Neuroscientists Finally Solve Astronomical Optical Illusion That Puzzled Galileo 400 Years Ago

By Ben Wolford on February 12, 2014 4:37 PM EST

Neuroscientists have solved an optical riddle that puzzled Galileo in the 16th century about why Venus appeared larger than Jupiter. Both are pictured here. (Photo: Shutterstock)
Neuroscientists have solved an optical riddle that puzzled Galileo in the 16th century about why Venus appeared larger than Jupiter. Both are pictured here. (Photo: Shutterstock)

Galileo Galilei knew his eyes were deceiving him. He had calculated that Jupiter was supposed to appear four times larger than Venus, but when he looked toward the heavens that's not what he saw. To his naked eye, Venus appeared eight to 10 times larger. He struggled to explain it: "Either because their light is refracted in the moisture that covers the pupil, or because it is reflected from the edges of the eyelids and these reflected rays are diffused over the pupil." He added helplessly, "Or for some other reason."

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It took science some 400 years to finally solve the puzzle. Turns out Galilei was right; this was an optical illusion. In the journal Proceedings of the National Academy of Sciences, a group of optometrists and neuroscientists described a phenomenon that explains why light objects and dark objects — and the background against which they appear — affect our perception of their size.

Humans (and cats and monkeys) have two neuron pathways, called ON and OFF channels, for seeing different brightnesses. The ON channels are for light objects, like Venus and Jupiter against a night sky, and the OFF channels are for dark objects, like most other things, including the words on this screen.

Scientists used to think we could see the same regardless of the pathway. In the new study, however, scientists wanted to test that assumption. By attaching electrodes to cats, monkeys, and human subjects, they measured brain activity. They found that "visual spacial resolution is higher for darks than lights," the scientists wrote in the paper, published Monday. In other words, we're better at seeing dark objects, while stars and other light objects appear larger and fuzzier — what Galileo called "a radiant crown."

"This effect is responsible for how we see everything from textures and faces — based on their dark parts in bright daylight — to why it is easier to read this very page with black-on-white lettering, rather than white-on-black (a well known, and until now, unexplained phenomenon)," the State University of New York College of Optometry said in a news release. The authors were surprised to find that the asymmetry of ON and OFF channels doesn't just affect vision, according to the paper. Smells hit our olfactory nerve in the form of chemicals and then also travel through either ON or OFF receptors. Like the dark objects, smells passed through OFF channels are perceived as stronger.

Why does this happen? The scientists say it has to do with something called spacial frequency. Dark objects have a low spacial frequency, which is picked up by the OFF channel; light objects have high spacial frequency, perceived by the ON channel. The authors speculate that OFF channels are more acute because they get more practice from infancy, when our eyes see everything blurry (low frequency). Later, when our eyes develop, OFF channels continue to receive more information because most of what we see is dark. Rest easy, Galileo. Problem solved.

Above photo courtesy of Shutterstock

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