Monkey See, Monkey Do?

Neuroscientists are looking at “mirror neurons” – found in both primate and human brains – as a possible missing link in the evolution of language.


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Prof. Michael A. Arbib
You've heard the old saying, “Monkey see, monkey do.” New research now indicates that the two activities are intimately linked – and that seemingly mundane insight may offer a stunning explanation of the origin of human speech.
Long before their throats were capable of speech, pre-human primates had the brain power to process gestural communication. So say computer scientist/biologist Michael A. Arbib and the University of Parma’s Giacomo Rizzolatti.
Indeed, the scientists claim they’ve identified the neural circuit responsible for the ancient gestural communication system used by our earliest ancestors. The team published its findings in a recent issue of the journal Trends in Neuroscience.

The communication system depends on so-called “mirror neurons.” Located in the premotor cortex, just in front of the motor cortex, these cells represent, according to Arbib, a mechanism for recognizing the meaning of others’ actions.
Rizzolatti first noticed mirror cells several years ago in an experiment to determine which neurons fire whenever a monkey grasps an object. Many neurons in the premotor cortex became active when a monkey performed a single task, say tearing or grasping. But the mirror neurons were special – they also fired if the monkey witnessed another monkey (or a human!) making the same moves.
Arbib calls this mechanism the pre-requisite for speech.
“For communication to succeed, both the individual sending a message and the individual receiving it must recognize the
significance of the sender’s signal. Mirror neurons are thus the missing link in the evolution of language. They provide a mechanism for the sharing of meaning.”
Do humans have a similar system? Using positron emission tomography (PET) scans to study human subjects as they executed and observed certain hand movements, the researchers did indeed find a similar neural circuit located in Broca’s area – the part of the human brain believed responsible for speech.
These findings support the theory that human speech evolved from an ancient gestural communication system rather than vocal communication evolving directly, as some scientists have speculated.
Analysis of fossil skulls suggests that, as the primate brain evolved, it was building a language mechanism even before there was a larynx capable of articulating speech. While monkeys and other non-human primates continue to lack the vocal hardware needed to articulate speech, they do have the brain circuits needed for simple communication.
“Monkey vocalizations are limited to calls and screeches, but they are able to communicate via a repertoire of oral-facial actions” such as tooth chattering and lip smacking, Arbib says. “The interesting point is that it is the oro-facial gestures, not the vocalizations, that link to the monkey’s ‘pre-Broca’s area.’ ”
Arbib and Rizzolatti are now charting the evolutionary steps that led from mirror neurons to human language.

 


 

 

 

Photo by Irene Fertik

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