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Unlike their visual cousins, the neurons that control movement arenot a predictable bunch. Scientists working to decode how suchneurons convey information to muscles have been stymied when tryingto establish a one-to-one relationship between a neuron's behaviorand external factors such as muscle activity or movement velocity. In an article published online June 3rd by the journal Nature , a team of electrical engineers and neuroscientists working atStanford University propose a new theory of the brain activitybehind arm movements. Their theory is a significant departure fromexisting understanding and helps to explain, in relatively simpleand elegant terms, some of the more perplexing aspects of theactivity of neurons in motor cortex. In their paper, electrical engineering Associate Professor KrishnaShenoy and post-doctoral researchers Mark Churchland, now aprofessor at Columbia, and John Cunningham of Cambridge University,now a professor at Washington University in Saint Louis, have shownthat the brain activity controlling arm movement does not encodeexternal spatial information -- such as direction, distance andspeed -- but is instead rhythmic in nature. Understanding the brain Neuroscientists have long known that the neurons responsible forvision encode specific, external-world information -- theparameters of sight. It had been theorized and widely suggestedthat motor cortex neurons function similarly, conveying specificsof movement such as direction, distance and speed, in the same waythe visual cortex records color, intensity and form. "Visual neurons encode things in the world. They are a map, arepresentation," said Churchland, who is first author of thepaper. "It's not a leap to imagine that neurons in the motorcortex should behave like neurons in the visual cortex, relating ina faithful way to external parameters, but things aren't soconcrete for movement." Scientists have disagreed about which movement parameters are beingrepresented by individual neurons. They could not look at aparticular neuron firing in the motor cortex and determine withconfidence what information it was encoding. "Many experiments have sought such lawfulness and yet nonehave found it. Our findings indicate an alternative principle is atplay," said co-first author Cunningham. "Our main finding is that the motor cortex is a flexiblepattern generator, and sends rhythmic signals down the spinalcord," said Churchland. Engine of movement To employ an automotive analogy, the motor cortex is not thesteering wheel, odometer or speedometer representing real-worldinformation. It is more like an engine, comprised of parts whoseactivities appear complicated in isolation, but which cooperate ina lawful way as a whole to generate motion. "If you saw a piston or a spark plug by itself, would you beable to explain how it makes a car move?" asked Cunninghamrhetorically. "Motor-cortex neurons are like that, too,understandable only in the context of the whole." In monitoring electrical brain activity of motor-cortex neurons,researchers found that they typically exhibit a brief oscillatoryresponse. These responses are not independent from neuron toneuron. Instead, the entire neural population oscillates as one ina beautiful and lawfully coordinated way. The electrical signal that drives a given movement is therefore anamalgam -- a summation -- of the rhythms of all the motor neuronsfiring at a given moment. "Under this new way of looking at things, the inscrutablebecomes predictable," said Churchland. "Each neuronbehaves like a player in a band. When the rhythms of all theplayers are summed over the whole band, a cascade of fluid andaccurate motion results." Dr. Daofen Chen, Program Director, Systems and CognitiveNeuroscience at the National Institute of Neurological Disordersand Stroke at the National Institutes of Health, said Shenoy andteam are working at the cutting edge of the field. "In tryingto find the basic response properties of the motor cortex, Dr.Shenoy and his colleagues are searching for the holy grail ofneuroscience," said Dr. Chen. "His team has beenconsistent in tackling important but tough questions, often inthought-provoking ways and in ambitious proposals. NIH is proud tosupport this kind of pioneering and transformative research." Precedents in nature In the new model, a few relatively simple rhythms explain neuralfeatures that had confounded science earlier. "Many of the most-baffling aspects of motor-cortex neuronsseem natural and straightforward in light of this model," saidCunnigham. The team studied non-rhythmic reaching movements, which made thepresence of rhythmic neural activity a surprise even though, theteam notes, rhythmic neural activity has a long precedence innature. Such rhythms are present in the swimming motion of leechesand the gait of a walking monkey, for instance. "The brain has had an evolutionary goal to drive movementsthat help us survive. The primary motor cortex is key to thesefunctions. The patterns of activity it displays presumably derivefrom evolutionarily older rhythmic motions such as swimming andwalking. Rhythm is a basic building block of movement,"explained Churchland. Reaching for the grail To test their hypothesis, the engineers studied the brain activityof monkeys reaching to touch a target. According to theresearchers, experiments show this 'underlying rhythm' strategyworks very well to explain both brain and muscle activity. In theirreaching studies, the pattern of shoulder-muscle behavior couldalways be described by the sum of two underlying rhythms. "Say you're throwing a ball. Beneath it all is a pattern.Maybe your shoulder muscle contracts, relaxes slightly, contractsagain, and then relaxes completely, all in short order,"explained Churchland. "That activity may not be exactlyrhythmic, but it can be created by adding together two or threeother rhythms. Our data argue that this may be how the brain solvesthe problem of creating the pattern of movement." "Finding these brain rhythms surprised us a bit, as thereaches themselves were not rhythmic. In fact, they were decidedlyarrhythmic, and yet underlying it all were these unmistakablepatterns," said Churchland. "This research builds on a strong theoretical framework andadds to growing evidence that rhythmic activity is important formany fundamental brain functions," said Yuan Liu of theNational Institute of Neurological Disorders and Stroke, NIH."Further research in this area may help us devise moreeffective technology for controlling prosthetic limbs." Liu isthe co-lead of the NIH-NSF Collaborative Research in ComputationalNeuroscience program. "In this model, the seemingly complex system that is the motorcortex can now be at least partially understood in morestraightforward terms. The motor cortex is an engine of movementthat obeys lawful dynamics," said Shenoy. Stanford post-doctoral fellow Matthew Kaufman, bioengineering PhDstudent and medical science training program student PaulNuyujukian, electrical engineering graduate student Justin Foster,and electrical engineering consulting assistant professor and PaloAlto Medical Foundation neurosurgeon Stephen Ryu were also authorson this paper. We are high quality suppliers, our products such as China 700MA Constant Current Driver , Constant Current Led Driver for oversee buyer. To know more, please visits Constant Current Led Driver.
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