Although a summertime chorus of frogs may sound like rhythmic cacophony, the nerve cells in each amphibian are decoding important messages. According to a new study, frogs can count the number of times a burst of sound is repeated in a call, an ability that researchers say helps them recognize friends and foes. The results, published in the 3 September online issue of Nature Neuroscience, also might help elucidate how humans understand speech.
In many frog species, males communicate using sound bites interspersed with regular pauses, such as the brief silence between "rih" and "bit." The length of the pause is full of meaning; male Pacific tree frogs, for example, use a slow call to intimidate other males and a fast trilling to attract females. Several years ago, researchers discovered individual nerve cells in the frogs that would fire in response to simulated frog calls of different lengths. In the current study, neuroethologist Gary Rose of the University of Utah, Salt Lake City, and colleagues wanted to determine what components of the sound the neurons were paying attention to.
The team played computer-generated calls to male frogs and recorded electrical impulses from neurons in the sound-processing region of their brains. They learned that each neuron was tuned to its own optimum number, between four and 12 pulses. But were the neurons calculating an average number of pulses over a set time, or just tallying the beats? When the researchers varied the length of pauses between tones, the frog neurons only discharged when the intervals were constant. That suggests the neurons count the spaces between throbs. In addition, a neuron that normally fired after nine pulses would fail to shoot if the team altered the interval length between pulse eight and nine. The neuron required another nine repetitions, suggesting it had been reset to zero.
"Very clever," says acoustic communications researcher Carl Gerhardt of the University of Missouri, Columbia. Although impressed, he would have liked to see whether female frogs responded preferentially to the constant interval calls. "It's important to verify that the [counting] neurons are playing a role in recognition by other frogs," he says. Also, it suggests a mechanism for how human brains might detect temporally patterned sound in voice recognition. Humans don't ribbit like frogs, but "we do use little bursts of sound that come out of the larynx," Gerhardt says.