Perhaps the finest achievement in sound processing is the ability to understand speech. Since this is a uniquely human trait, it would seem difficult to study in animals. Yet a researcher at Washington University in St. Louis believes it can be examinedby working with bats.
Bats navigate and locate prey by echolocation, a form of sonar in which they emit sound signals of their own and then analyze the reflected sounds. Nobuo Suga, who has spent nearly 20 years investigating the neural mechanisms used by bats to process the reflected signals, is convinced that such research can shed light on the understanding of human speech.
When Suga slowed down recordings of the high-frequency, short-duration sounds that bats hear, he found that their acoustic components were surprisingly similar to those of mammalian communication, including human speech. There were some constant frequencies and noise bursts, not unlike vowel and consonant sounds, as well as frequency-modulated components that were similar to those in combinations of phonemes such as "papa."
Suga further demonstrated that each of these acoustic elements is processed along a distinct pathway to higher-order neurons. Those neurons then combine and refine different aspects of the sonar pattern in much the same way that space-specific neurons combine the timing and intensity cues of sound signals.
Suga also identified maps of neurons in the bats' auditory cortex which register slight variations in each of these components of sound. The bat's brain uses such neuronal maps to register changes in its surroundings. Humans may use similar maps to process the basic acoustic patterns of speech, though speech requires additional, higher-level mechanisms, he points out.
"The ability to recognize variations in sound is what enables us to understand each other. No two people pronounce vowels and consonants in exactly the same way, but we are able to recognize the similarities," says Suga. He believes that neuronal maps may also play a role in human voice recognitionthe ability to recognize who is speaking as well as what is being said.
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