Monkey speak: Macaques have the anatomy, not the brain, for human speech
Monkeys known as macaques possess the vocal anatomy to produce "clearly intelligible" human speech but lack the brain circuitry to do so, according to new research.
Co-corresponding author Asif Ghazanfar, a Princeton University professor of psychology and the Princeton Neuroscience Institute, said that scientists across many disciplines have long debated if — and to what extent — differences between the human and primate vocal anatomy allow people to speak but not monkeys and apes.
"Now nobody can say that it's something about the vocal anatomy that keeps monkeys from being able to speak — it has to be something in the brain. Even if this finding only applies to macaque monkeys, it would still debunk the idea that it's the anatomy that limits speech in nonhumans," Ghazanfar said. "Now, the interesting question is, what is it in the human brain that makes it special?"
Ghazanfar and his co-authors investigated the range of movements that the primate vocal anatomy could produce. Previous examinations of primate vocal anatomy conducted on cadavers had concluded that monkeys and apes have a very limited range of sounds they could produce relative to humans.
|With their vocal tract, it would be easy for monkeys to produce many different language sounds|
[Credit: Tecumseh Fitch/University of Vienna]
Human speech stems from a source sound produced by the larynx that is changed by the positions of the vocal anatomy such as the lips and tongue, Ghazanfar said. For example, the same source sound lies behind the words "bat" and "bot" with the facial anatomy generating the different sound we hear.
The researchers plugged the source sound of a macaque's grunt call into their computer model of the primate's vocal anatomy. They found that a macaque could produce comprehensible vowel sounds — and even full sentences — with its vocal tract if it had the neural ability to speak. The researchers note, however, that while a macaque would be understandable to the human ear, it would not sound precisely like a human.
"This new result tells us that there's still a big mystery concerning where human speech came from," said Laurie Santos, a psychology professor at Yale University who is familiar with the research but had no role in it.
|Macaque MRIs used to convert the monkey's skull diameter measurements to create a vocal tract model |
[Credit: Fitch et al. Sci. Adv. 2016;2:e1600723]
"The paper opens whole new doors for finding the key to the uniqueness of humans' unparalleled language ability," Santos said.
"If a species as old as a macaque has a vocal tract capable of speech, then we really need to find the reason that this didn't translate for later primates into the kind of speech sounds that humans produce," she said. "I think that means we're in for some exciting new answers soon."
Because this work shows that macaques express nearly the same range of physical movements as humans during vocalization, primates could be used as models for understanding early human speech development and human speech evolution, Ghazanfar said.
"Their value as a model system for studying the parts of the brain that directly control the biomechanics of orofacial movements during speech and other vocal behaviors will increase," Ghazanfar said. "Moreover, it's going to force us to think more carefully about how speech evolved, how our brain is uniquely human and how we can use these model animals in the future to understand what goes wrong when we are unable to speak."
Author: Morgan Kelly | Source: Princeton University [December 09, 2016]