Critical periods in language acquisition and language attrition

The issue of the " critical period hypothesis for language acquisition " is the focus of a vast literature at the outset that the expression " Critical Period Hypothesis " (CPH) is used with two different meanings. The first meaning of the CPH corresponds to an empirical hypothesis according to which humans are more efficient at language learning in the first years of life. In other words, the CPH states that age of acquisition is an important predictor of ultimate proficiency: the older one starts to learn a language, the smaller the odds of reaching native­like proficiency. The expression " critical period hypothesis " is also sometimes used to refer to the concept that an age­related decline in neural plasticity is the cause of increasing difficulties in language learning (Penfield & Roberts 1959). According to this second meaning, the CPH is a potential explanation of age effects on language acquisition. The two meanings must be distinguished because there may be a critical period according to the first meaning, that is, a detrimental effect of age of acquisition on ultimate proficiency in a language, even if the explanation in terms of loss of neural plasticity is wrong. In other words, an adverse effect of age of acquisition (AoA) may have other causes than irreversible neural changes (Birdsong 1999b lists some alternative explanations). Another point is worth emphasizing. In the framework of the Critical Period Hypothesis (with the second meaning), it is often assumed that the putative loss of plasticity is due to maturational factors. Yet, it could also be the outcome of language acquisition itself, as suggested by the following quote from Wilder Penfield, one of the earliest advocates of the CPH: Before the child begins to speak and to perceive, the uncommitted cortex is a blank slate on which nothing has been written. In the ensuing years much is written, and the writing is normally never erased. After the age of ten or twelve, the general functional connexions have been established and fixed for the speech cortex. Language acquisition circuitry is not needed once it has been used. It should be dismantled if keeping it around incurs any cost [...] Greedy neural tissue lying around beyond its point of usefulness is a good candidate for the recycling bin (Pinker 1994: 294–