Animals and humans raise their voices, when trying to be heard over noise. It’s a very rapid feat from the ear to the brain and finally to vocalization. Johns Hopkins researchers are the first ever to measure how fast the action occurs in bats – 30 milliseconds. When compared to the blink of an eye that is just a tenth of the time.
The action, Lombard effect, occurs so quickly that the research team was capable of solving the longtime phenomenon regarding the neural mechanism linked to it. It was previously thought that it was a deeper cognitive behavior, which requires a longer processing time, but the researchers now conclude that it is a fundamental temporal reflex.
The study sheds light on the bases of human speech control and shows how species as diverse as frogs, fish, people and animals share the capabilities to be heard over external noises.
“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 Johns Hopkins graduate student in psychological and brain sciences and co-author Ninand Kothari. “We resolved this question.”
The new study could help develop assistive medical devices and lead to improved treatment for diseases such as Parkinson’s disease in which the Lombard effect can be amplified.
The Johns Hopkins researchers analyzed bats that rely on sonar-like echolocation -releasing sounds and listening to echoes – to detect, track down and capture prey. Bats are perfect for such a sensorimotor study, because human vocalizations are slow and long. The bat’s high-frequency chirrup is precise and quick and cannot be detected by the human ear, permitting scientists to test the limits of a mammalian brain.
Large brown bats were trained to stay perched on a platform, while tracking down insects attached to a tether that moved towards them. The research team utilized 14 microphones to record the bat’s vocalizations, while they hunted the insects. The researchers would sometimes permit the bats to hunt in silence, while playing bursts of disrupting white noise, at different intensities, other times.
The white noise disrupted the bat’s echolocation, causing their chirrups to increase in intensity, similar to a human attempting to be heard over a loud TV, then over a roaring chainsaw and then over a police siren. The bat would stop shouting once the noise stopped and began vocalizing at a more normal level.
For the Lombard effect, the researchers developed a computational model that applies to all vertebrates and concluded that the brain of a human, fish or bat constantly monitors background noise and makes vocal level adjustments as needed.
The auditory system is responsible for detecting the background noise, measuring the sound pressure level and adjusting the vocalization amplitude to compensate. As soon as the background noise stops, the sound pressure level vanishes, as well as the level of vocalization.
The researchers discovered that the entire process occurs in 30 milliseconds.
“Typically, we breathe every three to five seconds, our heart beats once per second and eye blinking takes on 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 Johns Hopkins postdoctoral fellow and lead author Jinhong Luo.
Previously, scientists believed that the Lombard effect was slower at nearly 150 milliseconds for bats and birds and about 150-175 milliseconds for humans.
The study was published online in the journal Proceedings of the National Academy of Sciences on June 5, 2017.