REHOVOT, Israel, October 14, 1997...Research conducted at the Weizmann Instituteof Science may give a whole new meaning to the phrase "stay tuned." Institute scientistshave found evidence that when the brain interprets sensory input, it uses a mechanism remarkably similarto that of an FM radio.
In a study reported in the October 14, 1997, issue of the Proceedings ofthe National Academy of Sciences (PNAS), the researchers describe how the brain uses this radio-likemechanism to "tune in" to a particular frequency, allowing information gathered through touch to betranslated into data about external objects.
This research provides a possible new explanation for the way the brainprocesses sensory information.
"We hope that our study will contribute to the deciphering of the neuralcode, the way in which information is encoded by the sensory organs and decoded by the brain," saysresearch team leader Dr. Ehud Ahissar of the Weizmann Institute's Neurobiology Department. He conductedthe study with departmental colleague Dr. Sebastian Haidarliu and Dr. Miriam Zacksenhouse ofthe Technion-Israel Institute of Technology.
"Cracking" the neural code would immensely advance brain research, justas the discovery of the genetic code revolutionized genetics and molecular biology.
Like an FM receiver
When we touch an object, the nerve endings in our skin send electric neuralsignals to the brain. Until now, scientists studying touch -- or, for that matter, other senses --have focused on identifying the brain cells that receive these signals and on assessing the signals' intensity.
However, according to Weizmann Institute researchers, this is not thewhole story of how the brain actually knows what it's being told by the senses. In the new study, they arguethat the timing of the signals also plays a crucial role in this process.
"We found that certain circuits in the brain work on the same principleas an FM radio," says Dr. Ahissar.
In frequency modulation (FM) receivers, the radio is tuned to a particular frequency, or station. During the broadcast this frequency is being constantly altered, or modulated,and the receiver translates these modulations into different sounds.
Similarly, the brain appears to be tuned to its own "radio stations." In the past decade, scientists discovered that the sensory cortical areas of the brain contain cells thatoscillate at regular frequencies due to intrinsic mechanisms that do not rely on external stimuli. In their study inPNAS, Ahissar and colleagues show that neural signals generated by touch modulate the oscillation frequencyof these cells.
Because the cortex oscillations are regular and persistent, they providethe brain with a "yardstick" against which the timing of incoming signals can be compared. The comparisonprobably takes place in the thalamus, which receives input both from the cortical areas containing theoscillating cells and from the external sensory stimuli.
It is this comparison that allows the brain to track the timing, orfrequency, of the incoming signals, enabling it to decode the information about the object being touched.
Imagine, for example, that you rub your finger against a ribbed surface,such as corduroy fabric. Nerve endings in the skin would send a signal to the brain every time they hitupon one of the fabric's ridges. The thinner and closer-spaced the ridges, the more frequent the signalswould be. Thus, the frequency of the signals encodes sensory information about the surface.
In fact, this may be the reason we need to move the finger over asurface in order to better assess its texture: movement allows us to assess the distribution of sensory input overtime and better define the object being touched. "The timing of the sensory signals appears to be aninherent part of the neural code," says Ahissar. "In fact, this timing contains so much information about theexternal world that it would be surprising if the brain made no use of it."
Clarifying the mechanism
The researchers conducted their study on rats, that twitch their whiskers whenscouring for food. The rats' brains translate the input from their whiskers into data about thelocation of objects. The whiskers twitch rapidly, at a rate of about eight motions per second. Thesemotions "notify" the oscillating neurons in the cortex to tune in to a "transmission frequency" ofabout 8 Hz. When the whiskers hit upon an object, they trigger additional neural signals to the brain, whichperturb, or modulate, the regular 8 Hz transmission. The timing of these perturbations is determined bythe object's location. Therefore, it allows the brain to create an internal representation of theobject's whereabouts.
"The brains of primates contain similar oscillating cells, which are tuned tothe characteristic frequencies generated when the fingertips rub against an external object," saysAhissar. "Thus, the human brain could use similar FM-radio-like mechanisms to process information obtainedthrough touch and perhaps through other senses as well."
In an extension of this research, Weizmann scientists are currentlyseeking to demonstrate that the same principle applies when the brain decodes information perceived throughother senses, particularly vision.
This research was funded in part by the Minna-James-Heineman Foundation,Germany; the Israel Science Foundation; and the United States-Israel Binational Science Foundation,Israel.
The Weizmann Institute of Science, in Rehovot, Israel, is one ofthe world's foremost centers of scientific research and graduate study. Its 2,500 scientists, students,technicians, and engineers pursue basic research in the quest for knowledge and the enhancement of the human condition. New ways of fighting disease and hunger, protecting the environment, and harnessing alternativesources of energy are high priorities.
Materials provided by Weizmann Institute. Note: Content may be edited for style and length.
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