Brownian thought space

Cognitive science, mostly, but more a sometimes structured random walk about things.

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Sunday, September 26, 2010

The question of dumb rats (and smart babies)

Some scientists seem to believe that rats predicting when the next foot shock happens is all we need to know to understand how babies learn language. Should you?
I came across an interesting blog post that is essentially more anti-Chomsky/Pinker rhetoric. It seems that some baby research likes to throw its little subjects out with their clichés, to describe «new and revolutionary ways» in thinking about language. So here is some attempt to restore balance to the universe.
The general line from that blog is something like this:
(1) Lack of negative evidence is critical to ChomskyPinkerian's position that there must be innate constraints on language acquisition.
(2) In a tone-shock associative task, rats are sensitive to the statistical pattern of presence and absence of the tone, and learn to predict the shock based on (what she calls) positive and negative evidence.
(3) Since rats can learn arbitrary relationships from such negative evidence, why do ChomskyPinkerians believe babies cannot, and instead invoke innate constraints? (followed by some wrist-slapping).
First of all, notice that the example is rotten - this is not the sense of negative evidence that's relevant at all. This is just saying that rats are sensitive to the probability of a shock given a tone. In the "negative example" case, it's just that the conditional probability p(shock|tone) is less than 1.0
The lack of negative evidence is a logical argument that's quite ridiculously simple: given finite data, you cannot deduce the true underlying generative system. Therefore, if all you're exposed to is a finite set of sentences, you cannot infer the true underlying (generative grammatical) system, unless you have something that guides you to the right solution space.
But you don't have to believe me when I say it's hard - look at the simple evidence: even with several bazillion sentences at our disposition, and all the fancy computational tools, no one has yet come up with an adequate description of a generative system that will produce all and only grammatical English sentences.
But a 5-year-old does end up knowing the generative system, and will produce sentences you sometimes wish she wouldn't, and will write little stories, making original sentences that she couldn't possibly have just overheard anywhere.
That is roughly the argument for why there must be something inside the baby that makes it such a genius at figuring out how the language system works. And this something (let's call it Flynn) is (part of) the reason why human babies, but not rat babies or komodo dragon babies or little guppies, end up learning language.
Does this mean there cannot be any general cognitive principles? Of course not, and no one claims there cannot be or that they don't affect language learning (that would be the Flab). What they are and how they do their job is an empirical question. Saying something is innate is merely a description - it doesn't tell you how the system works; that's what keeps researchers in business.
And coming to negative evidence, here's a fantastic quote from Roger Brown et al (quoted in Dan Slobin's Psychlinguistics):

What circumstances did govern approval and disapproval directed at child utterances by parents? Gross errors of word choice were sometimes corrected, as when Eve said What the guy idea. Once in a while an error of pronunciation was noticed and corrected. Most commonly, however, the grounds on which an utterance was approved or disapproved ... were not strictly linguistic at all. When Eve expressed the opinion that her mother was a girl by saying He a girl mother answered That's right. The child's utterance was ungrammatical but mother did not respond to the fact; instead she responded to the truth value of the proposition the child intended to express. In general the parents fit propositions to the child's utterances, however incomplete or distorted the utterances, and then approved or not, according to the correspondence between the proposition and reality. Thus Her curl my hair was approved because mother was, in fact, curling Eve's hair. However, Sarah's grammatically impeccable There's the animal farmhouse was disapproved because the building was a lighthouse and Adam's Walt Disney comes on, on Tuesday was disapproved because Walt Disney comes on, on some other day. It seems then, to be truth value rather than syntactic well-formedness that chiefly governs explicit verbal reinforcement by parents. Which render mildly paradoxical the fact that the usual product of such a training schedule is an adult whose speech is highly grammatical but not notably truthful (Brown, Cazden, and Bellugi, 1967, pp. 57-58).