In recent posts I have made a number of claims about language and the brain. Allow me to clarify and develop a couple of points.
I don't really want to buy into the debate about various versions of modularity or other theories of mental functioning. For one thing, I don't know the science well enough. I don't have a theory, but I don't know that I need one either.
Which is not to say that it is not important to have a basic understanding of how our minds work. My point is that such an understanding needn't take the form of a theory. It may simply develop from a general (or specialist) knowledge of pertinent disciplines (such as psychology or linguistics), and as a considered response to various kinds of evidence. I am particularly interested in the evidence provided by injuries and genetic disorders which affect cognitive and emotional functioning.
Certain genetically-caused disorders and brain injuries seem to provide evidence that language is in some sense a distinct system - or rather a set of systems - even if it interacts (as it obviously does) with non-linguistic processes. How else can you account for people who have a language deficit but can think well in other respects, or, conversely, who may be seriously cognitively impaired and yet maintain excellent language abilities?
Take Williams syndrome, for instance. It is a genetic disorder characterized by a range of medical problems, developmental delays and learning disabilities. Children with this condition seek interaction with others but are very vulnerable as they lack normal caution and social understanding. They are typically unable to cope with numbers and abstract reasoning. They also have impaired gross and fine motor skills.
On the positive side, they often have an affinity for music (and perfect pitch). And they also tend to do well linguistically, at least in certain respects.
Williams syndrome, like so many other conditions which impact on brain function, is selective in its effects. If specific aspects of thinking are adversely affected while other specific aspects are not affected or are enhanced, then this certainly supports the view that the brain consists of many (interacting) systems and sub-systems.
Linguists, of course, see language from various points of view corresponding to various sub-disciplines: phonetics (where the focus is on the actual sounds of language), phonology (more abstract), morphology and syntax, semantics, pragmatics, etc. In other words, language has many aspects, so it is misleading to talk about language ability without specifying exactly what one is talking about.
Likewise, it is not particularly helpful to talk about the brain's capacity for language per se. Better to focus on the particular processes which language use requires, like hearing (or seeing in the case of reading); interpreting the raw data (identifying phonemes and lexemes, parsing, etc.) and so understanding; or speaking (which involves not only mentation but also a very complex sequence of fine motor processes).
Children with Williams syndrome are typically slow to start speaking. This is presumably related at least in part to their fine motor problems. Most reference sources say that older children and adults with WS speak fluently and grammatically and have a good concrete, practical vocabulary (though abstract vocabulary remains deficient).
I picked Williams syndrome to focus on in this post because of an anecdotal report I remembered reading about a profoundly retarded girl with WS who nonetheless had an unusually extensive vocabulary and was able to invent strikingly original stories and fantasies. But the more I read about Williams syndrome the more complicated - and equivocal - the picture looks.
For example, consider this (from a recent research report* abstract): 'Williams syndrome (WS) is a neurodevelopmental genetic disorder, often referred [to] as being characterized by dissociation between verbal and non-verbal abilities, although a number of studies disputing this proposal is emerging.'
And in their own study the researchers found significantly more speech disfluencies (hesitations, repetitions, pauses) in the WS group than in a typically-developing group.
So the lesson of my story is that everything concerning the human brain is likely to be more complicated than it seems, and that only scientific findings - rather than models or theories - can give specific answers to specific questions. Of course, science requires its models and theories, but they are always provisional, a means to an end.
And, in the context of such reflections, it is hardly surprising that I find myself becoming more and more skeptical about certain Chomskian assumptions which have been part of my mental furniture since I took a linguistics course taught by one of the Master's protégés a couple of decades ago.
* Rossi, N.F. et al. 'Analysis of speech fluency in Williams syndrome.' Res. Dev. Disabil. 32(6) (2011): 2957-62.