A new study reveals a single neuron is capable of encoding information from two different sounds by switching between signals associated with one sound to that of the other.

Summary: A new studystudy reveals a single\nneuron is capable of encoding informationinformation from two different sounds by\nswitching between signals associated with one sound to that of the other.

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Source: Duke University.

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Back in the early days of\ntelecommunications, engineers devised a clever way to send multiple telephone\ncalls through a single wire at the same timetime. Called timetime-division\nmultiplexing, this techniquetechnique rapidly switches between sending pieces of each\nmessage.

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New researchresearch from Duke University shows\nthat neurons in the brainbrain may be capable of a similar strategystrategy.

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In anan experimentexperiment examining how monkeys\nrespond to sound, a team of neuroscientists and statisticians found that a\nsingle neuronneuron cancan encode informationinformation from two different sounds by switching\nbetween the signal associated with one sound and the signal associated with the\nother sound.

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“The question we asked is, how do\nneurons preserve informationinformation about two different stimuli in the world at one\ntime?” said Jennifer Groh, professor in the department of psychologypsychology and\nneuroscience, and in the department of neurobiology at Duke.

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“We found that there are periods of\ntime when a given neuronneuron responds to one stimulusstimulus, and other periods of timetime\nwhere it responds to the other,” Groh said. “They seem to be able to alternate\nbetween each one.”

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The results may explain how the brainbrain\nprocesses complexcomplex informationinformation from the world around usus, and may also provide\ninsight into some of our perceptual and cognitive limitations. The results\nappeared July 13 in Naturenature Communications.

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To make the discovery, Groh and her\nteam collaborated with Surya Tokdar, associate professor of statistical sciencescience\nat Duke, to develop and apply several new methods of analysisanalysis to their\nexperimental datadata.

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Most studies of single neuronneuron behaviorbehavior\ninvestigate only one stimulusstimulus at a timetime, looking at how anan individualindividual neuronneuron\nresponds when the subjectsubject is played a single note or shown a single imageimage.

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But realityreality is rarely so simple. Our\nbrains are capable of processing multiple stimuli at once — such as listeninglistening\nto a friend at a party with music playing in the backgroundbackground, or picking out the\nbuzz of a cicada from a symphony of trilling insects.

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“It is not obvious how you go from\nsingle neurons encoding single objects, to neurons encoding multiple objects,”\nsaid Valeria Caruso, a researchresearch scientist in Duke’s department of psychologypsychology\nand neuroscience. “We wanted to provide anan intermediate step, looking at how\nneurons encode small groups of objects.”

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To complicate matters, single-neuronneuron\nstudies have shown that many sensory neurons are broadly tuned, meaning each is\ncapable of responding to sounds at a range of different frequencies. For\nexample, the same neurons triggered by your friend’s voice may also be\ntriggered by the notes of your favorite tunes.

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“If II am a neuronneuron and II’m able to\nrespond to both anan imageimage of a pillow and the couch it is resting on, how does\nthe brainbrain infer that both the pillow and the couch are present?” Groh said.

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In the experimentexperiment, the researchers sat\nmonkeys in a darkened room and trained them to look in the direction of sounds\nthat they heard. The researchers played either one sound or two sounds, with\neach sound at a different frequency and coming from different locations.

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When the researchers played two sounds\ntogether, the monkeys looked first in the direction of one sound, and then in\nthe direction of the other sound, indicating that the monkeys recognized the\nexistence of two distinct sounds.

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To find out how the monkeys’ brains\nencoded both sounds simultaneously, the team used electrodes in the inferior\ncolliculus, a key point in the brainbrain’s auditory pathway, to measure the small\nspikes in the locallocal electric field caused by neurons firing.

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The researchers investigated the\nresponse of single neurons to both individualindividual sounds and to combined sounds.\nThe standard practicepractice in the field is to count how many spikes occur over a\nperiod of timetime and compute the average of a number of trials, Groh said. But\nthis methodmethod obscures any fluctuations in activityactivity that might indicate the\nneurons are switching back and forth between different stimuli.

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The team applied a combination of\nadvanced statistical methods, including a new methodmethod called a Dynamicdynamic Admixture\nPoint Processprocess modelmodel developed by Tokdar and hishis team, to extract more detailed\npatterns from the datadata.

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They found that a single neuronneuron could\nrespond to one sound with one firing rate, and a second sound with a different\nfiring rate. When both sounds were played simultaneously, it appeared to\nfluctuate between the two firing rates. Sometimes the fluctuations were fast\nenough that the neurons switched within a half second of the presentation of\nthe sound, and in other cases the switching was slower.

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The team repeated the statistical\nanalysis on datadata from experiments conducted by Winrich Freiwald, a professor of\nneurosciences and behaviorbehavior at The Rockefeller University. In these experiments,\nFreiwald investigated the firing rates of single neurons in a visual areaarea of\nthe cortex in responseresponse to images of one face or two faces. The analysisanalysis\nrevealed the same switching pattern when two faces were present.

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\"neurons\"

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A\nDuke team found that individualindividual neurons cancan encode informationinformation about multiple\nstimuli simultaneously, much the same way electronics like cellcell phones sort\nsignals by frequency. NeuroscienceNews.com imageimage is credited to Cruger\nCreations.

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These findings\nprovide clues to other circumstances where the brainbrain has to do more than one\nthing at a timetime with a limited setset of neurons. For example, our working memorymemory\n— the number of things we cancan hold in our minds at one timetime — is constrained to\naround five to seven items. While these experiments do not directly testtest\nworking memorymemory, the researchers think further studies may help explain these\nrestrictions.

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“Our working\nmemory system is quite limited and no one really knows why,” Groh said.\n“Perhaps that limit arises from some kind of cycling behaviorbehavior where you are\ncoding one thing at a timetime, and across a period of timetime, the number of things\nyou cancan represent depends on how long you needneed to represent each one and how rapidly\nyou cancan switch.”

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