Abstract
Inner speech emerges in early childhood, in parallel with the maturation of the dorsal language stream. To date, the developmental relations between these two processes have not been examined. We review evidence that the dorsal language stream has a role in supporting the psychological phenomenon of inner speech, before considering pediatric studies of the dorsal stream’s anatomical development and evidence for its emerging functional roles. We examine possible causal accounts of the relations between these two developmental processes and consider their implications for phylogenetic theories about the evolution of inner speech and the accounts of the ontogenetic relations between language and cognition.
Highlights
Studying inner speech in childhood is fraught with difficulty, there is a consensus that this pattern corresponds to the emergence of fully internalized inner speech as private speech “goes underground” (Vygotsky, 1987), and the findings suggest that children begin to understand the concept of inner speech in the preschool and middle school years (Flavell et al, 1993, 1997, 2001; Fernyhough, 2009)
If early humans had under-developed arcuate fasciculus (AF), and if highly developed AF is the neural substrate for inner speech production, one might suggest that early humans had no, or at least limited, inner speech
Neurodevelopmental studies have shown that humans are born with a dorsal language stream which is not fully developed and that it slowly matures throughout early childhood
Summary
Inner speech emerges in early childhood, in parallel with the maturation of the dorsal language stream. A study of 7- to 68-year-olds found similar results, showing that all three segments of the dorsal language stream (anterior, posterior, and direct) reach full maturation around age 20–30 (Hasan et al, 2010). By combining findings from different disciplines, we have presented evidence that the maturation of the dorsal language stream, especially the fronto-temporal and fronto-parietal segments, during childhood occurs in parallel with the development of inner speech. There is more specific experimental evidence to support this hypothesis: firstly, studies suggested that language learning in adults is mediated by subvocal rehearsal and is correlated with the integrity of the dorsal tracts (LopezBarroso et al, 2011, 2013); and secondly, children’s performance on phonological awareness tasks, often requiring inner speech, is correlated with dorsal pathway development (Yeatman et al, 2011; Vandermosten et al, 2012, 2015). Further research on the interplay between the ventral and the dorsal language streams may pay dividends for our understanding of functionally relevant distinctions between forms of inner speech, such as the distinction that can be made between subvocal rehearsal and planning (Alderson-Day and Fernyhough, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
