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Songbirds and Human Speech

A mockingbird learns its musical repertoire in much the same way a child learns to speak.

Songbirds -- a category embracing about 4,000 species of birds, including sparrows, ravens and grackles -- and humans demonstrate vocal learning. They imitate the sounds they hear from adult models rather than making sounds based on innate instinct. 
 
For associate professor of psychology Brenton Cooper, songbirds offer an analog for studying human speech development. Talking “really is one of the most challenging motor tasks around, yet you do it effortlessly,” he said. “You have to control breathing, your vocal organ, your tongue and jaw movements, your neck; and all of that alters the way the sound comes out of your mouth.”
 
Two discrete but connected areas of the brain are significant in human speech: Broca’s area, which governs speech production, and Wernicke’s area, which governs language comprehension. Songbirds show similar patterns with separate areas of the brain for the production and processing of song.

To understand the birds’ learning process, Cooper first had to understand their physical systems for sound production. A songbird’s syrinx, or vocal organ, has two sound sources: soft tissue masses on both the left and the right sides. The two sources can be used one at a time or simultaneously, allowing the birds to combine or switch between frequencies.
 
In most songbird species the sound source on the right side of the syrinx produces higher frequency sounds, and the source on the left produces lower frequency sounds, but in the Bengalese finch the opposite is true.
A mockingbird learns its musical repertoire in much the same way a child learns to speak.

Cooper wondered whether the frequency range from each source was determined by peripheral or neural constraints -- in other words, by the vocal organ tissues themselves, or by the nervous system’s capacity to learn.  The professor investigated the inquiry by surgically removing only one sound source in individual Bengalese finches, recording their songs after the operation, and studying the song as represented visually on a spectrogram. The spectrogram showed such great differences in frequency and amplitude between the left and right sides that Cooper knew anatomical differences were responsible for the difference in sound, not the nervous system. 
 
Cooper, whose work is supported by the Hearing Health Foundation, anticipates that syntax -- how birds arrange syllables and organize the structure of their song -- is likely defined by the nervous system, which would make that process similar to the one used by humans.
 
In another line of research, Cooper looked at whether the two brain hemispheres of the finches processed different auditory frequency ranges. Thus far, he is finding that the birds’ processing of different frequency ranges doesn’t seem to be based on the lateralization, or specialization of function, within hemispheres of the brain. These findings help define the limits of how analogous the songbird’s vocal learning system is to the human system.

— Robyn Ross



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