right yellow area defined by the black squiggly line.It is generally assumed that luminance is mediated primarily by the long and middle wavelength cones. There is little luminance contrast between the yellow and white areas. This implies that the excitations of L and M cones are approximately equal and the isoluminant contrast is mediated by the short wavelength cone system.
The filling-in phenomenon confirms that the spatial resolution of the short wavelength cone system is poor. Also, it demonstrates that areas of color, mediated by the y-b opponent system tend to be constrained by contours mediated by the long and middle wavelength sensitive cones.
Boynton predicted this illusion before he produced it for the first time.