Comprehension –knowledge: a primary broad ability or a product of the application of other cognitive abilities?

Comprehension knowledge is the breadth and depth of skills and knowledge that are valued by one's culture.  Comprehension knowledge includes the ability to understand spoken language, the breadth of a person's lexicon and general knowledge, and a person's awareness of grammatical aspects of language.

Is comprehension knowledge an independent ability or a product of the application of other cognitive abilities?  In other words, is it a cognitive ability, like fluid ability, short term memory and long term storage and retrieval or is it an area of achievement like reading, writing and math?

As will be discussed below, it's possible to conceptualize comprehension knowledge both as an ability and as an achievement.

Language impairment (LI) is an impairment in expressive and/or receptive language development  in the context of otherwise normal development (i.e., nonverbal IQ and self-help skills).   Language impairment interferes with activities of daily living and/or academic achievement.   LI children usually have poor comprehension knowledge, although LI will not always manifest in poor scores on intelligence subtest scores measuring comprehension knowledge.  The reason for that is that these tests do not assess all aspects of comprehension knowledge and especially not the child's command of grammar and syntax.  

A broad distinction can be drawn between two classes of LI model: those that regard the language difficulties as secondary to more general nonlinguistic deficits, and those that postulate a specifically linguistic deficit.

 The best known example of the first type of model is the rapid temporal processing (RTP) theory of Tallal and colleagues, which maintains that language learning is handicapped because of poor temporal resolution of perceptual systems.   The first evidence for the RTP theory came from a study where children were required to match the order of two tones.  When tones were rapid or brief, children with LI had problems in correctly identifying them, even though they were readily discriminable at slow presentation rates.  Children who have poor temporal resolution will chunk incoming speech in blocks of hundreds of milliseconds rather than tens of milliseconds, and this will affect speech perception and hence on aspects of language learning.

In CHC terms Tallal's theory can be conceptualized as poor auditory processing and maybe also  poor processing speed that underlie LI. 

Another theoretical account that stresses nonlinguistic temporal processing has been proposed by Miller, who showed that children with LI had slower reaction times than did control children matched on nonverbal IQ on a range of cognitive tasks, including some, such as mental rotation, that involved no language. Unlike the RTP theory, this account focuses on slowing of cognition rather than perception.

 A more specialized theory is the phonological short-term memory deficit account of LI by Gathercole & Baddeley. These authors noted that many children with LI are poor at repeating polysyllabic nonwords, a deficit that has been confirmed in many subsequent studies. This deficit has been interpreted as indicating a limitation in a phonological short-term memory system that is important for learning new vocabulary  and syntax.  

Ullman argues that our use of language depends upon two capacities: a mental lexicon of memorized words and a mental grammar of rules that underlie the composition of lexical forms into predictably structured larger words, phrases, and sentences.   On this view, the memorization and use of at least simple words depends upon an associative memory (the ability to learn links between pairs of stimuli, for example between the sequence of phonemes forming a word and its meaning.  Associative memory is a narrow ability within long term storage and retrieval).  The acquisition and use of grammatical rules depends upon procedural learning and memory (for instance, learning the rule/procedure of past tense as the addition of "ed" (walk – walked)).   Procedural memory enables us to learn many motor and cognitive "skills" and "habits" (e.g., from simple motor acts to skilled game playing).  LI, argues Ullman, is caused by poor associative and/or procedural memory.  LI is not a specifically linguistic disorder but is rather the consequence of an impaired system that will also affect learning of other procedural operations, such as motor skills.

To all this we can add Cattell's investment theory.  Cattell argued  that crystallized knowledge develops through the investment of fluid ability.  Babies are born with fluid ability only, which they use in their first encounters and experiences with the world.   Explicit and implicit memories formed in the baby's mind by these encounters and experiences gradually form his reservoir of crystallized knowledge.  From now on, the baby tackles new experiences equipped with both fluid ability and crystallized knowledge that he already acquired.  The more knowledge he acquires, the better is his ability to cope with situations he encounters, and the less fluid these situations become.

The theories presented above enable us to consider comprehension knowledge as an area of achievement.  When a child performs poorly in an area of achievement (reading decoding, reading comprehension, writing and spelling, math computations or math reasoning) we look for the cause of his poor performance among the cognitive abilities (fluid ability, short term memory, long term storage and retrieval, processing speed, visual processing, auditory processing, comprehension knowledge).  If comprehension knowledge is also an achievement area, when a child has poor comprehension knowledge, we should look for the reason for that in one of these cognitive abilities:  fluid ability, short term memory, long term storage and retrieval, processing speed, auditory processing and perhaps also visual processing (I don't know a theory linking LI to poor visual processing).

But comprehension knowledge is different than other achievement areas.  Reading, writing and math are acquired mostly through formal instruction (there are aspects of arithmetic that are acquired naturally without instruction – like counting procedures).  Native language is acquired mostly informally – mostly spontaneously/implicitly.  Formal instruction enriches and develops native language.

The authors above argued that domain-general deficits in cognitive and perceptual systems are sufficient to account for LI. This position differs radically from linguistic accounts of LI, which maintain that humans have evolved specialized language learning mechanisms and that LI results when these fail to develop on the normal schedule  . A range of theories of this type for LI focus on the syntactic difficulties that are a core feature of many children with LI. Children with LI tend to have problems in using verb inflections that mark tense, so they might say “yesterday I walk to school” rather than “yesterday I walked to school.” Different linguistic accounts of the specific nature of such problems all maintain that the deficit is located in a domain-specific system that handles syntactic operations and is not a secondary consequence of a more general cognitive processing deficit.

Pennington, B. F., & Bishop, D. V. (2009). Relations Among Speech, Language, and Reading Disorders. Annu. Rev. Psychol, 60, 283-306.‏  http://www.du.edu/psychology/dnrl/Relationsamongspeechlanguageandreadingdisorders.pdf

Schneider, W. J., & McGrew, K. S. (2012). The Cattell-Horn-Carroll model of intelligence. Contemporary intellectual assessment: Theories, tests, and, (3rd), 99-144.‏