Secret of Evolution May Be Buried Behind Our Eyes
Scientist uses new 3-D imaging to challenge decades of scientific belief of how our eyes and brains communicate
November 22, 1999
Though the idea may ruin the romance of watching a sunset, cells in your retina may be tearing up the beautiful image into pieces and dispersing them to different parts of the brain.
Scientists have long argued over whether evolution devised an efficient method of delivering images to the brain, or if more recently evolved aspects of sight - such as the ability to see red and green - were simply jammed into the older system. Since 60 percent of the human brain is devoted to processing vision, the answer could teach us much about the evolution and structure of our brain. One visual neuroscientist at the University of Rochester is using an advanced 3-D computer imaging technique to find the last piece of evidence that may settle nearly a half-century of scientific dispute.
Traditionally, researchers have thought that most of the information gathered by the eye was transmitted in bulk to the brain via a single type of nerve, called a P-cell, then it was up to the brain to decode the signal, extracting color, acuity, brightness and other bits of information to synthesize a whole picture. Imagine trying to paint a sunset when all your paints are mixed together, needing to be separated molecule by molecule before you could paint anything but brown. It would be much more efficient if the colors were delivered to you "presorted."
David J. Calkins, Ph.D., assistant professor of Neuroscience in the department of Ophthalmology and Center for Visual Science at the University of Rochester, is searching for the final piece of evidence, a pathway that carries red and green information to the brain. If he is successful, his findings will mean that within our brain lies an undiscovered section of the visual cortex.
Every mammal can see blue and yellow - the colors are carried by the same pathway - but primates like ourselves are the only mammals that can see red and green. Researchers have long thought that this ability evolved through a set of cells in the retina simply becoming more sensitive to red and green, and passing that information to the brain down the same nerves as other visual information. But Calkins suggests that when some cells evolved sensitivity to red and green light a whole new system of neurons - including new areas of the brain - had to evolve as well.
"Evolution has already found the most efficient way to wire the system," Calkins explains. "More pathways mean more information can be transmitted. Presorted information means less decoding work for the brain. The creator doesn't waste solder."
To find the elusive red-green carrying cells, Calkins sliced a retina into ultrathin layers ten thousand times thinner than a human hair and then scanned those layers into a computer just like you might scan a photograph on your home computer. He then digitally stacked the layers back together to form a 3-D image of the retina. He can zoom around the virtual retina to differentiate the neural cells that were once indistinguishable. Other scientists have used this process to peer inside the brain, but Calkins is the first to turn the imaging system on an eye. (Images @ http://urmc.rochester.edu/urmc/pr/eyes.html )
Calkins summed up his side of the pathway argument in the latest issue of Neuron. The review looks at several years of Calkins' research at the University of Rochester and John Hopkins University, several of his studies published in Nature, Journal of Neuroscience, Trends in Neuroscience, and dozens of other scientists' work on the subject supporting the theory of the retina's ability to presort vision information for the brain.
"David does a remarkable job of bringing together data from studies of the brain to the study of retinal synapses in a review that appears to set a new agenda," says Stewart Hendry, Ph.D., professor of Neuroscience at of Johns Hopkins University. "He was instrumental in showing that the pathway devoted to carrying the color blue remained separate all the way to the brain."