Abstract
We analyzed all published reports of individuals not exposed to syntactic language until puberty: two feral children, who grew up without hearing any language, and eight deaf linguistic isolates, who grew up communicating to their families using homesign or kitchensign, a system of gestures which allows them to communicate simple commands but lacks much in the way of syntax. A common observation in these individuals is the lifelong difficulty understanding syntax and spatial prepositions, even after many years of rehabilitation. This debilitating condition stands in stark contrast to linguistic isolates’ performance on memory as well as semantic tests: they could easily remember hundreds of newly learned words and identify previously seen objects by name. The lack of syntactic language comprehension in linguistic isolates may stem from inability to understand words and/or grammar or inability to mentally synthesize known objects into novel configurations. We have previously shown that purposeful construction of novel mental images is the function of the lateral prefrontal cortex (LPFC) ability to dynamically control posterior cortex neurons (Vyshedskiy et al. 2017). Here we have ranked all tests performed on linguistic isolates by their reliance on the LPFC control of the posterior cortex: a) the amount of posterior cortex territory that needs to be recruited by the LPFC and b) the number of disparate objects that have to be combined together by the LPFC in order to answer the test question. According to our analysis, linguistic isolates performed well in all tests that did not involve the LPFC control of the posterior cortex, showed decreasing scores in tests that involved greater recruitment of the posterior cortex by the LPFC, and failed in tests that involved greatest recruitment of posterior cortex necessary for mental synthesis of multiple objects. This pattern is consistent with inadequate frontoposterior connections in linguistic isolates. We discuss implications of these findings for the importance of early syntactic language exposure in formation of frontoposterior connections.
Highlights
There is general consensus that early time-sensitive exposure to syntactic language is important for acquisition of the full extent of a complex syntactic language (Chomsky 1959, Chomsky 1964, Wilder and Lamar 1959, Lenneberg 1967)
We noticed that mental synthesis of disparate objects is the function of the lateral prefrontal cortex (LPFC) ability to control the posterior cortex neurons (Vyshedskiy et al 2017) and ranked all verbal and nonverbal tests performed by linguistic isolates by: a) the number of disparate objects that have to be combined together by the lateral PFC (LPFC) in order to answer the question and b) the amount of physical territory within the posterior cortex that needs to be recruited by the LPFC
It is commonly believed that there is a strong association between early language acquisition and normal cognitive development (Chomsky 1959, Chomsky 1964, Wilder and Lamar 1959, Lenneberg 1967); there is no consensus on the neurological mechanism of this association
Summary
There is general consensus that early time-sensitive exposure to syntactic language is important for acquisition of the full extent of a complex syntactic language (Chomsky 1959, Chomsky 1964, Wilder and Lamar 1959, Lenneberg 1967). The mechanism of this process, remains unclear. Studying cognitive processes in adults and adolescents who have been linguistically isolated during their childhood could contribute to this discussion. Linguistically isolated children grow up with no verbal or sign communication to other humans. Isolated children, on the other hand, grow up communicating to other humans using simple signs void of syntax, grammar and spatial prepositions
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