"Let's go for a drive," my husband said, "I want to show you something."
We'd recently relocated to Hudson from D.C., and, because any trip was a welcome escape from unpacking boxes, I willingly tagged along.
The drive was short, not even a mile from the center of town. We turned onto Veterans Way, passing basketball courts on one side and a playground on the other. Our destination was 80 or so yards ahead on the right--a clock tower that resembled the one on the village green. But this wasn't a real clock. It was a piece of artwork created by Hudson notable Peter McDonald, who passed away in 2013.
"It's cute," I said. "But, um, so?"
And then we drove past it.
"Whoa," I said. "Did that clock tower just move?" We looped back and drove past again, this time with the structure on our left.
It definitely looked as if the tower had swiveled in our direction the moment we drove past. How did it do that? We parked the car and walked over to it.
The sculpture seemed simple--a half-inch-thick pliable board positioned into a wide-angled V, and standing open-book style on a mound. On the inside of the V was a drawing of two walls of a clock tower, facing inward. One tower wall was bright pink in color; the other deep red. A nearby plaque cryptically told us that the sculpture was named "Out Is In."
My curiosity piqued, I contacted°former colleagues°at the National Eye Institute, and asked if they knew of someone who could explain the illusion. They suggested Stephen Macknik and Susana Martinez-Conde, a husband-wife research team at the State University of New York, Downstate Medical Center, in Brooklyn. Macknik and Martinez-Conde are, arguably, the world's most knowledgeable authorities on the neuroscience behind optical illusions and the magical arts. They founded the Neural Correlate Society, which sponsors the Best Illusions of the Year contest. They regularly write an "Illusion Chasers" blog for Scientific American. And they knew instantly what was going on.
For starters, they explained, our eyesight isn't nearly as good as you might think. Only about 1/1000th of our visual field--the center of our gaze--is actually 20/20. The rest of our vision is quite poor. Our eyes move one to three times a second to capture high-quality images, and the brain then rifles through all the images--the few high-resolution ones and the many blurry ones--to make sense of it all.
"Your brain builds a model of the world based on very little information," said Macknik. "And it does a pretty good job of guessing what it would actually look like if you could have seen everything based on probabilities, because the world is fairly predictable."
According to the scientists, McDonald used shading and other techniques to trick our brains into thinking that the clock tower walls are convex, or jutting outward toward the viewer. Most objects that we come across in the world are convex, so the brain anticipates that the walls are that way too. In reality, however, the tower walls are concave, angled inward toward the back of the V.
"The brain seems to prioritize the 3-dimensional shading and convexity more than [the structure's] actual position," said Macknik.
What this means for people driving (or walking, running, or biking) on Veterans Way is that our brains may be telling us that the most proximal point to the viewer is at the center of the sculpture, where the two walls meet, but in reality, the center is the farthest point away. The illusion is called reverse perspective.
Because our brains think that the tower is convex, two phenomena occur: first, our brains conclude that the clocks must be facing outward, which is the reason behind the sculpture's name. And second, as we drive past, our brains register changes in perspective and other visual cues that wouldn't be possible if a convex structure was stationary. To resolve the discrepancy, our brains conclude that the sculpture is actively rotating to follow us.
All art is an illusion, the pair explained. Its purpose is to stir emotion, to move us. But perhaps more importantly, much of our knowledge about vision began with art.
"Artists have discovered a lot of the rules by which the brain builds our normal vision," said Martinez-Conde. "If we want to understand vision at the level of neuroscience, we need to understand what artists have discovered first."
Jennifer Wenger has been a science writer for more than 25 years and has worked in communication offices at universities, nonprofit organizations, and government agencies.