Thursday, October 8, 2009
Alison Barth and Colleagues Develop Method To Make Cortical Neuronal Firing More Efficient
PITTSBURGH—Neurons can be thought of like a light switch, turning on and off in response to a stimulus. Imagine having a light switch that only turned on the lights 50 or even 90 percent of the time, or that turned some of the lights on right away, others one or five minutes later, and occasionally not at all. To most people, this would be totally unacceptable. Neurons in areas of the brain like the brain stem and thalamus are extremely reliable, almost always firing in the same manner to the same stimulus. However, neuronal firing in the cerebral cortex, the part of the brain responsible for sensory perception and higher cognitive functions, such as planning, reasoning and language, is notoriously unreliable. In this area of the brain, a neuron will fire at varying rates — and sometimes not at all — in response to the same stimulus.
Carnegie Mellon University biologists have found that firing reliability in the cerebral cortex can be significantly improved through pattern sensory activation, indicating that this unreliability isn't a necessary component for cortical function. The researchers, led by Alison Barth, believe the discovery will become a cornerstone for furthering future research on brain functioning and perception. The study was published in the Sept. 23 issue of the Journal of Neuroscience.
"One of the most mysterious features of the cortex is how bad it is at transmitting information," said Barth, associate professor of biological sciences and a member of the Center for the Neural Basis of Cognition. "The cerebral cortex on a cell-to-cell basis tolerates great unreliability and impreciseness, yet still manages to function. This led us to ask, is this a bug or a feature?"
The Carnegie Mellon researchers sought to find out if this inefficiency was a feature of the wiring of the cerebral cortex. They believed that if the unreliability were indeed a feature, the variability in firing rates would be difficult to alter. The researchers tracked the neurons' plasticity, or their ability to change, to tactile information in the cerebral cortex in a humble laboratory mouse. To induce plasticity, the investigators designed an experiment in which the mouse senses its surroundings through only one whisker. Whiskers are useful in studying neuronal plasticity because, like human fingers, each whisker is linked to its own unique area of the brain's cortex, making it easy to monitor activity and changes in a single neuron in the correct region of the cerebral cortex.
Barth and colleagues found that, over time, animals that sensed the world through only one whisker showed a doubling in neuronal firing rates, with the frequency and timing of response much more reliable and precise. Improvements look longer to occur in older mice, but were still seen. These results indicated to the researchers that inefficiency in neuronal firing in the cerebral cortex could be easily corrected through repeated stimulation, indicating that it is not a fixed or necessary feature of the brain's circuitry.
According to Barth, the next step will be to investigate the molecular mechanisms behind cortical neuronal firing and what impact they have on perception.
"The brain is the most marvelous computer that has been conceived of and it works so well as a whole, even with sub-par pieces. We need to find how it can function given what appears to be rather shoddy components," Barth said.
The results will enable researchers to focus on the precise molecular mechanisms that underlie changes in neuronal reliability. In addition, they will help computational neuroscientists devise more accurate models of cortical function. Barth hopes to be able to directly address the role of precise firing in improved perception, using tactile stimulation delivered through the mouse's whiskers. "More reliable firing should mean that the animal can sense things that previously were undetectable," she said. "And if we can make the system work better, the fascinating question still remains: why isn't it optimized to do this in the first place?"
This research was funded by the National Institutes of Health and the Physiological Society.