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PHILADELPHIA — How do we hear?  More specifically, how does the auditory center of the brain discern important sounds – such as communication from members of the same species – from relatively irrelevant background noise?  The answer depends on the regulation of sound by specific neurons in the auditory cortex of the brain, but the precise mechanisms of those neurons have remained unclear.  Now, a new study from the Perelman School of Medicine at the University of Pennsylvania has isolated how neurons in the rat's primary auditory cortex (A1) preferentially respond to natural vocalizations from other rats over intentionally modified vocalizations (background sounds). A computational model developed by the study authors, which successfully predicted neuronal responses to other new sounds, explained the basis for this preference. The research is published in the Journal of Neurophysiology.

Rats communicate with each other mostly through ultrasonic vocalizations (USVs) beyond the range of human hearing.  Although the existence of these USV conversations has been known for decades, "the acoustic richness of them has only been discovered in the last few years," said senior study author Maria N. Geffen, PhD, assistant professor of Otorhinolaryngology: Head and Neck Surgery at Penn.  That acoustical complexity raises questions as to how the animal brain recognizes and responds to the USVs.  "We set out to characterize the responses of neurons to USVs and to come up with a model that would explain the mechanism that makes these neurons preferentially responsive to these relevant sounds."

Geffen and her colleagues obtained recordings of USVs from two rats kept together in a cage, then played the recordings to a separate group of male rats, while their neuronal responses were acquired and recorded.  The researchers also used USV recordings that were modified in several ways, such as having background sounds filtered out and being played backwards and at different speeds to mimic unimportant background noise.  "We found that neurons in the auditory cortex respond strongly and selectively to the original ultrasonic vocalizations and not the transformed versions we created," says Geffen.

Using the data collected on the responses of A1 neurons to various USVs, the researchers developed a computational model that could predict the activity of an individual neuron based on the pitch and duration of the USV.  Geffen observes that "the details of their responses could be predicted with high accuracy."  It was possible to determine which aspects of the acoustic input best drove individual neurons. Remarkably, it turned out that the acoustic parameters that worked best in driving the neuronal responses corresponded to the statistics of the natural vocalizations rats produce.

The work makes clear for the first time, says Geffen, "the mechanisms of how the auditory system picks out behaviorally relevant sounds, such as same species communication signals, and processes them more effectively than less relevant sounds.  This information is fundamental in understanding how sound perception helps animals survive. We conclude that neurons in the auditory cortex are specialized for processing and efficiently responding to natural and behaviorally relevant sounds.”

Other authors from Penn include Isaac M. Carruthers and Ryan G. Natan.

The research was supported by the Burroughs Wellcome Career Award; a Klingenstein Award in the Neurosciences; a Pennsylvania Lions Club Hearing Research Grant; Penn Center for Collaborative Neuroscience Pilot Grant; and a Computational Neuroscience and an IGERT Complex Scene Perception Training Grants.

Penn Medicine is one of the world's leading academic medical centers, dedicated to the related missions of medical education, biomedical research, and excellence in patient care. Penn Medicine consists of the Raymond and Ruth Perelman School of Medicine at the University of Pennsylvania (founded in 1765 as the nation's first medical school) and the University of Pennsylvania Health System, which together form a $5.3 billion enterprise.

The Perelman School of Medicine has been ranked among the top five medical schools in the United States for the past 18 years, according to U.S. News & World Report's survey of research-oriented medical schools. The School is consistently among the nation's top recipients of funding from the National Institutes of Health, with $373 million awarded in the 2015 fiscal year.

The University of Pennsylvania Health System's patient care facilities include: The Hospital of the University of Pennsylvania and Penn Presbyterian Medical Center -- which are recognized as one of the nation's top "Honor Roll" hospitals by U.S. News & World Report -- Chester County Hospital; Lancaster General Health; Penn Wissahickon Hospice; and Pennsylvania Hospital -- the nation's first hospital, founded in 1751. Additional affiliated inpatient care facilities and services throughout the Philadelphia region include Chestnut Hill Hospital and Good Shepherd Penn Partners, a partnership between Good Shepherd Rehabilitation Network and Penn Medicine.

Penn Medicine is committed to improving lives and health through a variety of community-based programs and activities. In fiscal year 2015, Penn Medicine provided $253.3 million to benefit our community.

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