The Arcuate Fasciculus and language origins: Disentangling existing conceptions that influence evolutionary accounts

Abstract

The Arcuate Fasciculus (AF) is of considerable interdisciplinary interest, because of its major implication in language processing. Theories about language brain evolution are based on anatomical differences in the AF across primates. However, changing methodologies and nomenclatures have resulted in conflicting findings regarding interspecies AF differences: Historical knowledge about the AF originated from human blunt dissections and later from monkey tract-tracing studies. Contemporary tractography studies reinvestigate the fasciculus' morphology, but remain heavily bound to unclear anatomical priors and methodological limitations. First, we aim to disentangle the influences of these three epistemological steps on existing AF conceptions, and to propose a contemporary model to guide future work. Second, considering the influence of various AF conceptions, we discuss four key evolutionary changes that propagated current views about language evolution: 1) frontal terminations, 2) temporal terminations, 3) greater Dorsal-versus Ventral Pathway expansion, 4) lateralisation. We conclude that new data point towards a more shared AF anatomy across primates than previously described. Language evolution theories should incorporate this continuous AF evolution across primates.

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

“…Therefore, a growing number of researchers proposed that language organisation took some of its phylogenetical roots into a gestural system across primate evolution ( Gentilucci and Dalla Volta, 2008 ; Corballis, 2015 ; Tomasello, 2008 ). Consequently, whereas comparative language research has focused on the potential continuities across primate brain circuitry (e.g., Balezeau et al, 2020 ; Becker et al, 2022 ) or vocal and auditory systems (e.g. Boë et al, 2017 ; Jarvis, 2019 ; Wilson et al, 2017 ), the research on gestural communication in apes and monkeys has historically shown a significant interest within this evolutionary framework.…”

Is Reduced Visual Processing the Price of Language?

“…The left-lateralized perisylvian network, then connects the auditory cortex to Wernicke’s and Broca’s areas through the Arcuate Fasciculus (AF), a bundle of fibers that have been the subject of recent scientific discourses with respect to neuroanatomical differences between the brain of macaque and that of human, within the frontotemporal area. The data favors a “progressive evolution of the AF during primate evolution rather than great leaps that would indicate categorial changes of the AF’s morphology between contemporary species.” [ 37 ] (p. 86). Furthermore, “recent works in human and non-human primates suggest that the AF course and terminations are more similar between species than described before” [ 37 ] (p. 86).…”

“…The data favors a “progressive evolution of the AF during primate evolution rather than great leaps that would indicate categorial changes of the AF’s morphology between contemporary species.” [ 37 ] (p. 86). Furthermore, “recent works in human and non-human primates suggest that the AF course and terminations are more similar between species than described before” [ 37 ] (p. 86).…”

“…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.…”