New research has shown that rhesus monkeys are able to call out or be silent on command, a find that suggests that the primates are able to consciously decide whether to make noise or not.
Researchers suggest that because the monkeys are able to instrumentalize the sounds they make in a targeted way, it's further evidence of the similarities between humans and primates.
"We want to understand the physiological mechanisms in the brain which lead to the voluntary production of calls because it played a key role in the evolution of human ability to use speech," said Steffen Hage, a neurobiologists at University of Tübingen's Institute for Neurobiology.
Writing in the journal Nature Communication, Hage, along with his colleague Andreas Nieder, demonstrated that nerve cells in the brain of the rhesus monkeys signal the targeted initiation of calls, which they say forms the basis of voluntary vocal expression.
Humans speak with purpose, intentionally saying what we think or consciously withholding information. But animals are not so calculating, usually making sounds associated with how they feel in a given moment.
"Even our closest relations among the primates make sounds as a reflex based on their mood," the researchers wrote in a news release, but rhesus monkeys, they said, "are able to call (or be silent) on command. They can instrumentalize the sounds they make in a targeted way, an important behavioral ability which [humans] also use to put language to a purpose."
For their research, the neuroscientists trained the rhesus monkeys to call out quickly when a dot appeared on a computer screen. In doing so they were able to find out how the neural cells in the brain become a catalyst for the production of controlled vocal noises.
"While the monkeys solved puzzles, measurements taken in their prefrontal cortex revealed astonishing reactions in the cells there," the researchers wrote. "The nerve cells became active whenever the monkey saw the spot of light which was the instruction to call out. But if the monkey simply called out spontaneously, these nerve cells were not activated. The cells therefore did not signaled for just any vocalization -- only for calls that the monkey actively decided to make."
The discovery could lead to future insights into the development and treatment of speech disorders, which have their origins in the prefrontal cortex.
"Disorders in this part of the human brain lead to severe speech disorders or even complete loss of speech in the patient," Nieder explained, adding that the results of their study could yield further information on how the production of sound in initiated in the brain, which may lead to a better understanding of speech disorders.
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