Can You Hear Me Now?

Summary: Researchers solve a long standing mystery of the neural mechanism behind the Lombard effect.

Source: Johns Hopkins University.

Astonishingly speedy brain mechanism helps bats get louder when necessary.

When trying to be heard over noise, humans and animals raise their voices. It’s a split-second feat, from ear to brain to vocalization, and Johns Hopkins University researchers are the first to measure just how fast it happens in bats: 30 milliseconds. That’s just a tenth of the time it takes to blink an eye, and a record for audio-vocal response.

Because this action, known as the Lombard effect, happens so very fast, the researchers were also able to solve a longtime mystery regarding the neural mechanism behind it. In a paper published online this week by the journal Proceedings of the National Academy of Sciences, they conclude it must be a fundamental temporal reflex rather than, as previously thought, a deeper cognitive behavior that would take more processing time. The findings, which shed light on the underpinnings of human speech control, also reveal how species as diverse as fish, frogs, animals and people share the ability to be heard over the fray.

“Scientists have been wondering for a century: Could there be a common auditory process to explain how this phenomenon happens in fish to frogs to birds to humans, species with wildly different hearing systems?” said co-author Ninad Kothari, a Johns Hopkins graduate student in psychological and brain sciences. “We resolved this question.”

The new information could lead to better treatment for diseases where the Lombard effect can be amplified, such as Parkinson’s disease, and could also help to build assistive medical devices.

The researchers studied bats, which rely on sonar-like echolocation — emitting sounds and listening for echoes — to detect, track, and catch prey. Unlike humans, whose vocalizations are comparatively long and slow, bats are ideal for such a sensorimotor study. Their high-frequency chirps, undetectable to the human ear, are quick and precise, allowing researchers to test the limits of a mammalian brain.

The team trained big brown bats to remain perched on a platform while tracking insects moving towards them on a tether. While the bat hunted the insect, the researchers recorded the bat’s vocalizations with an array of 14 microphones. Sometimes the researchers allowed the bat to hunt in silence, other times they played bursts of interfering white noise, at varying intensities, from a speaker positioned in front of the bat.

The white noise interfered with the bat’s echolocation and caused the bat to emit louder and louder chirps, not unlike someone trying to be heard, first over a loud radio, then over the clamor of a lawn mower and then over the blare of a passing fire engine. When the noise stopped, the bat would stop shouting, so to speak, and vocalize at a more typical level.

Image shows a bat.

This is a bat trained to stand on a platform and await delivery of its prey. NeuroscienceNews.com image is adapted from the Johns Hopkins University news release.

The researchers, who were able to create a computational model for the Lombard effect that applies to all vertebrates, concluded that the brain of a bat, or a person, or a fish, constantly monitors background noise and adjusts the vocal level as necessary.

First the auditory system detects background noise. The auditory system then measures the sound pressure level and adjusts the vocalization amplitude to compensate. When the background noise ends, the sound pressure level dissipates, and so does the level of vocalization.

This entire elaborate process happens in just 30 milliseconds, the authors found. Even in terms of near-instantaneous brain reactions, they call this reflex “remarkably short.”

“Typically, we breathe every three to five seconds, our heart beats once per second, and eye blinking takes one third of a second. If we believe that eye blinking is fast, the speed at which an echolocating bat responds to ambient noise is truly shocking: 10 times faster than we blink our eyes,” said lead author Jinhong Luo, a Johns Hopkins postdoctoral fellow.

Scientists had believed the Lombard effect was much slower: about 150 milliseconds for birds and bats and approximately150 to 175 milliseconds for humans.

“Our study features echolocating bats as valuable animal models for understanding connections between hearing and vocalizations, including speech control in humans,” said Cynthia Moss, a Johns Hopkins professor of psychological and brain sciences and of neuroscience and a co-author.

About this neuroscience research article

Funding: Support for this research came from the National Science Foundation (IOS-1010193 and IOS-1460149), the Human Frontiers Science Program (RGP0040 and LT000279/2016-L9, the Office of Naval Research (N00014-12-1-0339), and the Air Force Office of Scientific Research (FA9550-14-1-0398).

Source: Jill Rosen – Johns Hopkins University
Image Source: NeuroscienceNews.com image is adapted from the Johns Hopkins University news release.
Video Source: Video credited to JHU.
Original Research: Abstract for “Sensorimotor integration on a rapid time scale” by Jinhong Luo, Ninad B. Kothari, and Cynthia F. Moss in PNAS. Published online June 5 2017 doi:10.1073/pnas.1702671114

Cite This NeuroscienceNews.com Article
Johns Hopkins University “Can You Hear Me Now?.” NeuroscienceNews. NeuroscienceNews, 6 June 2017.
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Abstract

Sensorimotor integration on a rapid time scale

Sensing is fundamental to the control of movement: From grasping objects to speech production, sensing guides action. So far, most of our knowledge about sensorimotor integration comes from visually guided reaching and oculomotor integration, in which the time course and trajectories of movements can be measured at a high temporal resolution. By contrast, production of vocalizations by humans and animals involves complex and variable actions, and each syllable often lasts a few hundreds of milliseconds, making it difficult to infer underlying neural processes. Here, we measured and modeled the transfer of sensory information into motor commands for vocal amplitude control in response to background noise, also known as the Lombard effect. We exploited the brief vocalizations of echolocating bats to trace the time course of the Lombard effect on a millisecond time scale. Empirical studies revealed that the Lombard effect features a response latency of a mere 30 ms and provided the foundation for the quantitative audiomotor model of the Lombard effect. We show that the Lombard effect operates by continuously integrating the sound pressure level of background noise through temporal summation to guide the extremely rapid vocal-motor adjustments. These findings can now be extended to models and measures of audiomotor integration in other animals, including humans.

“Sensorimotor integration on a rapid time scale” by Jinhong Luo, Ninad B. Kothari, and Cynthia F. Moss in PNAS. Published online June 5 2017 doi:10.1073/pnas.1702671114

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