The idea of a critical period for language learning was first suggested in the work of Penfield – Roberts [Penfield, W. – Roberts, L. 1950. Speech and Brain Mechanisms. Princeton: Princeton University Press.] They argued that adults have difficulty in learning second languages because after puberty cortical lateralization is complete. This means that the language function becomes localized in the left hemisphere and the plasticity of the brain atrophies.
- Romaine (1988), Pag. 215

Geschwind [Geschwind, N. 1980. “Some comments on the neurology of language”, in Caplan, D. (ed.), Biological Studies of Mental Processes. Cambridge, Mass. : MIT Press.] […] stresses that it is functions that are lateralized, not skills as a whole, such as language. He adds that it should not be assumed that the lateralization of function is necessarily equivalent to lateralization of language. Part of the problem in evaluating the evidence of neurological studies in terms of its linguistic implications is that the result are often obtained from pathological cases […] or from populations of a different nature from those normally investigated by second language researchers.
- Romaine (1988), Pag. 216