Listen: Bat chirps reveal how we hear our name in a crowd

If the bat's entire brain paid equal attention to every sound, researcher Melville Wohlgemuth says, "it doesn't do an animal any good. You need the selectivity." (Credit: Daniel Neal/Flickr)

You’re at a crowded party, awash with chatter, loud music, and clinking glasses. But when someone behind you says your name, somehow you pick it out in all the noise and quickly turn in that person’s direction.

What’s up with that? The answer, scientists say, may be found in the way certain neurons fire in the midbrain of a brown bat.

Like humans, bats hear lots of things simultaneously and are able somehow to screen out the unimportant and pay attention to what they really need to hear.

“With so many stimuli in the world, the brain needs a filter to determine what’s important,” says Melville J. Wohlgemuth, a postdoctoral fellow in psychological and brain sciences at Johns Hopkins University. “The bat brain has developed special sensitivities that allow it to pick out sounds from the environment that are pertinent to the animal. We were able to uncover these sensitivities because we used the perfect stimulus—the bat’s own vocalizations.”

Wohlgemuth and coauthor Cynthia F. Moss, a Johns Hopkins professor and neuroscientist, described their research in an article published in the Proceedings of the National Academy of Sciences.

To navigate as they hunt in the dark, bats use echolocation. They make high-frequency sounds as they fly and then listen for echoes bouncing off objects in front of them to orient themselves.

Listen to the bats chirp

Wohlgemuth and Moss wanted to figure out which sounds would be deemed “important” enough to evoke responses from bat brain neurons involved in orienting behaviors, such as turning towards a sound.

The researchers experimented with five big brown bats, playing them a variety of sounds while monitoring their midbrain activity. They played recordings of natural chirps, the actual sounds bats made as they hunt. But they also played artificial white noise and sounds between the two extremes. All of the sounds were identical in amplitude, duration, and bandwidth.

Sensorimotor neurons in the bat midbrain reacted to all of the sounds, suggesting that the bats could “hear” it all. But the neurons involved in stimulus selection, those that actually guide orienting behaviors, fired selectively, reacting only to a subset of the natural chirps.

[Why nothing else sounds like a scream]

“That’s really important because it showed that if a neuron reacts to all stimuli, any sound in the environment, it doesn’t do an animal any good,” Wohlgemuth says. “You need the selectivity.”

Because all mammals share a basic brain organization, these findings suggest for the first time how mammals, including humans, likely choose which stimuli deserve attention, the researchers say.

“Bats produce the sounds that guide their behaviors, and consequently, we know what signals are important to them,” Moss says. “By comparing activity patterns of neurons to biologically natural and artificial sounds, we learn general principles of sensory processing that apply to a broad range of species.”

The National Institutes of Health, the National Science Foundation, and the Human Frontiers Science Program funded the study.

Source: Johns Hopkins University