• Choose a category
• All
• Classic-Fiction

By Root 671 0

that when our eyes are open, we are getting “real-time” access to the way the world is, here and now (minus the lag for the speed of light). But vision is in fact a matter of navigating the highway of life while keeping both eyes relentlessly focused on the rearview mirror. Human sight, which we think gets as close to foresight as anything, really just turns out to be hindsight. Vision is just another Darwinian process, one that produces the appearance of now-sight by filtering incoming variations for fitness to past environments. This filtering is not done just by our own past environments. The environments of distantly past human, mammalian, and vertebrate ancestors are still at it, filtering our current visual input. Despite appearances, vision is hindsight masquerading as now-sight. Seeing why this is so is both eye opening and fun.

The images in Figure 4 all trick the brain thoroughly. In each case, your brain is fooled into a set of optical illusions. It looks at the images and sees substantial brightness differences, between the squares on the left and right, between the upper and lower faces of the solid, between the centers of the two crosses, and between the light gray shadings on the puzzle piece. But there can’t be any brightness differences on the page, just color differences. These illusions illustrate how we can be deceived about things like brightness, color, distance and depth, geometry, and angles.

FIGURE 4. Brightness/darkness illusions from the website of Dale Purves (http://www.purveslab.net/seeforyourself/ )

These and other illusions reflect a few of the problems that the brain continually needs to solve to operate in its environment. The problems are those of figuring out how the local environment is arranged on the basis of completely insufficient data. Dale Purves, the neuroscientist whose work employs these illusions, calls the problem that the brain has to solve the “inverse optics” problem. The visual information that hits the retina at the back of the eye is only two-dimensional. Out of it the brain has to build a three-dimensional world. So how does the brain construct the third dimension? These illusions show how the brain goes about solving the inverse optics problem. They do it by revealing the ways it can sometimes get the solution wrong.

The illusions work because the same two-dimensional retinal pattern can be produced many different ways. What we see depends in large measure on three factors about our environment: the source of the light, the object it bounces off, and the medium it travels through to get to the eye. The source of light might be the sun, a candle, or a light bulb. It shines on an object—a building, a dice cube, a glass of water, the page you are looking at in this book—which reflects the light to our retinas through a medium—clear air or smoke, a dirty windowpane, or maybe sunglasses. Variations in sources produce differences in illumination, or luminance. Changes in objects’ surfaces (dusting it, cleaning it, polishing it) produce differences in reflectance. The condition of the space between the object and the eye produces differences in transmittance. The very same electrochemical reactions on the retina can be produced by different packages of these three variables. That’s why the illusions in Figure 4 work. The images trick you because they are each combinations of source, object, and medium that we rarely experience. These unusual packages produce the same retinal patterns as the quite different and much more frequent packages of source, object, and medium that normal life produces.

FIGURE 5.

Let’s focus on the sensation of the bright square in the middle of the left cross in Figure 5. Once you see how this illusion works, it’s not hard to apply the lesson to explaining the others. In normal life, this sensation is usually the result of a strong light source behind a cross with an open middle square. In Figure 5, obviously, there is no strong light behind the cross illuminating its center from the back. The luminance of the white in the middle