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After seven years of research, a team of neuroscientists has finally discovered how our brains process speech – and it’s not as we thought.
Instead of transforming the sound of someone speaking into words, as has long been assumed, our mind processes both sounds and words, but in two different places in the brain.
This discovery, say the researchers, could have implications for our understanding of hearing and language disorders, such as dyslexia.
Scientists’ ability to understand speech processing has been limited by topology: the region of the brain involved in speech processing, the auditory cortex, is deeply hidden between the frontal and temporal lobes of the brain.
Even if researchers could access this area of the brain, getting neurophysiological recordings of the auditory cortex would require a very high resolution scanner.
But advancements in technology, along with nine participants undergoing brain surgery, have enabled a team of neuroscientists and neurosurgeons from across Canada and the United States to answer the question of how we understand speech.
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“We undertook this study in the hope of finding evidence for the transformation of low-level representation of sounds into high-level representation of words,” said Dr. Edward Chang, one of the authors of the study at the University of California at San Francisco.
When we hear the sound of speech, the cochlea in our ear transforms it into electrical signals, which it then sends to the auditory cortex in the brain. Before their research, Chang explained, scientists believed that this electrical information needed to be processed by a specific area known as the primary auditory cortex, before it could be translated into syllables, consonants, and vowels, which make up the words we understand. .
“That is, when you hear your friend’s voice in a conversation, the different frequency tones of his voice are first traced in the primary auditory cortex … before they are transformed into syllables and in words in the primary auditory cortex.
“Instead, we were surprised to find evidence that the non-primary auditory cortex does not require inputs from the primary auditory cortex and is likely a parallel pathway for speech processing,” Chang said.
To test this, the researchers stimulated the primary auditory cortex in the participants’ brains with small, harmless electrical currents. If participants needed this area to understand speech, stimulating it would prevent or distort their perception of what they were being said.
Surprisingly, patients could still clearly hear and repeat all the words said to them.
Next, the team stimulated an area of the non-primary auditory cortex.
The impact on patients’ ability to understand what they were told was significant. “I could hear you talking, but I can’t make out the words,” one said. Another patient said it seemed like syllables were exchanged in the words they heard.
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“[The study] found evidence that the non-primary auditory cortex does not require inputs from the primary auditory cortex, which means that there is likely a parallel pathway for speech processing, ”explained Chang.
“[We had thought it was] a series track – like an assembly line. The parts are assembled and modified along a single path, and a step depends on the previous steps.
“A parallel lane is a lane where you have other lanes that also process information, which can be independent.”
The researchers warn that although this is an important step forward, they do not yet understand all the details of the parallel hearing system.
“It certainly raises more questions than answers,” Chang said. “Why has it evolved, and is it specific to humans? What is the anatomical basis of parallel processing?
“The primary auditory cortex may not have an essential role in understanding speech, but does it have other potential functions? “
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