Source: University Of California - San Diego
http://www.sciencedaily.com/releases/2004/06/040604033359.htm
Date: 2004-06-07
UCSD Study Shows How We Perceive World Depends On Precise Division Of Labor Among Cells In Brain
University of California, San Diego neurobiologists have uncovered evidence that sheds light on the
long-standing mystery of how the brain makes sense of the information contained in electrical
impulses sent to it by millions of neurons from the body.
In a paper published this week in the early on-line version of the journal Nature, a UCSD team led
by Massimo Scanziani explains how neurons, or nerve cells, in the brain sort out information before
deciding how to respond. The paper will appear in a forthcoming print issue of Nature.
Light, sound and odors, for example, are transformed by our sensory organs into a code made of
series of electrical impulses that travel along neurons from the body to the brain. Information
about the onset and the intensity of a stimulus is thought to be sent to the brain by the timing
and frequency of these electrical impulses. How information is sorted by the brain has been an open
question. The group discovered that different neurons in the brain are dedicated to respond to
specific portions of the information.
"Our work shows that deciphering the enormous amount of information that is conveyed to the brain
at any time-point is a matter of division of labor between specialized neurons," explains
Scanziani, an assistant professor of biology. "Each neuron literally 'picks' the type information
it is supposed to process, that it is competent for. Very much like each musician in an orchestra
only reads that part of the score of a symphony that was written for his or her own instrument."
Because they needed to see and record electrical impulses from individual nerve cells, the
researchers used slices of rat brain, which when bathed in an appropriate solution can be kept
alive under a microscope. To mimic incoming information, the first author on the paper, Frédéric
Pouille, a postdoctoral fellow in Scanziani's laboratory, provided an electrical
stimulus--analogous to the score in Scanziani's analogy--and then monitored which nerve cell read
which part of the information. Pouille and Scanziani found some nerve cells that were only
responsive to the first impulse that arrived, while other nerve cells only responded to multiple
electrical impulses arriving at certain frequencies.
"While some neurons only responded to the onset of each package of information, which, in other
words, means: Hey, something just arrived, other neurons actually looked into the package and
played the notes," says Scanziani.
Each of these specialized brain neurons has a highly branched structure where many neurons carrying
sensory information can form connections. At any moment, each of these specialized brain neurons
might be receiving multiple messages from multiple sources, but is only selectively responding to
certain information about the timing or frequency of the impulses it is receiving.
Why is the timing of information so important? Visual, tactile and auditory information needs to be
synchronized. If it were not, then one might, for example, perceive someone's lips move before
hearing the words being spoken--like a badly dubbed foreign film.
The brain also needs to know how intense a stimulus is because intensity will influence what action
needs to be taken. For example, an uncomfortable shoe will become more and more difficult to ignore
as your foot develops a blister. As the blister develops, the interval between subsequent
electrical impulses arriving at the brain would decrease; in other words, their frequency would
increase. Scanziani speculates that there might even be an "alarm neuron" in the brain that
responds to high frequency electrical impulses by triggering the appropriate muscle response to
escape the stimulus.
"This study advances our understanding of how the brain reads a code made of identical electrical
impulses, in order to produce a coherent perception of the world," he says. "Deciphering the
language of the brain will help us understand the neuronal basis for sensation and cognition and
their associated disorders."
In their paper, the UCSD researchers also determine a chain of physiological mechanisms working in
concert to allow these brain neurons to selectively respond to a specific pattern of incoming
electrical impulses. Communication across the connections between neurons is usually chemical
rather than electrical. The researchers found that the differences in the way the individual brain
neurons released and responded to these chemicals could explain their differing responses to
incoming information.
Scanziani and Pouille's experiments focused on the hippocampus--a region of the brain known to be
important in learning in memory. But they believe that other regions of the brain may also use the
same principles to sort information. However, the researchers point out that brain slices are a
simplified system, and more research is needed before they will understand the finer details of
this sorting.
"This is only part of the picture," cautions Scanziani. "We are not looking at the whole orchestra,
maybe only the violins and the oboes. But down the line we plan to look at further classes of nerve
cells."
The research study was initiated when Scanziani was an assistant professor at the Brain Research
Institute of the University of Zurich. The work was supported by the National Institutes of Health
and the Swiss National Science Foundation.
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