Thursday, April 28, 2011
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 at left, 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.
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.”
I'll take a brief break tomorrow (Friday) and finish this up on Saturday.
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"
Lines and the Brain Series,