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The brain generates three-dimensional images with the help of line patterns made of shading
Shading brings 3D form to life, beautifully carving out the shape of objects around us. Despite the importance of shading for perception, scientists have long been puzzled about how the brain actually uses it. Researchers from Justus-Liebig-University Giessen and Yale University recently came out with a surprising answer.
Previously, it has been assumed that one interprets shading like a physics-machine, somehow “reverse-engineering” the combination of shape and lighting that would recreate the shading we see. Not only is this extremely challenging for advanced computers, but the visual brain is not designed to solve that sort of problem. So, these researchers decided to start instead by considering what is known about the brain when it first gets signals from the eye.
“In some of the first steps of visual processing, the brain passes the image through a series of ‘edge-detectors’, essentially tracing it like an etch-a-sketch,” Professor Roland W. Fleming of Giessen explains, “we wondered what shading patterns would look like to a brain that’s searching for lines.” This insight led to an unexpected, but clever short-cut to the shading inference problem.
“It just so happens that shading patterns sketch blurry lines that follow the 3D curves of objects, so by measuring these lines, the brain can figure out 3D shape,” remarked Professor Fleming, “it turns out that the brain doesn’t need to know how light bounces off surfaces to understand shading.”
In fact, they found that the brain doesn’t care if shading is physically accurate at all. “We created artistic-renderings of ‘weird shading’—shading that breaks the rules of physics, but has the same line patterns as real shaded images,” says Professor Steven Zucker, a computer scientist at Yale University. “People identify the same 3D shapes from these weird images, which tells us that it’s the lines that matter.”
Using state-of-the-art computer models and experiments with human volunteers, the researchers tested this theory and confirmed a striking relationship between the 3D shapes people perceive and the 2D line patterns that shading creates. Most promisingly, this theory also accounts for shaded objects made from a range of materials—from matte, to glossy, all the way to polished chrome—something that traditional accounts of shading have struggled to do for decades.
Their findings suggest that the early-stage outputs of visual processing, where “edge detection” starts, may play a much larger role in perception than previously thought. This may explain why drawings, an attempt at representing the 3D world as an image, are built up with sketchy lines and contours.
"It’s remarkable because it doesn't just explain why different art techniques, like shading and crosshatching, are so compelling. It tells us exactly what information the brain is looking at in images to figure out the 3D structure of the world," says Celine Aubuchon, one of the scientists involved with the work.
The researchers recently published these findings in PNAS. They are continuing this work to see how learned relationships between line patterns and 3D objects can explain how we perceive the world, from shading and a host of other visual cues.
Prof. Dr. Roland W. Fleming
Kurt Koffka Professor of Experimental Psychology
Justus Liebig University Giessen
Phone: +49(0)641 99-26140
E-mail: roland.w.fleming@psychol.uni-giessen.de
C. Aubuchon, R. Vergne, S.A. Cholewiak, B. Kunsberg, D. Holtmann-Rice, S.W. Zucker, & R.W. Fleming, Orientation fields predict human perception of 3D shape from shading, Proc. Natl. Acad. Sci. U.S.A. 122 (28) e2503088122 (2025), https://doi.org/10.1073/pnas.2503088122
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