Scientists have shed new light on the underpinnings of human speech by identifying neural circuits in the brains of monkeys, which they say could represent a common evolutionary origin of social communication.
According to a study published in the journal Neuron, these circuits are involved in face recognition, facial expression, and emotion. And they may very well have given rise to our singular capacity for speech.
Working with rhesus macaque monkeys, the researchers from Rockefeller University in the US had previously identified neural networks responsible for recognising faces - networks that closely resemble ones found in the human brain.
In the latest study, Winrich Freiwald and colleagues investigated the patterns of activation that occur within and between various networks to better understand how the brain coordinates the intricate task of social communication.
They used a novel experimental setup to take Magnetic Resonance Imaging (MRI) scans of the brains of monkeys as they watched video clips of other monkeys making communicative facial expressions.
In some of the clips, the videotaped monkeys looked off to the side, mimicking a situation in which the subject monkeys were passively observing communication between other animals without participating in it.
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In others, the prerecorded animals appeared to be looking directly at the subject monkeys, simulating face-to-face social interaction.
These differences in social context proved to be significant. When the monkeys in the clips made a friendly lip-smacking gesture, the subject monkeys responded in kind - but only when their prerecorded peers appeared to be making direct eye contact with them.
Based on previous research, the scientists expected the face-perception regions of the monkeys' brains to simply feed information to a region associated with emotion, which would then stimulate the regions responsible for producing facial expressions.
All of those areas were indeed activated. However, much to the researchers' surprise, they did not shuttle information to one another in straightforward, sequential fashion, researchers said.
Videos that simulated social interaction through direct eye contact caused an unexpected third neural circuit to light up, they said.
This suggests that specific areas of the animals' brains are sensitive to social context, and perform the specialised cognitive functions necessary for social communication.
Producing facial expressions in response to the videotaped monkeys prompted an entirely different pattern of brain activation.
Generating a friendly lipsmack, in particular, activated a region that resembles Broca's area, a portion of the human brain concerned with the production of speech, Freiwald said.
This suggests that monkey facial expressions like lipsmacks might be evolutionary precursors to human speech - a possibility that some scientists had previously discounted on the grounds that such gestures were too simple or reflexive to pave the way for something as subtle and sophisticated as human verbal communication, he said.