Another intriguing effect is seen in l, invented by Italian psychologist Gaetano Kanizsa: the Swiss cheese ­­effect. When you glance at it casually, you see a large opaque rectangle with holes in it superimposed on a smaller gray rectangle sitting on a black background. But with some mental effort, you can start to imagine the light-gray rectangle behind the holes as actually a translucent white rectangle in front of the holes and then start to perceive a transparent rectangle through which you see black spots in the background. This illusion demonstrates the profound effect of “top down” influences on perception of surfaces; the transparency you see is not entirely driven bottom up through serial hierarchical processing of the physical input on the retina.

Taken collectively, these demonstrations allow us to conclude that a remarkable degree of “wisdom” about the statistics and physical laws of transparency are wired into visual processing, through a combination of natural selection and learning. Yet there are limits to this wisdom. The visual system seems tolerant of incompatible colors. It is incapable of applying the physics of color subtraction, partly because color perception evolved much later in primates and did not get wired in adequately and partly because in the luminance domain color overlap is much less common in the natural world than transparency and translucency are.

We may conclude that even though the visual system can make sophis­ticated use of such abstract properties as the physics of luminance ratios and the statistics of segmentation required for transparency, it is “dumb” with regard to other characteristics such as color ­because of the happenstance manner in which its hardware (or “squishy-ware”) evolved through natural selection—strong evidence against “intelligent design.”

This article was originally published with the title Transparently Obvious.

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