Brain Organization and Language Development
It is thought that if language is indeed an innate capacity, there must be a neural basis for this capacity that is present from birth. In this regard, there has been some debate about the concepts of equipotentiality and differential lateralization at birth.
Equipotentiality of the brain for language means that both sides are capable of performing linguistic functions. Equipotentiality also suggests plasticity of the brain and a ca-pacity for reorganization even after injury so that language functions can be transferred from one hemisphere to the other. Indeed, lesion data for childhood aphasia and earlier reports of hemispherectomy studies have suggested functional recovery with respect to language until puberty, as evidenced by ipsilat-eral and contralateral transfer of linguistic functions. Thus, it has been reported that if there is damage to the left side in infancy but the right side remains intact, the child can still develop normal language (Curtiss, 1989; Dennis and Whitaker, 1976).
In early childhood, after the onset of language but before age 4 years, damage to the language areas results in transient aphasia. After puberty, by about age 14 years, the prognosis begins to worsen, and similar lesions in adulthood can cause irreversible deficits (Lenneberg, 1967, 1969). Some authors, such as Lenneberg (1967), have taken these findings about the capacity for transfer of function in young children to mean that the brain is symmetrically organized to begin with and only gradually becomes asymmetri-cally specialized, resulting in a diminished capacity for recovery with age. However, there is now increasing evidence that the brain is asymmetrical with respect to linguistic ability from birth.
If the brain is lateralized for linguistic functions from birth, a question arises about the evidence for greater recoverability of language when damage occurs early enough in life. One interpre-tation of this recoverability is that there is greater neuroplasticity in the young brain, as evident from the capacity of surviving neu-rons to make new synaptic connections even after injury.
Lennebergâ€™s view of the critical period for language devel-opment (from birth to the early teens) was that both hemispheres are involved in language functions at first, but by puberty the left becomes more specialized. A later view relates the concept of critical period to the notion of neural plasticity. In any case, it is thought that the critical period is correlated with innate mecha-nisms, and that language development is most susceptible to the limiting effects of both biological and environmental factors if these extend beyond this critical period.