Broca's cerebral asymmetry reflects gestural communication's lateralisation in monkeys (Papio anubis)

Abstract

Manual gestures and speech recruit a common neural network, involving Broca's area in the left hemisphere. Such speech-gesture integration gave rise to theories on the critical role of manual gesturing in the origin of language. Within this evolutionary framework, research on gestural communication in our closer primate relatives has received renewed attention for investigating its potential language-like features. Here, using in-vivo anatomical MRI in 50 baboons, we found that communicative gesturing is related to Broca homologue's marker in monkeys, namely the ventral portion of the Inferior Arcuate sulcus (IA sulcus). In fact, both direction and degree of gestural communication's handedness - but not handedness for object manipulation - are associated and correlated with contralateral depth asymmetry at this exact IA sulcus portion. In other words, baboons that prefer to communicate with their right hand have a deeper left-than-right IA sulcus, than those preferring to communicate with their left hand and vice versa. Interestingly, in contrast to handedness for object manipulation, gestural communication's lateralisation is not associated to the Central sulcus depth asymmetry, suggesting a double dissociation of handedness' types between manipulative action and gestural communication. It is thus not excluded that this specific gestural lateralisation signature within the baboons' frontal cortex might reflect a phylogenetical continuity with language-related Broca lateralisation in humans.

Is Reduced Visual Processing the Price of Language?

“…We have presented evidence for language-associated capacities in other primates. There is evidence of lateralization connected to communicative gestures in baboons [ 62 ]. There are also similarities in the anatomical layout of the homologues of language-associated areas (for example, homologues of Broca’s and Wernicke’s areas).…”

“…Lately, studies [ 62 ] (inter al.) have shown brain lateralization related to communicative gestures in the Olive baboon, and likely such lateralization is present in other primates as well, but it is still unknown if similar lateralization is present in the above-mentioned wide variety of species (primates, birds, dogs).…”

“…We want to suggest that the homolog capacity for language is still present in many non-human primates [ 35 , 37 , 39 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]. However, this language capacity is flooded by other processes, so overt language does not result in animals.…”

“…In a series of studies, Becker et al [ 37 , 60 , 61 , 62 ] investigated language-relevant homologues in the Olive baboon ( Papio anubis ). First, the lateralization of the Planum Temporale (PT) is investigated in 35 newborn baboons, which found a significant leftward asymmetry present in the newborns, and this leftward asymmetry showed a significant increase when scanned later at 7 to 10 months.…”

“…Becker et al [ 62 ] show an association between brain asymmetry and the individual use of communicative gestures in the Olive baboon ( Papio anubis ). This study (ibid.)…”