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CAMBRIDGE, Mass. -- A Massachusetts Institute of Technology researcher reports in the April 20 issue of Nature that when an animal's brain is rewired so that visual input is directed to the auditory cortex, this part of the brain dedicated to hearing is able to respond to visual stimuli.
"This is a profound discovery that addresses age-old questions about whether the brain is genetically programmed or shaped by the environment," said Mriganka Sur, professor and head of the Department of Brain and Cognitive Sciences at MIT. "This provides dramatic evidence of the ability of the developing brain to adapt to changes in the external environment, and speaks to the enormous potential and plasticity of the cerebral cortex -- the seat of our highest abilities."
The research involved "rewiring" brains in very young mammals, so that inputs from the eye were directed to brain structures that normally process hearing. The animal's auditory cortex successfully interpreted input from its eyes. But it didn't do the job as well as the primary visual cortex would have, suggesting that while the brain's plasticity, or ability to adapt, is enormous, it is limited by genetic preprogramming.
Environmental input, while key to the development of brain function, does not "write on a blank slate," Sur said.
In addition to providing new evidence in the nature vs. nurture debate, this research could lead to treating brain disorders that are detected early by taking advantage of brain cells' apparent ability to switch gears in an early stage of development.
"I like to think of our experiment as highly optimistic," said Sur, who studies the development and function of the cortex. "It demonstrates the tremendous potential of the brain to transcend what is simply written in the genes."
WRITTEN IN THE TISSUE
In mammals, the brain develops from a few cells at the tip of the embryonic neural tube. Mainly guided by molecular cues early in development, the specific, detailed wiring of connections between neurons involves internal and external input.
Research indicates that the visual cortex in people who are blind from birth is involved in nonvisual tasks, but how this comes to pass is unknown. And it is known that visual deprivation early in life alters how certain brain pathways grow and develop. "The brain requires the right kind of input to develop certain types of function," Sur said. "One reason no two brains are alike is that they do not receive identical inputs during development."
Light that enters through the eye's retina projects to the visual thalamus, forming maps and modules there. The thalamus is a large, egg-shaped gray mass on either side of the third ventricle of the brain. The visual thalamus relays information to the visual areas of the cerebral cortex and works with the cortex to interpret visual stimuli.
The cortex is the outer layer of the brain, consisting of layers of nerve cells and the pathways that connect them. The cortex is the part of the brain in which higher processing, including thought, takes place.
Different parts of the thalamus and cortex are dedicated to highly specific tasks. "The function of specific brain cells appears to be 'written' in the tissue by adulthood, but how does it come to be written?" Sur said.
"The brain is a wonder of development that involves molecules, cells and inputs being in the right place at the right time. Connections between cells are the key to brain function. By altering the input to the tissue, the connections changed as a result, and we think the very molecules changed as well," Sur said.
Although our bodies are unable to replace damaged brain cells or grow more brain cells than the ones we are born with, environmental cues apparently play a large role in shaping the cells that are already there.
Sur's experiments show that "the effect of the environment can be enormous, but it is not entirely independent of a basic genetic program. The answer is not entirely genetic or entirely environmental," he said.
RESPONDING TO LIGHT AND SOUND
Sur and his colleagues published two papers in this week's Nature. In one, the researchers show that visual inputs re-specify the auditory cortex, altering the circuitry there to resemble circuitry and connectivity in the visual cortex. The researchers point out that the connections also retain some features typical of auditory cortex.
"These connections and networks do form, but remain something different from those in the primary visual cortex," Sur said. "There is less graceful organization in the rewired auditory cortex than in the primary visual cortex."
In the other article, "we show that the rewired animals behaviorally interpret visual inputs to the auditory cortex as a visual, rather than auditory, stimulus," he said.
By testing the animals, which had been trained to respond to light and sound, the researchers determined that the animals did see when visual input reached their auditory cortex. The inputs that normally transmit sound had been removed.
Other researchers involved in this work include Jitendra Sharma, research scientist in brain and cognitive sciences at MIT; former MIT graduate student Alessandra Angelucci; and former postdoctoral fellows Laurie von Melchner and Sandra Pallas.
This work is supported by the National Institutes of Health and the March of Dimes.