>>The reason for this is, as I see, that the more you are involved in an activity, the more neural connections are formed for that particular function.<<
Yeah, this is so well known it's cliché: as is the stuff I pasted.
>>This can also explain the ease with which people who have learnt a foreign language will learn a new one even more easily. But I guess this does not depend on age, or at least not completely.<<
It's like a triathlete adapting his physiology to middle-distance running, but for someone who's always been sedentary, this adaptation is a lot more difficult. So if your language base is European, for example, then it'll be easier to learn another European language than it would be an Asian language.
On the other hand, learning a second language for an adult means WORKING but at the same time CONTENDING with many more active neural connections of the brain, compared to a child; this could be advantageous in some ways, disadvantageous in others.
With a child, it's like a clean slate, with less resistance to oppose the input and mapping process. But it's clear the physiological and environmental differences mean the learning mechanisms for children and adults aren't the same.
"Native Japanese speakers, for example, typically do not discriminate the "r" from the "l" sounds that are evident to English speakers, and this ability is lost in early childhood because it is not in the speech that they hear."
This change is physiological in the trivial sense, i.e. in the same way that learning a new word results in a physiological change in the brain.
This does not prove that the inability to discriminate [r] and [l] is irreversible.
Neither does it prove that it is more difficult for an adult Japanese to recognize [r] and [l] than it is for an American infant.
"Neither does it prove that it is more difficult for an adult Japanese to recognize [r] and [l] than it is for an American infant."
But the study does point out that Japanese infants can easily discriminate "l" and "r" as opposed to their adult counterparts.
And why did you compare Japanese ADULTS to American INFANTS? Wouldn't it have been more fair to compare Japanese adults to American adults?
First of all, I want to make clear that I consider motivation probably the most important factor here. Nevertheless, I do not exclude the influence of other factors, particular, changes in the brain that are age related.
The fact of changes in language related areas is well established by many experiements, but this fact itself does not constitute the proof of inability for adults to acquire native proficiency or even that it is much more difficult for adults than children to learn a second language. If we want to establish whether it is more difficult or not, we have to consider differences in the learning process to assess what aspects are more difficult from one group than the other.
There are two obvious advantages that adults have over young children (under age 7): first, ability to abstract thinking (learn the rules of the language consciously); second, ability to focus conscious attention on something and to commit larger chunks of text in memory by rote. Naturally, every adult tries to use this advantage to learn a language faster. Whether this advantage is mostly short term, and may have a negative effect in the long run is an interesting question. I do not have the answer to it right now.
However, there is at least one area where it seems young children do much better than adults -- pronunciation. Different experiements have suggested that while children are worse at recognition phonemes of their native language than adults, they were better at discremination phonemes non-typical for their native tongue. The reason for that children use much more regions of brain than adults.
Harking back to the presentation. Tom, you wrote: "The older you are, the less likely your peers are to make fun of you because of your foreign accent or poor grammar, so you have fewer reasons to take English seriously." This statement is incorrect. In fact, peer pressure and bulling tend to grow in the beginning and reach the nadir about age 12. So the linear decline of grammar skills with age cannot be explained by peer pressure.
Nevertheless, I do not find the presented results as a sufficient proof that age is the only possible explanation here. If you read the text carefully, then you would notice "Compared to early-learners, late-learners show deviant activity (in the frontal lobe) during a grammatical task." That means that late-learners tried to use knowledge of grammar rules to detect an error, while their 'unconscious grammar' was undeveloped.
The developing of 'unconscious grammar' is a slow process that requires a lot of authentic input, but not any input will work. The input should be roughly a step ahead of the current level of 'unconscious grammar' of the learner. Obviously, late-learners were exposed to input that was not tuned to their needs. If you give an adult person too difficult input, s/he will use her/his analatical abilities (such as knowledge of grammar) to decode the message, but that will not contribute anything to developing 'unconscious grammar'. Another tendancy common among adult learners is what is called 'gestalt' perception. It is when a large fragment of speech is remembered as a fixed expression (or almost fixed expression with only one word to fill in). These fixed phrases are very useful for communication, but it seems they are not translated into unconscious grammar.
In the same time, the provided above explanation for grammar can be applied to pronunciation, and taking into accound some other data, I think that age is important in acquiring authentic pronunciation. It is not that it is impossible for adults, but it seems it more difficult for them because their brain are tuned to the specific phone-set, and speech habits are much more fixed.
Doing an imaging study of the brain is like taking an infrared photograph of the motherboard in a PC—in other words, it's just as imprecise and useless.
It might be imprecise compared to other imaging systems (and maybe even to the scalpel) and useless to you, but it proves physiological changes in the brain are an ongoing process -- it's no fiction. The detail provided by a microscope at the atomic level isn't necessary here.
The brain is organic like any other part of the body and it reacts like any other nerve or muscle: exercise a muscle and it increases in size. It's not so unbelievable but please, continue to argue away.