The World through a Whisker: Brain Plasticity in the Wake of Sensory Loss

Neuroscientists have long been fascinated by the brain's remarkable ability to adapt to traumatic changes in body structure,the loss of a limb, for instance. New findings show that the brain can also respond to the loss of sensory organs, and in unexpected ways. This finding could have a profound impact on the way doctors treat patients suffering from sensory damage following a stroke or other brain injury.

Led by Alison Barth, researchers at Carnegie Mellon University removed all but one whisker from a group of rats. Whiskers are an important sensory organ for rats; many species are nocturnal or live in burrows, where sight is of minimal importance. The team monitored neuronal activity in the rats' brains, to see what, if any, changes would occur. Neuroscientists call these adaptations in brain structure "plasticity," because the brain is able to mold itself into new configurations much like plastic.

Not only did the single whisker activate the expected neurons, it also stimulated surrounding clusters of neurons. "What this tells us," according to Barth, "is that the parts of the brain processing sensory information are extremely adaptive and can strengthen in the presence of limited sensory input." In essence, the rats' brains "compensated" for the loss of their whiskers by strengthening the connections between existing neurons and forging new ones.

In addition, the team also found that this effect was reduced when the rats were allowed a full compliment of whiskers on the opposite side of their face. The reasons for this unexpected result are somewhat complicated. In grammar school, children are taught that information from sense organs on one side of the body travels to portions of the brain on the opposite side. For example, if you drop a bowling ball on your left foot, the signal will travel to the right side of your brain. This concept is called contralateral control.

It is not so well known, however, that sensory input from one side of the body also has a minor effect on the same side of the brain. The sensory input from one side also inhibits (or tries to "block out") sensory input from the opposite side. So, when the rat had a full compliment of whiskers on one side, they acted to suppress input from the lone whisker on the other side,thereby reducing the plasticity effects observed earlier.

"These findings show us that a fully functioning set of whiskers on one side of the body dramatically inhibits the ability of a single whisker to remodel the brain," said Barth. "This finding suggests that we could boost the brain's plasticity if we 'turn off' sensory input from the opposite side of the body." 'Turning off' sensory input would allow the damaged side to send information to the brain uninhibited, thus strengthening neurons to a much greater degree.

Doctors could use this information to treat patients suffering from damage or stroke by temporarily eliminating any of their five senses. The damaged sensory organs on the opposite side would respond by strengthening their connections within the brain, as Barth observed in rats. Hopefully, this would result in greater sensibility in the long-term.

Barth has more basic concerns in mind for the future of her research. "We think that our well-designed model is extremely good for future in-depth studies of brain plasticity in response to changes in how an animal senses its environment. Ultimately, we want to understand at the molecular level the dynamic between sensory use and neural plasticity."

- By Kevin Rio.

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