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By Root 704 0

of the two crosses is the same. In Figure 5, an unusual package of source, object, and medium produces the same sensation that we usually experience from a bright source behind the cross. The illusion reveals a problem that the brain has to deal with and mostly gets right: there almost always is a bright source behind the square. How does the brain figure that out? How does it get from an ambiguous two-dimensional retinal image to the right three-dimensional situation out there in the world? The image produced by the package of luminance, reflectance, and transmittance is not enough. This is the inverse optics problem: the brain has to work back from the retinal image to its source in the local environment.

The brain can’t get the right answer about what’s out there from the incoming data by some fancy formula that extracts the package of luminance, reflectance, and transmittance from the retinal image. There are too many packages of light source, object, and atmosphere that give the same result, that produce the same retinal image. The way the brain solves this problem is by (continually) guessing. It treats incoming retinal data as the same package of source, object, and medium that was selected for in the brain’s past (and in the evolutionary past of the human brain when similar retinal stimuli were produced). It just assumes that what’s out there now is what was out there the last time it had that kind of retinal data.

Beginning at birth, each brain experiences and stores a whole set of retinal images and their associated packages of guesses about source, object, and medium. The brain just picks out of those stored packages the package that was best adapted in the past for meeting the brain’s needs—for avoiding injury, getting nourishment, getting pleasure, and so on. In other words, the visual cortex of the brain “looks” out at the world and just sees what it was reinforced for thinking was out there in its past. And the reinforcement is not just for success in the individual brain’s past—what the brain has experienced in its own lifetime. Our optical systems have been tuned through millions of years of dealing with packages of source, object, and medium. Long-past packages shaped the optical systems of our evolutionary ancestors and through them shaped our current visual experiences. We don’t “see” the way the world around us is now, we “see” the way it was most frequently in the past, including the evolutionary past.

This past trajectory—both personal and evolutionary—explains why illusions like the bright square in the cross of Figure 5 work so well. In the evolution of eyes on our planet, and in our own experiences since birth, we and our ancestors had to deal with things and their surroundings that didn’t usually differ hugely in luminance.

When something’s luminance is much greater than its surroundings, that is almost always the result of a bright light source. The cross on the left in Figure 5 is a rare case. There is a great luminance difference between the inside of the cross and its arms, even though there is no light source behind it. The white square on the left cross has exactly the same luminance as the white square on the right; it’s their surroundings that differ. The inside margins of the left cross are low in luminance; the inside margins (and the whole squares) on the right cross are high. In our experience, high luminance difference without a light source behind the object is rare. That’s why your brain gets it wrong—why it fails to solve the inverse optics problem correctly.

When high-luminance targets and low-luminance surroundings hit the retina, the brain responds in ways appropriate to a source of brightness. Our brain was selected for doing this, hardwiring the illusion in the last geological epoch. In our evolutionary past, the source of brightness was usually fire, and there was selection (genetic in the species, operant conditioning in the individual) for brightness sensations to trigger burn-avoiding behavior. Natural selection has operated on the genes that build the visual system