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Bat brains offer clues as to how we focus on some sounds and not others

GUMC neuroscientists find brains of bats tell some neurons to 'shush' and others to 'yell louder' in order for key sounds to be heard above background noise

Georgetown University Medical Center

San Diego - How do you know what to listen to? In the middle of a noisy party, how does a mother suddenly focus on a child's cry, even if it isn't her own?

Bridget Queenan, a doctoral candidate in neuroscience at Georgetown University Medical Center is turning to mustached bats to help her solve this puzzle.

At the annual meeting of the Society for Neuroscience in San Diego, Queenan will report that she has found neurons in the brains of bats that seem to "shush" other neurons when relevant communications sounds come in - a process she suggests may be working in humans as well.

In her investigations, she has also found that "some neurons seemed to know to yell louder to report communication sounds over the presence of background noise."

"So we can now start to piece together how the cells in your brain are able to deal with the complex sensory environment we live in," Queenan added.

To understand auditory brain function, bats are especially interesting animals to study because they process sound through echolocation, which is a kind of biological sonar. Bats call out and then listen to their own echoes produced when those calls bounce off nearby objects. Bats use these echoes to navigate and to hunt.

Not only do the brains of bats have to process a constant stream of pulses and echoes, they have to simultaneously process the bats' social communication, Queenan says.

"What we are trying to figure out is how a bat can fly around echolocating - screeching and listening to its own individual sounds bouncing back - amidst a whole colony of hundreds of other echolocating bats - and possibly hear another bat saying 'watch out! Bats actually do make these cautious calls quite a bit," she says. "In fact, bats have a whole host of communication sounds: angry sounds, warning sounds, and sounds that says 'please don't hurt me."

The auditory processing area in bats' brains is larger than other centers, just like the visual processing center in humans is large. "Humans operate predominantly by sight so a huge portion of our brain is devoted to vision processing. Bats, however, operate by sound," Queenan says.

In this study, Queenan and her colleagues presented different combinations of echolocation sounds with various communication sounds to awake bats to see how neurons in the bat brains were dealing with this incredible cacophony. The researchers found that some bats' neurons control the activity of other neurons when important sounds are perceived. These GUMC scientists also found other neurons that amp up perception of bat communication in the face of background noise. Working together, these clumps of neurons allow the bats to hear what is needed.

"All organisms are constantly assaulted by incoming stimuli such as sounds, light, vibrations, and so on, and our sensory systems have to triage the most relevant stimuli to help us survive," Queenan says. "As humans we are not only sensitive to a child's cry, but we notice flashing ambulance lights even though we are engrossed in something else. We want to know how that happens."

Queenan says her next task is to record brain neurons in bats that are not only awake, but flying.


About Georgetown University Medical Center

Georgetown University Medical Center is an internationally recognized academic medical center with a three-part mission of research, teaching and patient care (through MedStar Health). GUMC's mission is carried out with a strong emphasis on public service and a dedication to the Catholic, Jesuit principle of cura personalis -- or "care of the whole person." The Medical Center includes the School of Medicine and the School of Nursing and Health Studies, both nationally ranked, the world-renowned Georgetown Lombardi Comprehensive Cancer Center and the Biomedical Graduate Research Organization (BGRO). In fiscal year 2009-2010, GUMC accounted for 79 percent of Georgetown University's extramural research funding.

Abstract 275.21 B. N. QUEENAN, Z. ZHANG, J. MA, R. T. NAUMANN, S. MAZHAR, *J. S. KANWAL; Physiol. and Biophysics, Georgetown Univ. Med. Ctr., Washington, DC

Multifunctional neurons within the auditory cortex of mustached bats, Pteronotus parnellii, exhibit specializations for processing both echolocation and communication sounds (Ohlemiller et al., 1996; Kanwal et al., 2004). How such neurons detect communication (social) calls while processing the individual's own pulse-echo trains and/or in the presence of echolocation clutter within a dense colony of conspecifics remains unclear. We postulated the presence of neural mechanisms that potentiate single cortical neurons to respond to social calls in the midst of echolocation clutter. To test this hypothesis, we used a bundle of tungsten wires to record from neurons in the Doppler-shifted constant frequency processing (DSCF) area in the left primary auditory cortex of awake bats. A series of tones (20 to 30 ms in duration) corresponding to the CF in the echolocation signal and matching the facilitatory frequency tuning of DSCF neurons at the recording locus were presented at rates of 5, 10, 20 and 40 Hz. Each CF train was followed by the presentation of a call, such as the "rectangular broadband noise-burst (rBNB)" or the "bent upward frequency modulation (bUFM)" at various delays ranging from 1 to 400 ms. All sounds were presented at ~75 dB SPL. We also presented a series of rBNB syllables followed by a tone as a reverse condition. DSCF neurons recorded at different locations (n = 36) exhibited a multiplicity of response patterns. Within this stimulation paradigm, some neurons that did not respond to tones, exhibited an enhanced response to a call presented immediately after the tone sequence. The reverse condition did not produce an enhanced response to the tone. Enhancement of call responses may be explained by rebound excitation from lateral inhibition within a cortical network. This assumes neighboring neurons respond well to tones, and this was demonstrated via simultaneous recordings from adjacent electrodes. At high tone-repetition rates (> 20 Hz), response enhancement may also result from coincidence of a response to call onset with an offset response to a tone. Other neurons responded to calls alone and some responded to both, tones and social calls, but did not exhibit response enhancement. Adaptation to call and tone repetition was also observed. Our data suggest the presence of specialized cortical mechanisms and networks that enhance call perception within an acoustic clutter of echolocation sounds. Similar mechanisms in the brains of other social species, including humans, may allow mothers to hear the cry of their offspring or an alarm call within a cocktail party-like acoustic background or conspecific chatter.

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