The critical period for brain development and language learning


Critical period

Critical periods, the windows of plasticity of brain development. Adapted from Hensch, T.K. (2005). Critical period plasticity in local cortical circuits. Nature Reviews Neuroscience 6(11), 877-888.

Ask a Japanese native speaker to tell the difference between ‘Lion’ and ‘Ryan’–that’s gonna be a difficult task. To pronounce these two words correctly? Even harder. Why so?

The problem is, in Japanese, the language doesn’t make the distinction between the sounds /r/ and /l/, as a result, people who only speak Japanese tend to ‘generalise’ the range of sounds between /r/ and /l/ to only one sound. When the brain is tuned to only using this one language, the ability to tell these sounds apart is lost. Where for people who speak English, their brain is well trained to recognise this difference.

 

Similarly, for English speakers who try to learn Chinese, the ‘tones’ of words (e.g. mā, má, mǎ, mà) sound like incredibly subtle differences; and for a Chinese to learn German, it just seems never possible to get the rolling R right..

 

However, babies are naturally born with the ability to tell all these subtle differences from all the languages in the world.

 

Researchers have discovered a window of time that is vitally important in the development of children’s first language and that is called the “critical period”. In human, this critical period is closed at 8-12 months old. It is generally held that once this period is missed, it is utterly impossible to develop a native language after that. This is true for grammar (how to make sentence), phonology (perceiving tiny features of sounds) and other aspects of language.

 

Perineuronal Net (PNN) surrounding a nerve cell.  James M. Massey, et al., The Journal of Neuroscience 2006, 26(16): 4406-4414.

Perineuronal Net (PNN) surrounding a nerve cell. James M. Massey, et al., The Journal of Neuroscience 2006, 26(16): 4406-4414.

What is actually happening is that when the brain is within this ‘window of plasticity’ of the critical period, the excitatory neurons are very active, and the inhibitory neurons have not taken place yet, resulting in an overall high level of brain activity. What’s more, at this point, neurons have not developed the ‘perineuronal nets’–a layer of thick extracellular matrix that tightly covers the whole cell body. After the critical period passed, perineuronal nets are quickly produced, stabilising the whole neural network, but making it difficult for neurons to make new connections.

 

Phonology-wise, neonates, who are not yet exposed to their first language, possess the best sensitivity to all the features of sounds that human languages. This is a remarkable gift in stark contrast with adults who are well immersed in their first language and who have almost completely lost sensitivity to any other phonological features that do not exist in their first language. Therefore, as children develop their first language, they are more sensitive to the features of the sounds in the first language with a rapid decline in the gift to discern the unheard phonological features in other possible languages.

 

Therefore, a Japanese learner who started learning English will find it difficult to differentiate the sound /r/ from /l/ because both are regarded as the same sound in Japanese. However, although you may not be a multilingual family, it is still likely that a child can still gain advantages in learning foreign languages if the small window of the critical period can be exploited with early exposure to foreign language stimuli.

 

Most studies on early adopted children find that although they have not fully develop the language of their birthplace, short period of experience in early life can still leave trace which gives them some extra advantage if they would re-pick up their lost language especially when pronunciation is concerned. Following the same rationale, it is highly likely that early exposure to foreign languages can give infants an advantage if they want to pick them up at a later stage of life.