INVESTIGATION OF GESTURAL VS VOCAL ORIGINS OF LANGUAGE IN NONHUMAN PRIMATES: DISTINGUISHING COMPREHENSION AND PRODUCTION OF SIGNALS

Human language is an extraordinary and unique means of communication involving left-hemispheric specialization for both production and comprehension, and specific properties such as intentionality, flexibility, categorization, referential properties, etc. Since nonhuman primates are very close to humans from a phylogenetic standpoint, research on their communicative systems might provide elements for inferring the features of our ancestral communicative systems. There is a considerable debate about whether precursors of language may be found either in the gestural or in the vocal communicative systems of our primate cousins within theoretical framework about the origins of language (e.g., Ghazanfar & Hauser, 1999, Corballis, 2002).

In the present paper, we propose that the advocates of gestural vs vocal origins of language might be reconciled if we foresee distinguishing the origins of the linguistic perceptive system from the human speech production system. In fact, it turns out that most of the arguments proposed by the proponents of the vocal hypothesis come from the findings that are specifically related to the perception of vocalizations, that might involves amodal processes of categorization and understanding of the external world, rather than the vocal modality itself (Meguerditchian & Vauclair, 2008). It is largely admitted that conspecific vocalizations are referential since listeners are able to extract information from vocal signals such as the identity of the caller, the nature of the social relationships among conspecifics, matrilineal kin, dominance rank. In congruence with such a potential behavioral continuity with speech comprehension, behavioral asymmetries and neurobiological studies showed that perception of vocalizations involve a left-hemispheric dominance and a cerebral circuit that might be related to Wernicke's area in humans (involved in language comprehension). Concerning the production of vocal signals, contrary to speech production, although a certain degree of audience's effect and of plasticity of the vocal system have been demonstrated in nonhuman primates, (1) they fail to be dissociated from their appropriated emotional context and are related to a whole group rather than a specific recipient; (2) the vocal repertoire remains inextensible across groups of a given species and the subtle inter- or intra-group structural variations reported in some vocal signals concern only existing species-specific vocalizations of this repertoire; (3) there is no evidence that vocal production involves left-hemispheric specialization and homologous language's areas, but rather subcortical areas and the limbic system (related to emotions in humans). By contrast, as in human language production, the use of communicative gestures appears to be (1) much more flexible within a extensible gestural repertoire, (2) independent of a specific social context; (3) clearly intentional, exclusively directed to a specific recipient (Pika, 2008); (4) to involve left-hemispheric specialization and homologous of Broca's area in chimpanzees according to both behavioural asymmetries studies and neuroanatomical and neurofunctional imaging studies (Taglialatela et al., 2008), In conclusion, we suggest that the abilities in nonhuman primates for processing meaningful vocalizations are better related to their remarkable capacity to understand and categorize the external world, including vocalizations and visual events (Cheney & Seyfarth, 1990), rather than to their specific vocal production system. Then, such features might constitute the precursor of the representational processes involved in the comprehension of language in humans whereas the properties of gestural communication appear more convincing for inferring the prerequisites of the speech production system.

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