By Yannick Becker, Max Planck Institute for Human Cognitive and Brain Sciences, Adrien Meguerditchian, University of Aix-Marseille (AMU)
Bicêtre Hospital, Paris, mid-19th century: Doctor Paul Broca meets an extraordinary new patient. He seems perfectly normal, but when the doctor asks him about his illness, all he replies is “tan”!
Are the mysteries of the origin of our language hiding behind the eyes of this baboon? Yannick Becker and Adrien Meguerditchian Provided by the author
It turns out that “Mister Tan”, as he is later known, understands perfectly what he is being told, but he cannot answer with any other word than “tan”, which he tirelessly repeats. After his death, Broca discovered the reason for this loss of speech production: a lesion in the left forebrain.
Since this discovery, “Broca’s area” has gone down in history as an area of language production. And more recently, the function of this region has been refined to include other properties such as semantics, motor planning in gestures, or even syntax. Syntax concerns the grammatical rules that structure a sentence, but also so-called “motor” syntax, i.e. any sequence of nested actions, such as gestures, the use and manufacture of tools or driving a car. For all these promotions, your Broca space will be activated!
Such motor functions at the heart of this key area of language raise questions. Aren’t they traces of our ancient communication system inherited from our ancestors?
A gestural origin of language?
Answering this evolving question is not easy. Although soft tissues such as the brain do not fossilize, one can speculate about the cognitive functions of our ancestors, as paleoanthropologists do, based on the archaeological evidence they left, including the fossils of their bones, particularly their skulls (where their brains left precious imprints), their tools and other creations. Another complementary approach, valued by us primatologists, is to draw on the study of our primate relatives. In fact, we share with them relatively recent common ancestors in evolutionary history. Imagine if we discovered common points, we could reconstruct the cognitive traits inherited from these famous common ancestors.
Using this so-called “comparative” approach between species, researchers have discovered that monkeys’ gestural communication shares some interesting features with language. For example, monkeys produce gestures that fully meet the criteria of so-called “deliberate communication,” a trait central to language development in preverbal children. In fact, when the monkey slamming its hand on the ground to threaten a fellow human does not get the expected response, it tends to repeat or even vary its signal (open eyes, raise eyebrows, den shaking your head, jumping your feet together…) like a teacher trying to rephrase a statement to make it easier to understand.
A juvenile olive baboon deliberately communicates with a threatening gesture, the “handshake by repeatedly slamming the hand on the ground”. Yannick Becker and Adrien Meguerditchian Provided by the author
In our team, we have been studying this behavior closely for years with around a hundred baboons of all ages living in social groups at the CNRS primatology station near Aix-en-Provence. We have found that the majority of baboons (Papio anubis) prefer to use their right hand when communicating with their hands, and to a greater extent than when manipulating objects.
In other words, we have not only described that baboons can be right- or left-handed for complex object manipulation tasks, but also that hand-held communication alters these manual preferences, particularly in favor of the right hand.
This discovery put us on an interesting lead: Since the right hand is controlled by the left hemisphere and language also mobilizes the left hemisphere, ape gestural communication mobilizes the same hemispheric specialization as language—humans? Are they similar cerebral structures, especially the famous Broca’s area?
In order to examine this bold thesis, we were missing a key piece of the puzzle: the brain. But here it is: The brain is the most protected organ in an organism.
MRI is for monkeys too
Fortunately, thanks to the democratization of non-invasive brain imaging (imaging of the brain (also called neuro-imaging) denotes all techniques …) by resonance (resonance is a phenomenon by which certain physical systems …) magnetic ( MRI) , which even touched our psychology lab, we caught ourselves dreaming. Not putting a willing baboon into the narrow and noisy MRI scanner, much less asking it not to move an inch to produce command-pointing gestures (command: term used in many fields, usually denoting an order or a…) – we would have had trouble finding volunteers. But to make anatomical brain images of sleeping monkeys for the occasion, such as a 3D photograph of their brain.
The idea is to study not the cerebral activation in situations (In geography, the situation is a spatial concept that allows the relative localization of a…) of communication, but the morphology of language-homologous cerebral structures, specifically that of Broca’s area, to compare it with that of man. And so, every week we left the song of the cicadas in the countryside to go to the MRT center in Marseille, accompanied by our dear baboons. After drawing the portrait of their brains and making sure they were fine when they awoke from the machine, we rushed to take them back to the primatology station so they could find their kin in their social group. And with impatience we have revealed on the computer (A computer is a machine equipped with a processing unit that allows it …) one by one these brain images collected from 50 baboons to analyze and to trying to delineate the size of the homolog zone of the Broca area.
The monkey equivalent of this cerebral zone can be seen by following the natural demarcation of one of the folds, called the arcuate sulci, located in the anterior part of the brain. Thanks to software, we were able to extract and represent in 3D this groove in each of the two hemispheres – like cement poured into a mold – in order to measure its dimensions (in a general sense, the term dimension refers to the size; the dimensions of a piece…) from every angle, particularly the depth of that portion of the furrow that marks the boundary of Broca’s territory. For each baboon, we were thus able to quantify the differences in this border between the two hemispheres in order to determine the cerebral asymmetries of Broca’s area. We then compared these measurements between two groups of baboons: baboons who prefer to communicate with their right hand and those who prefer to communicate with their left hand.
Similarities between monkey and human brains
For the 28 baboons that prefer to communicate with the right hand (colored blue in the graph), the boundary of their Broca territory (colored red in the figure) was found to be deeper in the left hemisphere than that of the 22 baboons prefer their left hand (orange in the graph) and vice versa.
On the other hand, when a baboon manipulates objects without a communicative purpose (for example, when trying to remove a treat from a tube), its manual preference is unrelated to this cerebral asymmetry. Thus, only communicative gestures are associated with the homologous area of Broca’s area!
Summary of study results. Yannick Becker and Adrien Meguerditchian Provided by the author
This discovery in the baboon suggests that key regions of human language in monkeys may be specialized for gestural communication!
We hypothesize that language and its asymmetric cerebral organization may have been inherited from our distant ancestors’ gestural communication system shared with baboons, and thus predate the origin of language. So this system would not go back to the origin of hominids, but rather to the common ancestor (In phylogenetic history, an ancestor common to several species is the…) of much older ancient apes (The Old World). the part of the world known to Europeans…) and humans, 25-35 million years ago.
But then many questions arise: What about the other regions of the brain involved in human language? At what age do baboons develop such a brain-gesture connection? Is it early brain organization that leads to the development of gestures, or the emergence of gestural communication that influences the brain’s hemispheric specialization?
To try to answer these questions, we have started a new project which consists of tracing the evolution of gestural behavior in baboons within their social group since birth, while regularly collecting images of their developing brain. We’ll tell you more soon.
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