A Theory of Neurolinguistic Development

Copyright © John L Locke, Lehman College, The City University of New York. Reproduced with permission of the author.

Abstract

This article offers a developmental theory of language and the neural systems that lead to and subserve linguistic capabilities. Early perceptual experience and discontinuities in linguistic development suggest that language develops in four phases that occur in a fixed, interdependent sequence. In each phase of language, a unique ontogenetic function is accomplished. These functions have proprietary neural systems that vary in their degree of specialization. Of particular interest is an analytical mechanism that is responsible for linguistic grammar. This mechanism is time-locked and can only be turned on in the third phase. Confirming evidence is provided by children who are delayed in the second phase of the language learning process. These children store insufficient lexical material to activate their analytic mechanism. Inactivation behaves like damage, shifting language functions to homologous mechanisms in the nondominant hemisphere, thereby increasing functional and anatomical symmetry across the hemispheres. This atypical assembly of neurolinguistic resources produces functional but imperfect command of spoken language and may complicate learning of written language. The theory thus offers a different role for genetics and early experience, and a different interpretation of neuroanatomic findings, from those entertained in most other proposals on developmental language disorders.

I thank Michael Studdert-Kennedy and Noam Chomsky for helpful comments on the manuscript.

Thanks are also due Dorothy Bishop, Leslie Brothers, Marcy Dorfman, Paul Macaruso, Aniruddh Patel, Elena Plante, and Michael Smith for their assistance. I am greatly indebted to Patrick Bateson for discussions of the critical period concept.

OVERVIEW

This article offers a theory according to which neurolinguistic capacity develops in individual phases that occur in a fixed and overlapping sequence.

In each phase, a unique function is accomplished, and each phase has its own commitment of neural resources. The first phase is indexical and affective; the infant is strongly oriented to the human face and voice, and learns caregivers' superficial vocal characteristics. The second phase is primarily affective and social: its function is to collect utterances, a responsibility that is subserved largely by mechanisms of social cognition sited principally in the right hemisphere. The third phase is analytical and computational. Previously stored forms are decomposed into syllables and segments, a process that facilitates discovery of regularities and is thereby responsible for the child's discovery and subsequent application of grammatical rules. This phase is active for a finite period and is largely served by left hemisphere mechanisms that make possible phonology, morphology, and syntax. The fourth phase is integrative and elaborative. In collaboration with acquisitive dispositions, this phase enables extensive lexical learning. Children who are delayed in the second phase have too little stored utterance material to activate their analytic mechanism at the optimum biological moment, and when sufficient words have been learned, this modular capability has already begun to decline. Inactivation has the same effect as damage. Compensatory use of homologous right hemisphere structures, set in motion by lexical delay, causes increases in functional and anatomical symmetry across the hemispheres.

The resulting neurolinguistic resources, not being specialized for phonological operations, are minimally adequate, but not optimal, for development of spoken language and may also disfavor phonological encoding and decoding operations associated with written language.

A theory of neurolinguistic development must deal with several questions about the ontogeny of linguistic capacity. First, what neural mechanisms support the development of language? Second, when and under what neurogenic and behavioral circumstances do these mechanisms become active?

Finally, when does the activation period for these mechanisms draw to a close? As the literature on sensitive periods is (indirectly) relevant to these questions, it is necessary first to examine possible contributions of this literature to an ontogenetic theory of language.

A SENSITIVE PERIOD FOR LANGUAGE

The literature contains four types of evidence for a sensitive period (see reviews in Locke, 1993a, 1994b). First, there are the usually dramatic case studies of individuals who were socially deprived in their infancy (cf. Curtiss, 1977). The theoretical impact of these studies has been limited, presumably because there are few well-documented cases and no reports in which significant linguistic deprivation occurred without extreme neglect and abuse.