The throat and facial movements that twist the air pushing through your vocal cords into words could be rooted in the well-meaning expressions primates exchange with each other, according to two recent studies based at Princeton University.
The researchers found that the oral-facial component of human speech mirrors the rhythm, development and internal dynamics of lip smacking, a friendly back-and-forth gesture performed by primates such as chimpanzees, baboons and macaques. The studies also show that the mechanics of primate lip smacking are distinct from those of chewing, similar to the separate mechanics of human speech and chewing.
These parallels suggest that in primates chewing and lip smacking — as with chewing and speech-related facial movement in humans — have separate neural controls, explained Asif Ghazanfar, an associate professor of psychology and the Princeton Neuroscience Institute, and a lead researcher for both studies. With further study, the neural pathway in primates from the brain to facial mechanics could help illuminate the neurological basis of speech disorders in humans, he said.
Ghazanfar and his colleagues first reported in the journal Developmental Science that lip smacking undergoes the same developmental trajectory from infancy to adulthood in rhesus macaques that speech-related mouth movement does in humans. Infant macaques smacked their lips slowly and with an inconsistent rhythm, similar to the documented pace of babbling in human infants. By adulthood, however, lip smacking has a distinct rhythm and a faster pace averaging 5 hertz, or cycles per second — the same as adult humans producing speech. Ghazanfar worked with lead author Ryan Morrill, who received his undergraduate degree from Princeton in 2010; Annika Paukner, a research fellow at the National Institutes of Health; and Pier Ferrari, an assistant professor of biology at the University of Parma in Italy.
In the second paper, published in the journal Current Biology, Ghazanfar and co-author W. Tecumseh Fitch, a professor of cognitive biology at the University of Vienna in Austria, used X-ray movies to film adult rhesus macaques as they smacked their lips or as they chewed food. The researchers observed that during lip smacking, internal structures such as the tongue and hyoid, which houses the larynx, move in pace with the lips with a rhythm of 5 hertz — again, just as in human speech. Also similar to humans, chewing produced a slow, tightly coordinated movement of these components in macaques, while lip smacking resulted in faster, loosely coordinated movement. Ghazanfar, as lead author, wrote the paper with Fitch, Princeton postdoctoral fellow Daniel Takahashi and Neil Mathur, who received his undergraduate degree from Princeton in 2011.
Ghazanfar explains the findings of both papers as follows:
"This research gives us insight into methods of exploring the neural basis of not only facial expression production but also its evolution and relationship to speech. Exploring the neural side of speech production and development can give us a handle on what can go wrong neurophysiologically in human communication disorders. We have few testable ideas about the neural mechanisms that go awry because there is very little work on the production side of communicative expressions.
"In our research, we found that primate lip smacking and the facial component of human speech have the same frequency range, developmental trajectory and involve a similar interplay of the lips, tongue and hyoid. So, if the neural controls for lip smacking are the same as for human speech, then further study of lip smacking in monkeys could reveal more about the brain mechanisms behind human speech.