However you define them, lines or edges are an exciting topic now because of new breakthroughs in vision science. Neuroimaging and other tools used to analyze brain function are rapidly increasing the understanding of how the visual system interprets the lines we encounter in the world around us.
The primary visual cortex, which lies at the back of the brain, has about 140 million neurons. These neurons are organized in groups that specialize in sorting the information into various properties. Some groups of neurons called orientation columns respond preferentially to vertical lines, some to horizontal lines.
In the image abovve, created from the visual cortex of a macaque monkey, neural clusters with different functions are grouped by color. Other clusters of neurons respond to size, color, and shape. Some are tuned to respond to vertical movement, and others to radial movement.
Do verticals stay vertical even when your head is tilted?
I asked Carl Schoonover: Do the orientation columns perceive a vertical line as vertical even if the head is tilted or if the lines are receding in three point perspective?
“Orientation columns perceive orientation relative to the patterns of light that hit the retina. So if look at a vertical line and tilt your head 90 degrees, neurons that respond to vertical lines will go silent (aka that orientation column will go silent), whereas previously 'horizontal-preferring' silent neurons/orientation columns will then be activated.
“However, you may still perceive the vertical line as vertical, even if your head is tilted. This is because there's a lot more to the visual system than just one-to-one representation of visual space onto cortical space. In higher areas of processing, it is possible to maintain a more flexible representation of one's environment, irrespective of the exact pattern of light hitting the retina.”
“This is thanks in part to 'Helmholtzian' signals” (a centuries-old hypothesis that posits the stability of images despite our head movements). “This is useful for many reasons--for one, our heads our constantly moving as we walk, as our eyes saccade across visual space... but nonetheless our visual experience remains quite stable.”
Illusory shapes and lines
So if the visual brain uses a lot of cues, not just lines, to piece together its understanding of the world, can we perceive shapes without lines?
Even in the absence of outlines, we easily and automatically register shapes. The Kanisza square illusion shows that certain arrangements of forms create the perception of a shape where no line exists.
Coles Phillips was a Golden Age illustrator who used this phenomenon as a compositional device, making our brains see what isn’t there. He became known for the “Fadeaway Girl,” who disappears into the background.
More on orientation columns from the Journal of Comparative Neurology
Orientation columns image by Yevgeniy B. Sirotin and Aniruddha Das
Painting by John Berkey for a 1979 Brown and Bigelow calendar. Thanks, Jim Pinkowski, who has a big website of Berkey images.
Thanks, Carl Schoonover. More in his book "Portraits of the Mind"
Visual stimuli and the way the brain interprets them is so cool. I think the more we know about the brain, the more we can exploit those perceptional tendencies to create a believable illusion of a 3-dimensional space on a 2-dimensional surface.
I love this visual science stuff. Fun to geek out on