Human brains decode the complex sounds of speech
A composite sound such as a vowel sound in human speech usually has three dominant frequency components. The movement of the eardrum and ear bones receiving such a sound is very complex, but when the sound reaches the basilar membrane, the frequency components are sorted out. Each of the frequency components sets off a separate travelling wave and each wave produces its peak at the position on the membrane that responds best to that frequency. Next, the hair cells on the membrane at each peak send signals indicating to higher centers that a certain frequency of sound has been detected.
What happens when signals from language sounds are sent to higher centers? So far, this has not been an easy question to answer. When researchers try to find which cortical centers and cells are involved in neurological functions, they often inject tracer materials into animal brains, perform experiments, and then dissect the brain to find out what areas were affected. Language processing has been difficult to study because it is a uniquely human trait and such experiments cannot be done on people. Studies of sonar and echolocation in bats have provided many insights into the processing of complex sounds.
Another way scientists find out about brain areas used in language is to study people with language problems. They then either study those patients' brains after natural death, or before death with techniques such as magnetic resonance imaging (MRI), which allows them to see the damaged areas and hypothesize about the functions of those areas.
Researchers are now also using positron emission tomography (PET) to study brain function during language processing in normal individuals. PET is a non-invasive procedure that shows local changes in blood flow and metabolism that occur when the brain is working-in this case, working to interpret spoken language. Through studies such as these, scientists have discovered many brain subdivisions for processing different aspects of language. The area for comprehending spoken language, for example, contains separate areas for decoding the meaning of words and for understanding the relationship of words in a sentence. These studies are opening new windows on how we decode language.
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