Linguistically deprived children: meta-analysis of published research underlines the importance of early syntactic language use for normal brain development

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 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.

Language evolution to revolution: the leap from rich-vocabulary non-recursive communication system to recursive language 70,000 years ago was associated with acquisition of a novel component of imagination, called Prefrontal Synthesis, enabled by a mutation that slowed down the prefrontal cortex maturation simultaneously in two or more children – the Romulus and Remus hypothesis

“…In other words, deaf linguistic isolates grow up exposed to a non-recursive communication system with large number of sign-words. Our analysis shows that these individuals deprived of recursive syntactic conversations until puberty performed poorly in all PFS tests, both verbal and nonverbal (such as those shown in Figure 1) despite focused multi-year post-pubertal rehabilitation efforts 3 . The consistent observation of PFS disability in these individuals stands in stark contrast to their performance on memory as well as semantic tests: they could easily remember hundreds of newly learned words and recall previously seen images from memory but had real difficulty in any tasks requiring them to mentally combine these images into novel configurations.…”

“…Note that patients with PFC disability commonly have no problem with simpler performance IQ tasks, such as integration of modifiers 4 . ; and the LPFC that synthesizes the objects from memory into a novel mental image according to grammatically imposed rules 3,4 . Crucially, PFS definition leaves out interpretation of grammar in the Broca's area and leaves out linking words with objects in the Wernicke's area.…”

“…Less common is realization that understanding of full recursive syntactic language depends on the lateral prefrontal cortex (LPFC). Wernicke's area primarily links words with objects 2 , Broca's area interprets the grammar and assigns words in a sentence to a grammatical group such as noun, verb, or preposition 2 , but only the LPFC can synthesize the objects from memory into a novel mental image according to grammatically imposed rules 3,4 . The latter function is commonly called imagination.…”

Novel Linguistic Evaluation of Prefrontal Synthesis (LEPS) test measures prefrontal synthesis acquisition in neurotypical children and predicts high-functioning versus low-functioning class assignment in individuals with autism

“…Propensity to acquire PFS seems to start before the age of two (Bick et al, 2015), reduces notably after five years of age (Basser, 1962;Boatman et al, 1999;Krashen & Harshman, 1972;Lenneberg, 1967;Pulsifer et al, 2004;Vyshedskiy et al, 2019), and ceases completely after puberty (Vyshedskiy, Mahapatra, et al, 2017). As the result of this strong and short critical period, the ability of children to acquire PFS can be significantly diminished by the time they enter the public school system.…”

“…Only by defining PFS as a separate neurological mechanism, were we able to discover the strong critical period for PFS acquisition. Specifically, individuals who have not acquired PFS in early childhood cannot develop PFS later in life despite years of therapy Vyshedskiy, Mahapatra, et al, 2017). Strong critical periods are not unusual in central nervous system development.…”