Neuroscientists usually explain color illusions in mechanistic terms: They arise because of the way cells in the retina and the brain respond to certain wavelengths of light. Those explanations miss the larger point, says Beau Lotto, a brain research at University College London. We misperceive colors and shapes because our visual sense has been molded by evolutionary history.
The blue dot on the top of the cube and the one on the darker side are exactly the same hue, reflecting light with identical intensity, or luminance. But because your visual system interprets the two-dimensional drawing as three-dimensional reality, you see the right side "in shade" and so perceive the right-side dot as bright, even glowing. In everyday life this perceptual bias is useful; it is what normally allows you to understand how distant objects occupy space.
This simplified view clearly shows that the blue dot on the top of the cube and the one on the darker side are exactly the same hue, reflecting light with identical intensity, or luminance.
Beau Lotto's lab has created dozens of optical illusions to study the systematic goofs that evolution has built into our minds. He proposes that visual glitches are the result of a brain that is beautifully calibrated to detect biologically relevant information--responding effectively to motion (which might indicate a predator on the run) while messing up on simple color matching, which generally has little relevance to survival.
How many blue tiles are on the top surface of the cube on the left? And how may yellow tiles on the top of the right cube? Four and seven, respectively, right? The correct answer for each question is none--both sets of tiles are gray.
The image on the right appears to be bathed in short-wavelength blue light, so our vision compensates by perceiving gray, a neutral shade, as longer-wavelength yellow. The opposite holds true on the left.
The fact that we can perceive colors consistently at all is astonishing, considering that our brains must take into account not only the color reflected from the object but also the color of the illumination and the context. Each parameter influences the frequency of the light as it reaches the eye, yet we still see blue as blue whether we're in brilliant sunshine, under fluorescent light, or looking through water. We are capable of distinguishing roughly 1 million colors.
This color-simplified view shows that the four "blue" tiles in the left image and the seven "yellow" tiles in the right image are in fact all the same shade of gray.
"The point is, we want to see the relationships between things if we are to survive," Lotto says. "The problem is that we can't see objects or conditions of the world directly, except through the light that falls on the eye--which is not the same thing as the objects in the world that reflect that light. That is the fundamental challenge the brain evolved to solve."
At first glance, the green table appears longer and thinner than the red table, which looks to be more or less square. Yet the two actually have the same length and width, if the red table were rotated one quarter turn.
They seem so different because the angle at the lower left corner of the green table makes it appear more "vertical," as though its back edge is receding into the distance. In response, our brains alter the real information to fit what we expect to see: a three-dimensional scene in which perspective is paramount.
The highlighted guide lines reveal the counterintuitive geometry of the two tables. The two blue lines are the same length, as are the two yellow lines; hold a reference object up to the screen if you don't believe it.
Beau Lotto theorizes that the reason humans are vulnerable to optical illusions is that we are not really responding to the physical properties of the object we are looking at. Rather, we "see" things based on what other things similar to the current image have typically turned out to be in the past.
The implication: The visual cortex is evolutionarily adapted to the sensory information it receives, not to the world beyond. Such forced interpretations allow us to recognize important objects (fruit in a tree, a tiger in the grass) quickly, but at the cost of total accuracy. Basically, we do not see the world the way it really is because our brains will not let us.
The top and bottom planes of this shape seem to be distinct, separated by a thin band of shadow. In reality, both are the same absolute color.
Your brain perceives the gradients where the tiles meet as curves, and you surmise that light hits the large, flat surface of the top tile and fades to shadow at its curved bottom edge; for the bottom tile, the opposite holds. Automatically your mind fits the drawing to a physical object that makes sense based on your experience, altering your visual perception to match expectations.
This simplified image shows that the two faces of the "object" are indeed the same shade.
The center square of the disk on the left (marked with a dot) appears green, while the center square of the disk on the right appears orange. The two are identical in hue.
The two center squares are precisely the same color, as seen in this cutaway view.