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25 October 1997
CIRCUITS in the brain can pick up the senses just like a living FM radio, scientists in Israel claim. They think that we can feel textures because the brain tirelessly monitors the changing frequencies of neurons.
According to the conventional “passive” model of the brain, sensory information such as touch passes as electrical pulses from nerve endings in the skin to the brain stem. Pulses then travel to the thalamus, a pea-shaped structure just above, and end up in a processing centre in the cortex.
Now Ehud Ahissar of the Weizmann Institute of Science in Rehovot and his colleagues say that’s not the whole story. Seven years ago, they found that a monkey’s cortex has certain neurons that continuously oscillate. “It was not at all clear what their role might be,” says Ahissar.
His team decided to study rats to see if these oscillations have a role in sensory perception. Rats find out about objects around them by touching them with quivering whiskers.
The researchers monitored neurons in the cortex that receive information from whiskers. They found that even when the rats were not moving their whiskers or touching anything, a tenth of the neurons had an intrinsic frequency of about 10 hertz. When the whiskers touch an object, the frequency of the neuron oscillation alters.
This means that the brain interprets the signals like an FM radio, says Ahissar in the current issue of Proceedings of the National Academy of Sciences (vol 94, p 11633). Frequency modulation (FM) transmitters send out a “carrier” radio wave at the channel frequency. Sounds are encoded on this wave as alterations in frequency.
So in rats’ brains, the natural frequency of neurons in the cortex can be compared to the frequency of the FM channel, while information about the object a rat is touching is encoded like the sound. Ahissar speculates that rather than simply relaying pulses to the cortex, neurons in the thalamus act as an FM receiver by interpreting the frequency changes, from which rats perceive texture.
The same may be true for humans. Our fingertips have two main types of receptors, and Ahissar suspects that while one sends pulses to the brain in the conventional way, the other works like the FM system. “They are probably decoded in parallel in the human brain,” says Ahissar. “Together they provide the whole picture.”