Information coding in the brain
- From: "Lance" <lachenicht@xxxxxxxxxx>
- Date: 16 Apr 2006 15:01:52 -0700
Brain Communicates in Analog and Digital Modes Simultaneously
New Haven, Conn. - Contrary to popular belief, brain cells use a mix
of analog and digital coding at the same time to communicate
efficiently, according to a study by Yale School of Medicine
researchers published this week in Nature.
This finding partially overturns a longstanding belief that each of the
brain's 100 billion neurons communicate strictly by a digital code.
Analog systems represent signals continuously, while digital systems
represent signals in the timing of pulses. Traditionally, many
human-designed circuits operate exclusively in analog or in digital
modes.
"This study reveals that the brain is very sophisticated in its
operation, using a code that is more efficient than previously
appreciated," said David McCormick, professor in the Department of
Neurobiology and senior author of the study. "This has widespread
implications, not only for our basic understanding of how the brain
operates, but also in our understanding of neuronal dysfunction."
"It's as if everyone thought communication in the brain was like a
telegraph, but actually it turned out to be more similar to a
telephone," he said.
Neurons receive input from other cells largely through synaptic
contacts on their dendrites and cell bodies. The release of
neurotransmitters at these synapses causes the voltage inside the cell
receiving the transmitters to fluctuate continuously. Once this voltage
passes a threshold, an action potential is generated. The action
potential is a specialized waveform known to be able to travel down the
axon, or output portion of the cell.
Due to its length and thinness, the nerve axon has been believed to be
impassable to the smaller analog voltage deflections that gave rise to
action potential. As this action potential reaches the synaptic
terminals of the axon, it causes the release of a transmitter onto the
next neurons in the chain. So, although signals in the cell body are
represented in an analog fashion, they were thought to be transmitted
between cells solely through the rate and timing of the action
potentials that propagated down the axon, that is, in a digital
fashion.
McCormick's group demonstrated that the analog signal present in the
cell body also propagates down the axon and influences synaptic
transmission onto other neurons. As the voltage on the sending cell
becomes more positive, the amplitude of the subsequent transmission to
the receiving cell, mediated by an action potential, is enhanced. This
means that the waveform generated in the receiving neuron is not just
determined by the digital pattern of action potentials generated, but
also by the analog waveform occurring in the sending neuron.
For example, McCormick said, epileptic seizures and the aura of
migraine headache both involve large changes in the voltage inside
neurons. He said this study indicates that these abnormal patterns of
activity may be directly communicated to nearby neurons, even in the
absence of the generation of the digital code of action potential
activity.
McCormick said future investigations and models of neuronal operation
in the brain will need to take into account the mixed analog-digital
nature of communication. Only with a thorough understanding of this
mixed mode of signal transmission will a truly in depth understanding
of the brain and its disorders be achieved, he said.
The first author is Yousheng Shu of Yale. Co-authors are Andrea
Hasenstaub, Alvaro Duque and Yuguo Yu of Yale.
Nature: Published online April 12, 2006 (DOI 10.1038/nature04720)
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